Functional expression of a human C5a receptor clone in Xenopus oocytes requires additional RNA

Characterization of the human fMLP receptor in neutrophils and in Xenopus oocytes

1. N-formyl peptides (e.g. fMLP; N-formyl-L-methionyl-L-leucyl-phenylalanine) are potent mediators for inflammatory reactions. We report functional expression in Xenopus oocytes of human fMLP-R98 cDNA, without co-expression of the promiscuous G-protein subunit, Galpha-16. 2. Stimulation of voltage-clamped oocytes (-70 mV) with fMLP produced a dose-dependent biphasic inward current with fast and slow components. Analysis using GTP-gamma-S and cholera and pertussis toxins suggested these currents are mediated by an endogenous G-protein of the Gq family. 3. The fast current reversed at -25 mV and was blocked by SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid), suggesting the current is carried by Cl(-). The slow current showed weak inward rectification, was Ca(2+)-dependent and blocked by Cd(2+), 4-AP (4-aminopyridine) and haloperidol, suggesting activation of a mixed population of cation channels. 4. Comparative experiments with human neutrophils using flow cytometric analysis showed that the proportion of neutrophils activated by fMLP was reduced in the presence of SITS, in the absence of external calcium and in the presence of Cd(2+), TEA (tetraethylammonium) and haloperidol but not 4-AP. In addition, the oxidative burst from activated neutrophils was reduced by SITS and by the absence of external calcium but not by Cd(2+), TEA, 4-AP or haloperidol. 5. We suggest that in human neutrophils activation by fMLP is dependent on store-operated calcium influx that appears to be regulated by Cl(-) channels and linked, in part, to non-selective cation channels.

A high affinity nitrate transport system from Chlamydomonas requires two gene products

A nitrate-regulated cluster of genes involved in nitrate transport and assimilation has been identified in Chlamydomonas reinhardtii. Mutant strains of the alga, which are defective in some aspect of transport and assimilation have been used to assign functions to these genes. This analysis has suggested that two gene products are necessary to obtain a functional high affinity nitrate system in Chlamydomonas [Quesada et al. (1994) Plant J. 5, 407-419]. In this paper we have tested this hypothesis by injecting Xenopus oocytes with mRNA prepared from these two cDNAs, Nrt2;1 and Nar2, and then assaying the oocytes for nitrate transport activity. Oocytes injected with single types of mRNA did not show any nitrate transport activity. Furthermore, Nar2 mRNA was toxic to oocytes, with nearly 60%, of the oocytes dead 3 days after the injection. However, when oocytes were injected with a mixture of two mRNAs prepared from Nrt2;1 and Nar2, a high affinity nitrate transport activity could be measured. However, the Km for nitrate of this transport system was 28 microM which is higher than the value of 1.6 microM which had been obtained by the analysis of mutant phenotypes. The pH-dependence of the nitrate-elicited currents was consistent with a proton-cotransport mechanism. These results prove that two gene products are required to produce a functional high affinity nitrate transport system and that this process does not involve transcriptional regulation.

In Xenopus oocytes the human C3a and C5a receptors elicit a promiscuous response to the anaphylatoxins

The Xenopus laevis oocyte has been widely utilized for cloning and functional expression of G-protein coupled receptors (GPCR). This system was used for the functional expression and characterization of the recently identified human C3a receptor. Complementary RNA from the human C3a receptor was transcribed in vitro and microinjected into Xenopus oocytes for functional characterization. A positive response to a synthetic C3a peptide agonist and to C3a, but not to platelet activating factor or fMetLeuPhe was detected. In addition, a response of approximately one third the amplitude obtained with C3a was obtained with rC5a. Conversely, oocytes co-injected with the C5a receptor and total RNA isolated from U937 cells responded to C5a as well as to C3a and the C3a synthetic peptide. A functional response with the anaphylatoxin C3a receptor in oocytes was dependent on co-injection of a pertussis toxin sensitive complementary human factor which could be supplied by co-injection of total RNA isolated from U937 cells. Oocytes expressing the anaphylatoxin C3a and C5a receptors responded to both agonists, in each case the response to the cognate ligand was substantially more robust than the response elicited by the other anaphylatoxin.

1,25(OH)2D3 and cAMP synergistically induce complement 5a receptor messenger RNA

Complement 5a receptor (C5aR) mediates both acute and chronic participation of monocytes in the immune response. In the human U937 monoblast, C5aR is maximally expressed 4 days after treatment with 1,25(OH)2D3 (or cycloheximide) and prostaglandin E2 combined. The authors asked whether these agents altered expression of C5aR messenger RNA (mRNA). Unstimulated U937 cells expressed neither C5aR mRNA nor C5a binding. Complement 5aR mRNA rose 3 hours after prostaglandin E2 application and fell to basal levels by 12 hours. This early rise in C5aR mRNA did not cause an acute rise in C5a binding, which gradually increased between 1 and 4 days. Neither 1,25(OH)2D3 nor cycloheximide induced expression of C5aR mRNA in the absence of prostaglandin E2 but did enhance prostaglandin E2-stimulated C5aR mRNA expression and C5a binding. The authors observed a late increase in C5aR mRNA at day 3 in treated cells. Inhibition of this late rise in mRNA with 5,6-dichlorobenzimidazole riboside attenuated C5a binding by 65%, indicating its importance in the generation of C5a binding sites. The expression of functional C5aR is, therefore, a complex process involving regulation at transcriptional and posttranscriptional levels.

Site-directed mutagenesis of conserved charged residues in the helical region of the human C5a receptor. Arg2O6 determines high-affinity binding sites of C5a receptor

The human C5a receptor (C5aR) belongs to the family of G-protein-coupled receptors with seven transmembrane helices. This part of the molecule is thought to contain part of the ligand-binding pocket, specifically to bind the C-terminal Arg of human C5a. Guided by sequence similarity and molecular modelling studies, several residues including polar (Asn119, Thr168, Gln259) as well as all conserved charged amino acids in the upper transmembrane region of the C5aR (Asp37, Asp82, Arg175, Arg2O6, Asp282) were exchanged by site-directed mutagenesis. Receptor mutants were transiently expressed in COS cells and analyzed for altered binding behaviour and/or localization at the cell surface by immunofluorescence. For all residues, suitable mutants could be found that exhibited wild-type affinity towards the ligand, providing evidence against a major contribution of these residues to high-affinity ligand binding. Some mutants, however, exhibited a complete (Asp282-->Ala) or partial loss of ligand-binding capacity (Arg175-->Ala, Arg2O6-->Gln) despite adequate expression levels on the cell surface. This phenotype was further analyzed in the [Gln2O6]C5aR mutant: quantitative flow cytometric analysis of epitope-tagged receptor derivatives in 293 cells confirmed an equal level of wild-type and mutant C5aR on the cell surface. Competitive binding curves revealed the presence of only a small population (<10%) of high-affinity sites (Kd approximately 2 nM), which was functionally active at 20 nM in the heterologous Xenopus oocyte expression system after coexpression of G alpha-16. The number of high-affinity sites of wild-type and [Gln2O6]C5aR in 293 cells could be up-regulated by coexpression of Gi alpha-2 and down-regulated by GTP[gamma S]-mediated uncoupling of the G-protein receptor interaction in membrane preparations. These findings are compatible with a model in which the Arg2O6 residue located in the upper third of transmembrane helix V determines high-affinity binding in the human C5aR by affecting the intracellular G-protein coupling.

G alpha-16 complements the signal transduction cascade of chemotactic receptors for complement factor C5a (C5a-R) and N-formylated peptides (fMLF-R) in Xenopus laevis oocytes: G alpha-16 couples to chemotactic receptors in Xenopus oocytes

The human leukocyte chemoattractant receptors for complement factor C5a (C5a-R) and N-formylated peptides (fMLF-R) are important members of the superfamily of G-protein coupled receptors (GPCR). Uniquely among the GPCR, these two receptors cannot be expressed in a functionally active form in the oocytes of the frog Xenopus laevis, but require substitution of total RNA of the myelomonocytic U-937 or HL-60 cell lines, respectively. Recently, it was reported that the C5a-R may couple to the alpha subunit of G-16. We have tested this G-protein for its ability to complement the signal transduction cascade of the C5a-R and fMLF-R in Xenopus oocytes. Injection of cRNA for the C5a-R in combination with G alpha-16 led to expression of a functional C5a-R as measured by ligand-induced whole cell current. In contrast to a previous report, the fMLF-R exhibited some residual functional activity when transiently expressed in Xenopus oocytes the extent of which could, however, substantially be increased by coexpression of G alpha-16. Thus, G alpha-16 complements the signal transduction cascade of both receptors in Xenopus laevis oocytes and is most likely the complementing factor present in the U-937 and HL-60 cell lines.

Xenopus oocytes as a heterologous expression system for plant proteins

The Xenopus oocyte is a robust and convenient system for the transient expression of many different animal proteins and it has recently been demonstrated that oocytes can also translate, process, and target plant proteins. This expression system can also be used to clone genes, characterize function, and study posttranslational processing of proteins. Here we describe the methodology for the expression of plant proteins, in particular membrane proteins, in Xenopus oocytes.

Amino acids 327-350 of the human C5a-receptor are not essential for [125I]C5a binding in COS cells and signal transduction in Xenopus oocytes

The anaphylatoxic peptide C5a is an important inflammatory mediator of the complement system. We have generated human C5a-receptor (hC5aR) mutants with truncation of its cytosolic carboxyl-terminus (C-terminus). Both mutants were analysed for C5a-binding in transiently expressing COS cells, and one mutant additionally for GTP-binding regulatory protein (G-protein) coupling in cRNA-injected Xenopus oocytes. Our data suggest that (a) amino acids (aa) 314 to 326 as part of the C-terminus are necessary for proper receptor folding or expression and (b) the receptor C-terminus distal from position 327 is not critical for receptor expression, folding, binding and G-protein coupling.

Site-specific mutagenesis of residues in the human C5a anaphylatoxin which are involved in possible interaction with the C5a receptor

To check and clarify existing data on receptor-interacting residues in the human C5a anaphylatoxin, we tested mutant C5a proteins obtained by site-directed mutagenesis of a recombinant human C5a (rhC5a) cDNA clone for structural and functional integrity. Amino acid positions in three different regions of the molecule were investigated: Arg74 at the C-terminus, Arg40 and Pro45 located in the core region, and Lys14 and Lys19, Lys20 in the N-terminus. Des-Arg74-rhC5a displayed only a residual 3-4% functional activity in the myeloperoxidase-release assay from human granulocytes while retaining the three-dimensional solution structure of wild-type (wt)-rhC5a as shown by circular dichroism (CD) spectroscopy. Des-Arg74-rhC5a was able to activate the human C5a receptor transiently expressed in Xenopus oocytes, but was inactive in the heterologous guinea pig (gp) ileum-contraction assay. These results reveal profound differences between the guinea pig and human C5a-receptor ligand-binding characteristics. Exchange of the core residue Arg40 by a glycine did not significantly affect functional C5a activity, in contrast to a previous observation [Mollison, K. W., Mandecki, W., Zuiderweg, E. P., Fayer, L., Fey, T. A., Krause, R. A., Conway, R. G., Miller, L., Edalji, R. P., Shallcross, M. A., Lane, B., Fox, J. L., Greer, J. & Carter, G. W. (1989) Identification of receptor-interacting residues in the inflammatory complement protein C5a by site-directed mutagenesis, Proc. Natl Acad. Sci. USA 86, 292-296], nor did exchange of the conserved Pro45 residue by the C3a analogue glutamic acid, a mutation expected to alter the whole geometry of the loop connecting helix III-helix IV (including Arg40) of the C5a molecule. Thus, participation of this loop in receptor interaction appears unlikely. While exchange of the N-terminal Lys14 residue by alanine did not significantly affect functional activity, a double replacement of Lys19 and Lys20 by alanine residues reduced activity more than 30-fold. These results confirm Lys19 and/or Lys20 as a putative receptor-interacting site, although we could not obtain a CD spectrum of this important mutant due to poor expression.