A long-acting heat-stable enterotoxin analog of enterotoxigenic Escherichia coli with a single D-amino acid

The Molecular Basis of Heat-Stable Enterotoxin for Vaccine Development and Cancer Cell Detection

Heat-stable enterotoxin (ST_a) produced by Enterotoxigenic E. coli is responsible for causing acute diarrhea in infants in developing countries. However, the chemical synthesis of ST_a peptides with the native conformation and the correct intra-molecular disulfide bonds is a major hurdle for vaccine development. To address this issue, we herein report on the design and preparation of ST_a analogs and a convenient chemical method for obtaining ST_a molecules with the correct conformation. To develop an ST_a vaccine, we focused on a structure in a type II β-turn in the ST_a molecule and introduced a D-Lys residue as a conjugation site for carrier proteins. In addition, the -Glu-Leu- sequence in the ST_a molecule was replaced with a -Asp-Val- sequence to decrease the toxic activity of the peptide to make it more amenable for use in vaccinations. To solve several issues associated with the synthesis of ST_a, such as the formation of non-native disulfide isomers, the native disulfide pairings were regioselectively formed in a stepwise manner. A native form or topological isomer of the designed ST_a peptide, which possesses a right-handed or a left-handed spiral structure, respectively, were synthesized in high synthetic yields. The conformation of the synthetic ST_a peptide was also confirmed by CD and NMR spectroscopy. To further utilize the designed ST_a peptide, it was labeled with fluorescein for fluorescent detection, since recent studies have also focused on the use of ST_a for detecting cancer cells, such as Caco-2 and T84. The labeled ST_a peptide was able to specifically and efficiently detect 293T cells expressing the recombinant ST_a receptor (GC-C) protein and Caco-2 cells. The findings reported here provide an outline of the molecular basis for using ST_a for vaccine development and in the detection of cancer cells.

Modified heat-stable toxins (hSTa) of enterotoxigenic Escherichia coli lose toxicity but display antigenicity after being genetically fused to heat-labile toxoid LT(R192G)

Enterotoxigenic Escherichia coli (ETEC) strains are a major cause of diarrhea in humans and animals. Heat-stable (STa) and heat-labile (LT) enterotoxins produced by ETEC disrupt fluid homeostasis in host small intestinal epithelial cells and cause fluid and electrolyte hyper-secretion that leads to diarrhea. ETEC strains producing STa or LT are sufficiently virulent to cause diarrhea, therefore STa and LT antigens must be included in ETEC vaccines. However, potent toxicity and poor immunogenicity (of STa) prevent them from being directly applied as vaccine components. While LT toxoids, especially LT(R192G), being used in vaccine development, STa toxoids have not been included. A recent study (IAI, 78:316-325) demonstrated porcine-type STa toxoids [pSTa(P12F) and pSTa(A13Q)] elicited protective anti-STa antibodies after being fused to a porcine-type LT toxoid [pLT(R192G)]. In this study, we substituted the 8th, 9th, 16th, or the 17th amino acid of a human-type STa (hSTa) and generated 28 modified STa peptides. We tested each STa peptide for toxicity and structure integrity, and found nearly all modified STa proteins showed structure alteration and toxicity reduction. Based on structure similarity and toxic activity, three modified STa peptides: STa(E8A), STa(T16Q) and STa(G17S), were selected to construct LT₁₉₂-STa_-toxoid fusions. Constructed fusions were used to immunize mice, and immunized mice developed anti-STa antibodies. Results from this study provide useful information in developing toxoid vaccines against ETEC diarrhea.

In vitro and in vivo Evaluation of 111In-labeled E. coli Heat-Stable Enterotoxin Analogs for Specific Targeting of Human Breast Cancers

Research into the interaction between the E. coli heat-stable enterotoxin (STh) and the guanylin receptor guanylate cyclase C (GC-C) has generated >100 synthetic analogs of the peptide, several of which have been investigated as imaging or therapeutic agents for colorectal cancers. The evidence presented here suggests that in addition to STh binding to GC-C expressing cell lines derived from human colon, STh also specifically binds to an as yet unidentified receptor expressed in high densities on the surface of cell lines derived from human breast cancers. In vitro whole-cell crosslinking studies using 125I-labeled F19-STh(1-19) demonstrate that the putative STh binding protein migrates as an approximately 120-125 kDa species by SDS-PAGE, significantly smaller than the glycosylated GC-C molecule found in the T84 human colon cancer cell line. RT-PCR using total RNA isolated from breast and colon cancer cell lines indicates that GC-C transcripts are undetectable in human breast cancer cell lines and abundant in human colon cancer cell lines. In vitro competitive binding studies using STh analogs and the estrogen receptor positive (ER+) T-47D cell line demonstrated IC50 values between 2.6 and 8.5 nM. Similar studies on the estrogen receptor negative (ER-) cell line MDA-MB-231 showed IC50's between 5.6 and 9.9 nM. Saturation binding analysis revealed receptor expression to fall between 40,000 and 120,000 sites per cell in these cell lines, receptor abundances equal to or greater than the abundance of GC-C in colorectal cancer cell lines. STh binding to these cells, although of similar affinity to STh binding to GC-C, is distinguishable from it on the basis of its ligand specificity. The characteristics of STh analogs as radiopharmaceutical agents were tested in an in vivo model utilizing T-47D human breast cancer cell xenografts in SCID mice. Clearance of STh analogs was rapid, primarily via renal excretion into the urine, with >85% ID excreted into the urine at 1 h p.i. Tumor uptake at 1 h p.i. in T-47D tumor cell xenografts was 0.67+/-0.23% ID/g, and was significantly decreased (p<0.05) upon co-administration of 4 mg/kg unlabeled STh. These results suggest that STh may find application for the imaging and treatment of breast cancer.

A comparative molecular field analysis (COMFA) of the structural determinants of heat-stable enterotoxins mediating activation of guanylyl cyclase C

The heat-stable enterotoxin binds to and activates guanylyl cyclase C (GC-C), regulating fluid and electrolyte secretion in intestinal epithelial cells. A COMFA model was developed to predict the primary interactions between GC-C agonists and their receptor. This model predicts that the amide backbone of Cys(5)-Cys(6)-Glu(7)-Leu(8), the beta carbon atoms of Cys(5)-Cys(6), and the side chains of Pro(12), Ala(13), and Ala(15) comprise the primary interactions of GC-C agonists with the receptor surface.

Determination of the binding site on the extracellular domain of guanylyl cyclase C to heat-stable enterotoxin

Guanylyl cyclase C, one of the family of membrane-bound guanylyl cyclases, consists of an extracellular domain and an intracellular domain, which are connected by a single transmembrane polypeptide. The extracellular domain binds unique small polypeptides with high specificity, which include the endogenous peptide hormones, guanylin and uroguanylin, as well as an exogenous enterotoxigenic peptide, heat-stable enterotoxin, secreted by pathogenic Escherichia coli. Information on this specific binding is propagated into the intracellular domain, followed by the synthesis of cGMP, a second messenger that regulates a variety of intracellular physiological processes. This study reports the design of a photoaffinity labeled analog of heat-stable enterotoxin (biotinyl-(AC(5))(2)-[Gly(4), Pap(11)]STp(4-17)), which incorporates a Pap residue (p-azidophenylalanine) at position 11 and a biotin moiety at the N terminus, and the use of this analog to determine the ligand-binding region of the extracellular domain of guanylyl cyclase C. The endoproteinase Lys-C digestion of the extracellular domain, which was covalently labeled by this ligand, and mass spectrometric analyses of the digest revealed that the ligand specifically binds to the region (residue 387 to residue 393) of guanylyl cyclase C. This region is localized close to the transmembrane portion of guanylyl cyclase C on the external cellular surface. This result was further confirmed by characterization of site-directed mutants of guanylyl cyclase C in which each amino acid residue was substituted by an Ala residue instead of residues normally located in the region. This experiment provides the first direct demonstration of the ligand-binding site of guanylyl cyclase C and will contribute toward an understanding of the receptor recognition of a ligand and the modeling of the interaction of the receptor and its ligand at the molecular level.

Dual regulation of heat-stable enterotoxin-mediated cGMP accumulation in T84 cells by receptor desensitization and increased phosphodiesterase activity

We report the regulation of cGMP accumulation induced by the heat-stable enterotoxin, STh, in the T84 human colonic cell line. STh binding to its receptor, guanylyl cyclase C (GCC), leads to elevated intracellular levels of cGMP. Prolonged exposure of T84 cells to STh induced refractoriness to further cGMP accumulation, without significant receptor internalization, but with reduced STh-induced cGMP synthesis by the receptor. Significantly, increased degradation of cGMP by a cGMP-specific phosphodiesterase was observed in desensitized cells. This is the first report on the desensitization of GCC, as well as the role of the Type V phosphodiesterase in inducing cellular refractoriness.

Identification of ligand recognition sites in heat-stable enterotoxin receptor, membrane-associated guanylyl cyclase C by site-directed mutational analysis

Guanylyl cyclase C (STaR), a receptor protein for heat-stable enterotoxin (STa) elaborated by Escherichia coli, is associated with and spans the plasma membrane of mammalian intestinal cells. The extracellular domain functions in the binding of STa and the association of each domain to an oligomeric form. Two amino acid residues, Arg-136 and Asp-347, were identified as the residues binding to STa in the extracellular domain of pig STaR by site-directed mutagenesis and analysis of expression on 293T cells. Replacement of these residues by other amino acid residues resulted in the loss of binding of pig STaR to STa, and as a result, STa-induced guanylyl cyclase activity was eliminated. Furthermore, mutation in a region (from Asp-347 to Val-401) which is close to the transmembrane domain caused a significant reduction in both STa-binding activity and guanylyl cyclase catalytic activity. These results suggest that the region adjacent to the transmembrane domain plays an important role in facilitating a favorable conformation of STaR for STa binding.

Pig intestinal membrane-bound receptor (guanylyl cyclase) for heat-stable enterotoxin: cDNA cloning, functional expression, and characterization

A cDNA encoding the receptor protein for a heat-stable enterotoxin (STa) produced by enterotoxigenic Escherichia coli was cloned from intestinal epithelial cells of a 10-week-old pig. The cDNA had an open reading frame of 3,219 base pairs and coded for a protein with 1,073 amino acid residues. The mature protein consisted of 1,050 amino acid residues with a molecular mass of ca. 121 kDa and was 87% and 82% identical with the human and rat protein, respectively. The CHO cell line overexpressing the pig recombinant STa receptor specifically bound to a photoaffinity-labeled analog of STa and showed marked elevation of the cellular content of cGMP in response to STa.

The Escherichia coli heat-stable enterotoxin is a long-lived superagonist of guanylin

The mechanism by which bacterial heat-stable enterotoxins (ST I STA) cause diarrhea in humans and animals has been linked to the activation of an intestinal membrane-bound guanylate cyclase. Guanylin, a recently discovered rat intestinal peptide, is homologous in structure to ST I and can activate guanylate cyclase present on the human colonic carcinoma cell line T84. To directly test the mechanistic association of guanylate cyclase activation with diarrhea, we synthesized guanylin and a guanylin analog termed N9P10 guanylin and compared their biological activities with those of a synthetic ST I analog, termed ST Ib(6-18). We report that guanylin is able to inhibit the binding of a radiolabeled ST I analog to rat intestinal cells but causes diarrhea in infant mice only at doses at least 4 orders of magnitude higher than that of ST Ib(6-18). In contrast, N9P10 guanylin was enterotoxic in mice at much lower doses than guanylin but proved to be a weaker inhibitor of radiolabeled ST I than guanylin in the receptor binding assay. The pattern of guanylate cyclase activation observed for ST Ib(6-18) and the two guanylin analogs parallels the results observed in the receptor binding assay rather than those observed in the diarrheal assay. Treatment of guanylin with chymotrypsin or lumenal fluid derived from newborn mouse intestines resulted in a rapid loss of binding activity. Together, these results suggest that ST I enterotoxins may represent a class of long-lived superagonists of guanylin.

Pharmacologic action of Escherichia coli heat-stable (STa) enterotoxin

Escherichia coli produces a heat-stable (STa) enterotoxin that belongs to a family of peptides that mediate several diarrheal diseases, including traveler's diarrhea and epidemic diarrhea in infants and newborns. The STa enterotoxin consists of 18 or 19 amino acids and is encoded by genes specified on a transposon. Intestinal secretion is induced by specific binding to high affinity receptors that reside on the brush border cell membrane of the small intestine. Receptor activation by STa enterotoxin induces a sequence of events that culminate in the release of fluid and electrolytes into the intestinal lumen. These events include the stimulation of particulate guanylate cyclae and subsequent increase of intracellular cyclic GMP, involvement of particulate protein kinase, elevation of intracellular calcium, and activation of the phosphatidylinositol pathway. The release of archidonic acid and production of prostaglandins and/or leukotrienes have also been implicated in the action of STa. Evidence indicates that the STa enterotoxin receptor may be a single multifunctional membrane protein.