F4-related mutation and expression analysis of the aminopeptidase N gene in pigs

Swine enteric colibacillosis: Current treatment avenues and future directions

Enteric colibacillosis is a common disease in nursing and weanling pigs. It is caused by the colonization of the small intestine by enterotoxigenic strains of Escherichia coli (ETEC) that make use of specific fimbria or pili to adhere to the absorptive epithelial cells of the jejunum and ileum. Once attached, and when both the immunological systems and the gut microbiota are poorly developed, ETEC produce one or more enterotoxins that can have local and, further on, systemic effects. These enterotoxins cause fluid and electrolytes to be secreted into the intestinal lumen of animals, which results in diarrhea, dehydration, and acidosis. From the diversity of control strategies, antibiotics and zinc oxide are the ones that have contributed more significantly to mitigating post-weaning diarrhea (PWD) economic losses. However, concerns about antibiotic resistance determined the restriction on the use of critically important antimicrobials in food-producing animals and the prohibition of their use as growth promoters. As such, it is important now to begin the transition from these preventive/control measures to other, more sustainable, approaches. This review provides a quick synopsis of the currently approved and available therapies for PWD treatment while presenting an overview of novel antimicrobial strategies that are being explored for the control and treatment of this infection, including, prebiotics, probiotics, synbiotics, organic acids, bacteriophages, spray-dried plasma, antibodies, phytogenic substances, antisense oligonucleotides, and aptamers.

Heat-labile enterotoxin enhances F4-producing enterotoxigenic E. coli adhesion to porcine intestinal epithelial cells by upregulating bacterial adhesins and STb enterotoxin

As one of the crucial enterotoxins secreted by enterotoxigenic Escherichia coli (ETEC), heat-labile enterotoxin (LT) enhances bacterial adherence both in vivo and in vitro; however, the underlying mechanism remains unclear. To address this, we evaluated the adherence of LT-producing and LT-deficient ETEC strains using the IPEC-J2 cell model. The expression levels of inflammatory cytokines and chemokines, and tight-junction proteins were evaluated in IPEC-J2 cells after infection with various ETEC strains. Further, the levels of adhesins and enterotoxins were also evaluated in F4ac-producing ETEC (F4 + ETEC) strains after treatment with cyclic AMP (cAMP). The adherence of the ΔeltAB mutant was decreased compared with the wild-type strain, whereas adherence of the 1836-2/pBR322-eltAB strain was markedly increased compared with the 1836-2 parental strain. Production of LT up-regulated the expression of TNF-α, IL-6, CXCL-8, and IL-10 genes. However, it did not appear to affect tight junction protein expression. Importantly, we found that cAMP leads to the upregulation of adhesin production and STb enterotoxin. Moreover, the F4 + ETEC strains treated with cAMP also had greater adhesion to IPEC-J2 cells, and the adherence of ΔfaeG, ΔfliC, and ΔestB mutants was decreased. These results indicate that LT enhances the adherence of F4 + ETEC due primarily to the upregulation of F4 fimbriae, flagellin, and STb enterotoxin expression and provide insights into the pathogenic mechanism of LT and ETEC.

Resistance to ETEC F4/F18-mediated piglet diarrhoea: opening the gene black box

Diarrhoea, a significant problem in pig rearing industry affecting pre- and post-weaning piglets is caused by enterotoxigenic Escherichia coli (ETEC). The ETEC are classified as per the fimbriae types which are responsible for bacterial attachment with enterocytes and release of toxins causing diarrhoea. However, genetic difference exists for susceptibility to ETEC infection in piglets. The different phenotypes found in pigs determine their (pigs') susceptibility or resistance towards fimbrial subtypes/variants (F4ab, F4ac, F4ad and F18). Specific receptors are present on intestinal epithelium for attachment of these fimbriae, which do not express to same level in all animals. This differential expression is genetically determined and thus their genetic causes (may be putative candidate gene or mutations) render some animals resistant or susceptible to one or more fimbrial subtypes. Genetic linkage studies have revealed the mapping location of the receptor loci for the two most frequent variants F4ab and F4ac to SSC13q41 (i.e. q arm of 13th chromosome of Sus scrofa). Some SNPs have been identified in mucin gene family, transferring receptor gene, fucosyltransferase 1 gene and swine leucocyte antigen locus that are proposed to be linked mutations for resistance/susceptibility towards ETEC diarrhoea. However, owing to the variety of fimbrial types and subtypes, it would be difficult to identify a single causative mutation and the candidate loci may involve more number of genes/regions. In this review, we focus on the genetic mutations in genes involved in imparting resistance/susceptibility to F4 or F18 ETEC diarrhoea and possibilities to use them as marker for selection against susceptible animals.

Binding determinants in the interplay between porcine aminopeptidase N and enterotoxigenic Escherichia coli F4 fimbriae

The binding of F4⁺ enterotoxigenic Escherichia coli (ETEC) and the specific receptor on porcine intestinal epithelial cells is the initial step in F4⁺ ETEC infection. Porcine aminopeptidase N (APN) is a newly discovered receptor for F4 fimbriae that binds directly to FaeG adhesin, which is the major subunit of the F4 fimbriae variants F4ab, F4ac, and F4ad. We used overlapping peptide assays to map the APN-FaeG binding sites, which has facilitated in the identifying the APN-binding amino acids that are located in the same region of FaeG variants, thereby limiting the major binding regions of APN to 13 peptides. To determine the core sequence motif, a panel of FaeG peptides with point mutations and FaeG mutants were constructed. Pull-down and binding reactivity assays using piglet intestines determined that the amino acids G159 of F4ab, N209 and L212 of F4ac, and A200 of F4ad were the critical residues for APN binding of FaeG. We further show using ELISA and confocal microscopy assay that amino acids 553–568, and 652–670 of the APN comprise the linear epitope for FaeG binding in all three F4 fimbriae variants.

Animal Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is the most common cause of E. coli diarrhea in farm animals. ETEC are characterized by the ability to produce two types of virulence factors: adhesins that promote binding to specific enterocyte receptors for intestinal colonization and enterotoxins responsible for fluid secretion. The best-characterized adhesins are expressed in the context of fimbriae, such as the F4 (also designated K88), F5 (K99), F6 (987P), F17, and F18 fimbriae. Once established in the animal small intestine, ETEC produce enterotoxin(s) that lead to diarrhea. The enterotoxins belong to two major classes: heat-labile toxins that consist of one active and five binding subunits (LT), and heat-stable toxins that are small polypeptides (STa, STb, and EAST1). This review describes the disease and pathogenesis of animal ETEC, the corresponding virulence genes and protein products of these bacteria, their regulation and targets in animal hosts, as well as mechanisms of action. Furthermore, vaccines, inhibitors, probiotics, and the identification of potential new targets by genomics are presented in the context of animal ETEC.

Proteome analysis for the global proteins in the jejunum tissues of enterotoxigenic Escherichia coli -infected piglets

Enterotoxigenic Escherichia coli (ETEC) is a common cause of diarrhea in humans and livestock. In this study, isobaric tags for relative and absolute quantitation (iTRAQ) combined with multidimensional liquid chromatography (LC) and MS analysis was used for screening the differentially expressed proteins in piglet jejunum after ETEC infection. Totally 1,897 proteins were identified with quantitative information in piglet jejunum. We identified 92 differentially expressed proteins in ETEC-induced diarrhea, of which 30 were up regulated and 62 down regulated. Most of the differentially expressed proteins were involved in intestinal function of binding, metabolic process, catalytic activity and immune responses. The inhibition of intestinal immune responses in the jejunum in ETEC-induced diarrhea was also validated by immunobloting and RT-PCR. Our study is the first attempt to analyze the protein profile of ETEC-infected piglets by quantitative proteomics, and our findings could provide valuable information with respect to better understanding the host response to ETEC infection.

Porcine aminopeptidase N binds to F4+ enterotoxigenic Escherichia coli fimbriae

F4⁺ enterotoxigenic Escherichia coli (ETEC) strains cause diarrheal disease in neonatal and post-weaned piglets. Several different host receptors for F4 fimbriae have been described, with porcine aminopeptidase N (APN) reported most recently. The FaeG subunit is essential for the binding of the three F4 variants to host cells. Here we show in both yeast two-hybrid and pulldown assays that APN binds directly to FaeG, the major subunit of F4 fimbriae, from three serotypes of F4⁺ ETEC. Modulating APN gene expression in IPEC-J2 cells affected ETEC adherence. Antibodies raised against APN or F4 fimbriae both reduced ETEC adherence. Thus, APN mediates the attachment of F4⁺E. coli to intestinal epithelial cells.

Variation in 12 porcine genes involved in the carbohydrate moiety assembly of glycosphingolipids does not account for differential binding of F4 Escherichia coli and their fimbriae

Background

Glycosphingolipids (GSLs) are important membrane components composed of a carbohydrate structure attached to a hydrophobic ceramide. They can serve as specific membrane receptors for microbes and microbial products, such as F4 Escherichia coli (F4 ETEC) and isolated F4 fimbriae. The aim of this study was to investigate the hypothesis that variation in genes involved in the assembly of the F4 binding carbohydrate moiety of GSLs (i.e. ARSA, B4GALT6, GAL3ST1, GALC, GBA, GLA, GLB1, GLB1L, NEU1, NEU2, UGCG, UGT8) could account for differential binding of F4 ETEC and their fimbriae.

Results

RT-PCR could not reveal any differential expression of the 12 genes in the jejunum of F4 receptor-positive (F4R⁺) and F4 receptor-negative (F4R^-) pigs. Sequencing the complete open reading frame of the 11 expressed genes (NEU2 was not expressed) identified 72 mutations. Although some of them might have a structural effect, none of them could be associated with a F4R phenotype.

Conclusion

We conclude that no regulatory or structural variation in any of the investigated genes is responsible for the genetic susceptibility of pigs towards F4 ETEC.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-014-0103-x) contains supplementary material, which is available to authorized users.