Identification of dynorphin a from zebrafish: a comparative study with mammalian dynorphin A

We report the cloning and molecular characterization of the zfPDYN. The complete open reading frame for this propeptide is comprised in two exons that are localized on chromosome 23. zfPDYN cDNA codes for a polypeptide of 252 amino acids that contains the consensus sequences for four opioid peptides: an Ile-enkephalin, the neo-endorphins, dynorphin A and dynorphin B. Upon comparison between zebrafish (zfDYN A) and mammalian dynorphin A (mDYN A) it has been stated that these two peptides only differ in two amino acids: the Leu(5) is replaced by Met(5) and the Lys(13) by Arg(13). Taking into consideration that mDYN A is able to bind to the three mammalian opioid receptors, we have compared the pharmacological profile of zfDYN A and mDYN A on the zebrafish opioid receptors. By means of radioligand binding techniques, we have established that these two dynorphins bind and activate all of the cloned opioid receptors from zebrafish (delta-, mu- and kappa-like), although with different affinities. zfDYN A and mDYN A displace [(3)H]-diprenorphine binding with K(i) values on the nanomolar range, showing greater affinity for zebrafish opioid receptor (ZFOR) 3 (kappa) receptor. ZFOR1 (delta) and ZFOR4 (delta) present higher affinity for zfDYN A than for mDYN A, while the opposing behavior is observed in ZFOR2 (mu). Functional [(35)S]guanosine 5'-[gamma-thio]triphosphate (GTPgammaS) stimulation experiments indicate that these two peptides fully activate the zebrafish opioid receptors, although the mean effective dose (EC(50)) values obtained for ZFOR2 and ZFOR3 receptors are lower than those seen for ZFOR1 and ZFOR4. A comparative study indicates that mammalian and zebrafish opioid receptors might bind their corresponding dynorphin A in a similar fashion, hence suggesting an important role of the opioid system through the vertebrate evolution.

Binding Profile of the Endogenous Novel Heptapeptide Met-Enkephalin-Gly-Tyr in Zebrafish and Rat Brain

Zebrafish is considered a model organism, not only for the study of the biological functions of vertebrates but also as a tool to analyze the effects of some drugs or toxic agents. Five opioid precursor genes homologous to the mammalian opioid propeptide genes have recently been identified; one of these, the zebrafish proenkephalin, codes a novel heptapeptide, the Met-enkephalin-Gly-Tyr (MEGY). To analyze the pharmacological properties of this novel ligand, we have labeled it with tritium ([(3)H]MEGY). In addition, we have also synthesized two analogs: (d-Ala(2))-MEGY (Y-d-Ala-GFMGY) and (d-Ala(2), Val(5))-MEGY (Y-d-Ala-GFVGY). The binding profile of these three agents has been studied in zebrafish and rat brain membranes. [(3)H]MEGY presents one binding site in zebrafish, as well as in rat brain membranes, although it shows a slight higher affinity in zebrafish brain. The observed saturable binding is displaced by naloxone, thus confirming the opioid nature of the binding sites. Competition binding assays indicate that the methionine residue is essential for high-affinity binding of MEGY and probably of other peptidic agonists in zebrafish, whereas the change of a Gly for a d-Ala does not dramatically affect the ligand affinity. Our results show that the percentage of [(3)H]MEGY displaced by all the ligands studied is higher than 100%, thus inferring that naloxone (used to determine nonspecific binding) does not bind to all the sites labeled by [(3)H]MEGY. Therefore, we can deduct that some of the MEGY binding sites should not be considered classical opioid sites.

Lectin purified human class I MHC-derived peptides: evidence for presentation of glycopeptides in vivo

Previously, using synthetic glycopeptides carrying a natural cytosolic type of monosaccharide O-beta-linked N-acetylglucosamine (GlcNAc) glycosylation of serine residues, we have shown that glycopeptides act as suitable substrates for TAP-mediated transport into the endoplasmic reticulum (ER), and that they bind efficiently to class I major histocompatibility complex (MHC) molecules and can elicit glycopeptide-specific cytotoxic T-lymphocyte (CTL) responses in mice. Recently, we have reported that peptides presented by human class I MHC molecules in vivo encompass a small but significant amount of peptides which seem to be carrying O-beta-linked monosaccharide GlcNAc. In the present report we provide further evidence that glycosylated peptides are indeed presented by class I MHC molecules in vivo. Thus, peptides derived from HLA-A*0201 were purified by wheat germ agglutinin (WGA) lectin affinity chromatography as previously described. Subsequently, the peptides contained in the WGA-eluate were subjected to sequence analysis by Edman degradation. It was found that the peptides derived from HLA-A*0201 which had been retained by the O-GlcNAc-binding lectin WGA did indeed carry a HLA-A*0201 binding motif. Furthermore, using an enzymatic labeling procedure we present evidence that the HLA-A*0201-derived peptides which bind to the WGA lectin are glycosylated with terminal GlcNAc residues. Together, these data provide further evidence for the natural presentation by human class I MHC of glycopeptides carrying terminal O-GlcNAc residues in vivo.

Phosphorylated peptides can be transported by TAP molecules, presented by class I MHC molecules, and recognized by phosphopeptide-specific CTL

CTL recognize short peptide fragments presented by class I MHC molecules. In this study, we examined the effect of phosphorylation on TAP transport, binding to class I MHC molecules, and recognition by CTL of peptide fragments from known phosphorylated oncogene proteins or virus phosphoproteins. We show that phosphopeptides can be efficiently transported from the cytosol to the endoplasmic reticulum by the TAP. Furthermore, we show that phosphorylation can have a neutral, negative, or even a positive effect on peptide binding to class I MHC. Finally, we have generated phosphopeptide-specific CTL that discriminate between the phosphorylated and the nonphosphorylated versions of the peptide. We conclude that phosphopeptide-specific CTL responses are likely to constitute a subset of the class I MHC-restricted CTL repertoire in vivo.

Presentation of cytosolic glycosylated peptides by human class I major histocompatibility complex molecules in vivo

Antigens presented by class I major histocompatibility complex (MHC) molecules for recognition by cytotoxic T lymphocytes consist of 8-10-amino-acid-long cytosolic peptides. It is not known whether posttranslationally modified peptides are also presented by class I MHC molecules in vivo. Many different posttranslational modifications occur on cytoplasmic proteins, including a cytosolic O-beta-linked glycosylation of serine and threonine residues with N-acetylglucosamine (GlcNAc). Using synthetic glycopeptides carrying the monosaccharide O-beta-GlcNAc substitution on serine residues, we have shown that glycopeptides bind efficiently to class I MHC molecules and elicit a glycopeptide-specific cytotoxic T lymphocyte response in mice. In this study, we provide evidence that peptides presented by human class I MHC molecules in vivo encompass a small, significant amount of glycopeptides, constituting up to 0.1% of total peptide. Furthermore, we find that carbohydrate structures present on glycopeptides isolated from class I MHC molecules are dominated by the cytosolic O-beta-GlcNAc substitution, and synthetic peptides carrying this substitution are efficiently transported by TAP (transporter associated with antigen presentation) into the endoplasmic reticulum. Thus, in addition to unmodified peptides, posttranslationally modified cytosolic peptides carrying O-beta-linked GlcNAc can be presented by class I MHC molecules to the immune system.

Crystal structures of two H-2Db/glycopeptide complexes suggest a molecular basis for CTL cross-reactivity

Two synthetic O-GlcNAc-bearing peptides that elicit H-2Db-restricted glycopeptide-specific cytotoxic T cells (CTL) have been shown to display nonreciprocal patterns of cross-reactivity. Here, we present the crystal structures of the H-2Db glycopeptide complexes to 2.85 A resolution or better. In both cases, the glycan is solvent exposed and available for direct recognition by the T cell receptor (TCR). We have modeled the complex formed between the MHC-glycopeptide complexes and their respective TCRs, showing that a single saccharide residue can be accommodated in the standard TCR-MHC geometry. The models also reveal a possible molecular basis for the observed cross-reactivity patterns of the CTL clones, which appear to be influenced by the length of the CDR3 loop and the nature of the immunizing ligand.

Peptide anchor residue glycosylation: effect on class I major histocompatibility complex binding and cytotoxic T lymphocyte recognition

This study extends our previous observation that glycopeptides bind to class I major histocompatibility complex (MHC) molecules and elicit carbohydrate-specific CTL responses. The Sendai virus nucleoprotein wild-type (WT) peptide (FAPGNYPAL) binds H-2Db using the P5-Asn as an anchor. The peptide K2 carrying a P5 serine substitution did not bind Db. Surprisingly, glycosylation of the serine (K2-O-GlcNAc) with N-acetylglucosamine (GlcNAc), a novel cytosolic O-linked glycosylation, partially restored peptide binding to Db. We argue that the N-acetyl group of GlcNAc may fulfil the hydrogen bonding requirements of the Db pocket which normally accomodates P5-Asn. Glycosylation of the P5-Asn residue itself abrogated binding similar to K2, probably for steric reasons. The peptide K2-O-GlcNAc readily elicited Db-restricted cytotoxic T lymphocytes (CTL), which did not cross-react with K2 or WT. However, all Db-restricted CTL raised against K2-O-GlcNAc cross-reacted strongly with another glycopeptide, K3-O-GlcNAc, where the GlcNAc substitution is on a neighboring P4-Ser. Furthermore, Db-restricted CTL clones raised against K2-O-GlcNAc or K3-O-GlcNAc displayed a striking TCR conservation. Our interpretation is that the carbohydrate of K2-O-GlcNAc not only mediates binding to Db, but also interacts with the TCR in such a way as to mimic K3-O-GlcNAc. This unusual example of molecular mimicry extends the known effects of peptide glycosylation from what we and others have previously reported: glycosylation may create a T cell neo-epitope, or, conversely, abrogate recognition. Alternatively, glycosylation may block peptide binding to MHC class I and finally, as reported here, restore binding, presumably through direct interaction of the carbohydrate with the MHC molecule.

Recognition of carbohydrate by major histocompatibility complex class I-restricted, glycopeptide-specific cytotoxic T lymphocytes

Cytotoxic T cells (CTL) recognize short peptide epitopes presented by class I glycoproteins encoded by the major histocompatibility complex (MHC). It is not yet known whether peptides containing posttranslationally modified amino acids can also be recognized by CTL. To address this issue, we have studied the immunogenicity and recognition of a glycopeptide carrying an O-linked N-acetylglucosamine (GlcNAc) monosaccharide-substituted serine residue. This posttranslational modification is catalyzed by a recently described cytosolic glycosyltransferase. We show that glycosylation does not affect peptide binding to MHC class I and that glycopeptides can elicit a strong CTL response that is glycopeptide specific. Furthermore, glycopeptide recognition by cytotoxic T cells is dependent on the chemical structure of the glycan as well as its position within the peptide.

Synthetic glycosylation of proteins using N-(beta-saccharide) iodoacetamides: applications in site-specific glycosylation and solid-phase enzymic oligosaccharide synthesis

A simple and efficient synthetic glycosylation method suitable for use in solid-phase enzymic oligosaccharide synthesis and site-specific glycosylation of recombinant proteins to produce defined glycoforms is described. This strategy utilizes N-(beta-saccharide) haloacetamides for attaching oligosaccharides specifically to cysteine residues of proteins in solution to form neoglycoproteins. The alkylation reaction was tested using N-(beta-chitotriose) bromoacetamide and an unprotected synthetic hexapeptide containing a single cysteine residue. The glycosylated product was confirmed by amino acid and hexosamine analyses as well as laser desorption mass spectrometry. Similarly N-(beta-chitotriose) iodoacetamide was covalently linked to non-reduced BSA to produce a defined glycoform of this protein. The specific attachment of chitotriose at the single cysteine residue in non-reduced serum albumin was suggested by Ellman's assay for free thiols. This was verified by amino acid sequencing of tryptic glycopeptide derived from this neoglycoprotein. Multiple sugar attachment was accomplished using fully reduced serum albumin as demonstrated by the formation of two neoglycoproteins using iodoacetamide derivatives of galactose beta 1-3-N-acetylgalactosamine (Gal beta 1-3GalNAc) and the major xylose/fucose-class plant-type oligosaccharide of horseradish peroxidase. These two neoglycoproteins with an average of 18-21 sugar residues attached were assayed positively for binding to peanut agglutinin and a sugar-specific anti-(horseradish peroxidase) monoclonal antibody YZ1/2.23 respectively. Sialylation of the neoglycoprotein containing Gal beta 1-3GalNAc was accomplished using alpha-2,3-sialyltransferase and radiolabelled CMP-N-acetylneuraminic acid. Significantly, glycan attachment using this conjugation method is reversible as demonstrated by the release of oligosaccharides from these two neoglycoproteins using hydrazinolysis. Therefore this method could provide invaluable reagents for many glycobiological studies.

9-Fluorenylmethoxycarbonyl (Fmoc)-glycine coupling of saccharide beta-glycosylamines for the fractionation of oligosaccharides and the formation of neoglycoconjugates

We have devised a new saccharide derivatization scheme to provide not only the temporary attachment of a chromophore for detecting and facilitating the chromatographic separation of carbohydrates, but also the intermediates for further derivatization to produce neoglycoconjugates. Several neutral unprotected saccharide beta-glycosylamines were formed by the direct condensation of the reducing saccharides with aqueous ammonium bicarbonate. The beta-glycosylamine derivatives of N-acetylglucosamine, di-N-acetylchitobiose, and asialo-, digalactosylated biantennary complex oligosaccharide were N-acylated separately with excess 9-fluorenylmethoxycarbonyl (Fmoc)-glycine. The Fmoc-glycinamido beta-derivatives of these unprotected saccharides were well separated by normal-phase high-performance liquid chromatography and detected by ultraviolet absorption. Similar derivatization and fractionation of a partial acid hydrolyzate of chitin were equally successful resulting in the separation of Fmoc-glycinamido derivatives of di-N-acetylchitobiose to hepta-N-acetylchitoheptaose in the hydrolyzate. The reversibility of the Fmoc derivatization was demonstrated by treating the Fmoc-glycinamido derivative of N-acetylglucosamine with piperidine to generate its 1-N-glycyl-beta-saccharide derivative. The structure and stereochemistry of this product was confirmed by proton nuclear magnetic resonance spectroscopy. The 1-N-glycyl-beta-saccharide derivatives are stable intermediates for the formation of asparagine-linked neoglycoconjugates.