Analysis of receptor/ligand interactions using whole-molecule randomly-mutated ligand libraries

Characterisation of receptor binding by the chemotaxis inhibitory protein of Staphylococcus aureus and the effects of the host immune response

The chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS) is reported to bind to the receptors for C5a and formylated peptides and has been proposed as a promising lead for the development of new anti-inflammatory compounds. Here we have examined the receptor specificity and mode of action of recombinant CHIPS_28–149 and also the immune response to CHIPS_28–149 in patients with S. aureus infections and in uninfected controls. Recombinant CHIPS_28–149 bound with high affinity to the human C5a receptor (C5aR), but had low affinity for the second C5a receptor, C5L2, and the formyl peptide receptor, FPR. Although ligand binding to C5aR was potently inhibited, CHIPS_28–149 had much weaker effects on ligand binding to C5L2 and FPR. Similarly, CHIPS_28–149 potently inhibited the ligand-induced activation of C5aR but was less potent at inhibition via FPR. NMR studies showed that CHIPS_28–149 bound directly to the N-terminus of C5aR but not C5L2, and CHIPS_28–149 residues involved in the interaction were identified by chemical shift analysis. All human sera examined contained high titres of IgG and IgA reactivity against CHIPS_28–149, and no correlation was observed between infection status at the time of serum collection and antibody titre. Individual serum samples promoted or inhibited the binding of CHIPS_28–149 to C5aR, or had no effect. IgG depletion of serum samples abrogated the effects on CHIPS binding, demonstrating that these were antibody mediated. Sera from infected individuals were more likely to inhibit CHIPS_28–149 binding than sera from healthy controls. However, high antibody titres correlated well with both inhibition and enhancement of CHIPS_28–149 binding to C5aR; this suggests that the inhibitory effect relates to epitope specificity rather than greater antibody binding. We conclude that CHIPS is likely to be too immunogenic to be used as an anti-inflammatory treatment but that some antibodies against CHIPS may be useful in the treatment of S. aureus infections.

S19 ribosomal protein dimer augments metal-induced apoptosis in a mouse fibroblastic cell line by ligation of the C5a receptor

To analyze the role of S19 ribosomal protein (RP S19) in apoptosis, murine NIH3T3 were transfected with either hemagglutinin peptide-tagged (HA) wild-type human RP S19 or a mutant (Gln137Asn) that is resistant to transglutaminase-catalyzed cross-linked-dimerization. Transfection with the mutant HA-RP S19 inhibited manganese (II) (Mn II)-induced apoptosis whereas the wild-type HA-RP S19 augmented apoptosis and a mock transfection had no effect. Release of the wild-type HA-RP S19 dimer but not the mutant HA-RP S19 was observed during the apoptosis. The reduced rate of apoptosis of the cells transfected with the mutant HA-RP S19 was overcome by addition of extracellular wild-type RP S19 dimer. The apoptosis rates in cells transfected with either form of human HA-RP S19 and in mock transfectants were reduced to about 40% by the presence of anti-RP S19 antibody in the culture medium. Immunofluorescence staining and fluorescence-activated cell sorting (FACS) analysis showed that the cell surface expression of the receptor for cross-linked RP S19 dimer, C5a receptor, increased during apoptosis, concomitant with phosphatidylserine exposure. The expression of the C5a receptor gene also increased twofold. Apoptosis rates in the transfected and control cell lines were also reduced by the presence of an anti-mouse C5a receptor monoclonal antibody or of a peptide C5a receptor antagonist. These results indicated the presence of an RP S19 dimer- and C5a receptor-mediated autocrine-type augmentation mechanism during Mn II-induced apoptosis in the mouse fibroblastic cell line. In contrast to the RP S19 dimer, C5a actually inhibited apoptosis, suggesting that signaling through the C5a receptor varies depending on the ligand bound.

Comparative agonist/antagonist responses in mutant human C5a receptors define the ligand binding site

The C terminus is responsible for all of the agonist activity of C5a at human C5a receptors (C5aRs). In this report we have mapped the ligand binding site on the C5aR using a series of agonist and antagonist peptide mimics of the C terminus of C5a as well as receptors mutated at putative interaction sites (Ile(116), Arg(175,) Arg(206), Glu(199), Asp(282), and Val(286)). Agonist peptide 1 (Phe-Lys-Pro-d-cyclohexylalanine-cyclohexylalanine-d-Arg) can be converted to an antagonist by substituting the bulkier Trp for cyclohexylalanine at position 5 (peptide 2). Conversely, mutation of C5aR transmembrane residue Ile(116) to the smaller Ala (I116A) makes the receptor respond to peptide 2 as an agonist (Gerber, B. O., Meng, E. C., Dotsch, V., Baranski, T. J., and Bourne, H. R. (2001) J. Biol. Chem. 276, 3394-3400). However, a potent cyclic hexapeptide antagonist, Phe-cyclo-[Orn-Pro-d-cyclohexylalanine-Trp-Arg] (peptide 3), derived from peptide 2 and which binds to the same receptor site, remains a full antagonist at I116AC5aR. This suggests that although the residue at position 5 might bind near to Ile(116), the latter is not essential for either activation or antagonism. Arg(206) and Arg(175) both appear to interact with the C-terminal carboxylate of C5a agonist peptides, suggesting a dynamic binding mechanism that may be a part of a receptor activation switch. Asp(282) has been previously shown to interact with the side chain of the C-terminal Arg residue, and Glu(199) may also interact with this side chain in both C5a and peptide mimics. Using these interactions to orient NMR-derived ligand structures in the binding site of C5aR, a new model of the interaction between peptide antagonists and the C5aR is presented.

Characterisation of C5a receptor agonists from phage display libraries

C5a des-Arg(74) has a 10- to 100-fold lower receptor binding affinity than intact C5a and is only a partial agonist. We have used phage display selection from randomly mutated C5a des-Arg(74) libraries to isolate variant proteins that can activate C5a receptors with similar potency to C5a. Here we explore the interactions of three variants (V1-3) with C5aR mutated at residues involved in the differential response. The mutant Asp(282)Arg-C5aR is preferentially activated by C5a des-Arg(74), probably due to repulsion between Arg(74) of C5a and the substituent Arg(282). In accordance with this hypothesis, V2 (with a polar C-terminus which has no Arg residue) but not V1 (with a C-terminal Arg residue at position 73) could activate Asp(282)Arg-C5aR. V3, with a very hydrophobic C-terminus, was the most potent agonist at Asp(282)Arg-C5aR. Arg(175) is a potential counterion for the C-terminal carboxylate of C5a. C5aR mutated to either Ala or Asp at this position lost nearly all responsiveness to both C5a and C5a des-Arg(74), suggesting that mutation of Arg(175) caused a non-specific loss of receptor conformation and a loss of signalling capacity. However, V3 could still activate Arg(175)Asp/Ala-C5aR with the same potency as wild-type C5aR, demonstrating that the mutant receptors retained high signalling capability and showed a specific loss of responsiveness. Thus C5a des-Arg(74) variants produced by phage display are potentially useful tools for the dissection of ligand-receptor interactions.

The orphan receptor C5L2 has high affinity binding sites for complement fragments C5a and C5a des-Arg(74)

The substantial variations in the responses of cells to the anaphylatoxin C5a and its desarginated form, C5adR(74), suggest that more than one type of cell surface receptor for these ligands might exist. However, only a single receptor for C5a and C5adR(74), CD88, has been characterized to date. Here we report that the orphan receptor C5L2/gpr77, which shares 35% amino acid identity with CD88, binds C5a with high affinity but has a 10-fold higher affinity for C5adR(74) than CD88. C5L2 also has a moderate affinity for anaphylatoxin C3a, but cross-competition studies suggest that C3a binds to a distinct site from C5a. C4a was able to displace C3a, suggesting that C5L2, like the C3a receptor, may have a low binding affinity for this anaphylatoxin. Unlike CD88 and C3a receptor, C5L2 transfected into RBL-2H3 cells does not support degranulation or increases in intracellular [Ca(2+)] and is not rapidly internalized in response to ligand binding. However, ligation of C5L2 by anaphylatoxin did potentiate the degranulation response to cross-linkage of the high affinity IgE receptor by a pertussis toxin-sensitive mechanism. These results suggest that C5L2 is an anaphylatoxin-binding protein with unique ligand binding and signaling properties.

The use of phage display in the study of receptors and their ligands

Phage display technology presents a rapid means by which proteins and peptides that bind specifically to predefined molecular targets can be isolated from extremely complex combinatorial libraries. There are several important ways by which phage display can provide impetus to receptor-based research. Firstly, phage display can be applied, alongside transcriptome and proteome expression profiling techniques, to the identification and characterisation of receptors whose expression is specific to either a cell lineage, a tissue or a disease state. Secondly, specific monoclonal antibodies that enable researchers to identify, localize and quantify receptors can be produced very rapidly (weeks). Thirdly, it should be possible to apply phage display to the matching of orphan ligands and receptors. Finally, phage display can be used to identify proteins and peptides that modulate receptor activity. As well as being useful in the study of receptor function, biologically active proteins and peptides could also be used therapeutically, or as leads for drug design. Hence phage display is ready to play a central role in the study of receptors in the post-genome era. This review outlines the ways in which phage display has been applied to the study of receptor-ligand systems, and discusses how new developments in the technology may be of even greater utility to the field in the next decade.

Modulation of ligand selectivity by mutation of the first extracellular loop of the human C5a receptor 1 1Abbreviations: C5aR, human complement fragment 5a receptor; WT, wild-type; G105D, C5aR mutated to aspartate at glycine105; P103Y, C5aR mutated to tyrosine at proline105; P103Y/G105D, C5aR containing both substitutions; C5adR74, des arginated C5a; F-[OPchaWR], phenylalanine [l-ornithine-proline-d-cyclohexylalanine-tryptophan-arginine]; MeFKPchaWr, N-methyl-l-phenylalanine-lysine-proline-d-cyclohexylalanine-tryptophan-d-arginine; PMN, polymorphonuclear leukocytes; PCR, polymerase chain reaction; YSFKPMPLaR, l-tyrosine-serine-phenylalanine-lysine-proline-methionine-proline-leucine-d-alanine-arginine; and YSFKD(MeNle)PIAR, l-tyrosine-serine-phenylalanine-lysine-aspartate-N-methylnorleucine-proline-d-leucine-alanine-arginine

The cyclic C5a receptor antagonist, phenylalanine [L-ornithine-proline-D-cyclohexylalanine-tryptophan-arginine] (F-[OPchaWR]), has approximately 1000-fold less affinity for the C5a receptor (C5aR) on murine polymorphonuclear leukocytes than on human. Analysis of C5aR from different species shows that a possible cause of this difference is the variation in the sequence of the first extracellular loop of the receptor. The mouse receptor contains Y at a position analogous to P(103) in the human receptor, and D at G(105). To test this hypothesis, we expressed human C5aR mutants (P(103)Y, G(105)D and the double mutant, P(103)Y/G(105)D) in RBL-2H3 cells and investigated the effects of these mutations on binding affinity and receptor activation. All three mutant receptors had a higher affinity for human C5a than the wild-type receptor, but showed no significant difference in the ability of F-[OPchaWR] to inhibit human C5a binding. However, all of the mutant receptors had substantially lower affinities for the weak agonist, C5a des Arg(74) (C5adR(74)), and two altered receptors (G(105)D and P(103)Y/G(105)D) had much lower affinities for the C-terminal C5a agonist peptide analogue, L-tyrosine-serine-phenylalanine-lysine-proline-methionine-proline-leucine-D-alanine-arginine (YSFKPMPLaR). Although it is unlikely that differences at these residues are responsible for variations in the potency of F-[OPchaWR] across species, residues in the first extracellular loop are clearly involved in the recognition of both C5a and C5a agonists. The complex effects of mutating these residues on the affinity and response to C5a, C5adR(74), and the peptide analogues provide evidence of different binding modes for these ligands on the C5aR.

Combinatorial alanine-scanning

Combinatorial libraries of alanine-substituted proteins can be used to rapidly identify residues important for protein function, stability and shape. Each alanine substitution examines the contribution of an individual amino acid sidechain to the functionality of the protein. The recently described method of shotgun scanning uses phage-displayed libraries of alanine-substituted proteins for high-throughput analysis.

Selection of novel ligands from a whole-molecule randomly mutated C5a library

Novel antagonists of the proinflammatory leukocyte chemoattractant C5a have been produced from a phage display library of whole-molecule random mutants. The cDNA for the inflammatory polypeptide C5adR(74) was used as template in a PCR reaction doped with the mutagenic nucleoside triphosphates dPTP [dP: 6-(2-deoxy-beta-D-ribofuranosyl)-3,4-dihydro-8H-pyrimido-[4,5-c][1,2]oxazin-7-one] and 8-oxodGTP (8-oxodG: 8-oxo-2'-deoxyguanosine) to allow the introduction of mutations in a highly controlled manner throughout the cDNA. The resultant library of mutants was displayed on bacteriophage M13 using a jun/fos linker sequence. Functional polypeptides were isolated by several rounds of selection against the receptor for C5a expressed on the surface of CHO cells. From this selection procedure, a limited number of variants of C5adR(74) were obtained. When expressed as free polypeptide, the binding affinities of the selected C5adR(74) sequences were increased 5-fold relative to wild-type protein. Site-directed mutagenesis of the C-terminus of these variants resulted in the production of antagonists of C5adR(74) activity.