Rat and human neutrophil N-formyl-peptide chemotactic receptors. Species difference in the glycosylation of similar 35-38 kDa polypeptide cores

The leukocyte chemotactic receptor FPR1 is functionally expressed on human lens epithelial cells

BACKGROUND

Lens degeneration in Fpr1(-/-) mice prompted us to search for functional FPR1 expression directly on lens epithelial cells.

RESULTS

FPR1 is functionally expressed on human lens epithelial cells but has atypical properties compared with hematopoietic cell FPR1.

CONCLUSION

Lens epithelial cell FPR1 may be involved in development and maintenance of the lens.

SIGNIFICANCE

This is the first link between non-hematopoietic expression of FPR1 and an ophthalmologic phenotype. Formyl peptide receptor 1 (FPR1) is a G protein-coupled chemoattractant receptor expressed mainly on leukocytes. Surprisingly, aging Fpr1(-/-) mice develop spontaneous lens degeneration without inflammation or infection (J.-L. Gao et al., manuscript in preparation). Therefore, we hypothesized that FPR1 is functionally expressed directly on lens epithelial cells, the only cell type in the lens. Consistent with this, the human fetal lens epithelial cell line FHL 124 expressed FPR1 mRNA and was strongly FPR1 protein-positive by Western blot and FACS. Competition binding using FPR1 ligands N-formyl-Nle-Leu-Phe-Nle-Tyr-Lys (Nle = Norleucine), formylmethionylleucylphenylalanine, and peptide W revealed the same profile for FHL 124 cells, neutrophils, and FPR1-transfected HEK 293 cells. Saturation binding with fluorescein-labeled N-formyl-Nle-Leu-Phe-Nle-Tyr-Lys revealed ~2500 specific binding sites on FHL-124 cells (K(D) ~ 0.5 nm) versus ~40,000 sites on neutrophils (K(D) = 3.2 nm). Moreover, formylmethionylleucylphenylalanine induced pertussis toxin-sensitive Ca(2+) flux in FHL 124 cells, consistent with classic G(i)-mediated FPR1 signaling. FHL 124 cell FPR1 was atypical in that it resisted agonist-induced internalization. Expression of FPR1 was additionally supported by detection of the intact full-length open reading frame in sequenced cDNA from FHL 124 cells. Thus, FHL-124 cells express functional FPR1, which is consistent with a direct functional role for FPR1 in the lens, as suggested by the phenotype of Fpr1 knock-out mice.

Critical role of N-terminal N-glycosylation for proper folding of the human formyl peptide receptor

The human formyl peptide receptor (FPR) is N-glycosylated and activates phagocytes via G(i)-proteins. The FPR expressed with G(i)alpha(2)beta(1)gamma(2) in Sf9 insect cells exhibits high constitutive activity as assessed by strong inhibitory effects of an inverse agonist and Na(+) on basal guanosine 5(')-O-(3-thiotriphosphate) (GTPgammaS) binding. The aim of our study was to analyze the role of N-glycosylation in FPR function. Site-directed mutagenesis of extracellular Asn residues prevented FPR glycosylation but not FPR expression in Sf9 membranes. However, in terms of high-affinity agonist binding, kinetics of GTPgammaS binding, number of G(i)-proteins activated, and constitutive activity, non-glycosylated FPR was much less active than native FPR. FPR-Asn4Gln/Asn10Gln/Asn179Gln and FPR-Asn4Gln/Asn10/Gln exhibited similar defects. Our data indicate that N-glycosylation of N-terminal Asn4 and Asn10 but not of Asn179 in the second extracellular loop is essential for proper folding and, hence, function of FPR. FPR deglycosylation by bacterial glycosidases could be a mechanism by which bacteria compromise host defense.

The N-formyl peptide receptor: a model for the study of chemoattractant receptor structure and function

N-formyl peptides, such as fMet-Leu-Phe, are one of the most potent chemoattractants for phagocytic leukocytes. The interaction of N-formyl peptides with their specific cell surface receptors has been studied extensively and used as a model system for the characterization of G-protein-coupled signal transduction in phagocytes. The cloning of the N-formyl peptide receptor cDNA from several species and the identification of homologous genes have allowed detailed studies of structural and functional aspects of the receptor. Recent findings that the receptor is expressed in nonhematopoietic cells and that nonformylated peptides can activate the receptor suggest potentially novel functions and the existence of additional ligands for this receptor.

Internalization of N-formyl peptide chemotactic receptor-ligand complex by human neutrophils. The role of the receptor's 2-kDa external domain and carbohydrates

We treated human neutrophils with papain to remove the external 2-kDa domain and along with it the two oligosaccharide side chains of the N-formyl peptide chemotactic receptor and investigated what effect their absence has on the ligand-receptor complex internalization. After prelabeling of the cells with 125I-hexapeptide for 5 min at 22 degrees C, about 95% of the bound radioactivity was located on the cell surface. During the first 5-min incubation at 37 degrees C both the control and papain-treated cells internalized 73% of the receptor-ligand complexes suggesting that internalization is very rapid in human neutrophils and that removal of the external domain and the carbohydrates of the receptor does not affect the rate. However, the truncated receptor-ligand complexes were degraded at a faster rate because the radioactivity released into the medium was significantly higher and correspondingly the acid-resistant radioactivity significantly lower in the papain-treated neutrophils than in control cells already at 5 min and all subsequent time points. The radioactivity accumulated in the medium of the control and papain-treated neutrophils represented inactivated 125I-hexapeptide as less than 5% of it at 5, 30 and 120 min were capable of rebinding. No receptor recycling was detected in either cells. These results indicate that removal of the 2-kDa external domain and the carbohydrates of the N-formyl chemotactic receptors has little effect on the internalization rate of the receptor-ligand complexes but accelerates markedly their intracellular degradation.

Glycosylation of VIP receptors: a molecular basis for receptor heterogeneity

Apparent molecular weights of VIP-binding proteins differ greatly according to species and to tissue. In this study, we used plasma membranes from various species (human, rat, pig) and tissues (melanoma, intestine, liver), which display major 125I-VIP-labeled components with molecular weights ranging from M(r) = 51,800 to 66,800. With the exception of porcine receptor, the various VIP receptors had similar apparent molecular weights after removal of their N-linked carbohydrates. In addition to differences in the amount of asparagine-linked glycans, our results also revealed differences in the composition of the oligosaccharide chains, which can also account for the heterogeneity in the molecular weights of the VIP receptor.

Covalent cross-linking of radiolabeled N-formylated hexapeptide to its specific receptor on rat and human neutrophils: evidence for a ligand induced complex formation

The structure of the rat and human neutrophil receptor for N-formylated chemotactic peptides was characterized using 125I-labeled N-formyl-Nle-Leu-Phe-Nle-Tyr-Lys hexapeptide as a ligand and an affinity cross-linking technique. 125I-hexapeptide bound to purified rat peritoneal neutrophils was time, temperature and pH-dependent. The average receptor number per cell was about 67,000 and dissociation constant (Kd) 0.41 nM. Formyl-MLLP, fMLP, fNLP, were 750%, 15%, 8.6% respectively and Boc-MLP, Boc-NLP, and MLP 0.6% as potent as the hexapeptide in inhibiting the binding of 125I-hexapeptide to rat neutrophils. The same correlation was found between these peptides in their potency to induce chemotaxis. This indicates that the rat neutrophil chemotactic receptor is like human receptor also a highly stereoselective and requires a N-formylated ligand for high affinity binding. Affinity cross-linking of 125I-hexapeptide to rat and human neutrophil chemotactic receptor with glutaraldehyde revealed on SDS-PAGE a 85-kDa and 62-kDa major complex and a 170-kDa and 120-kDa minor complex, respectively. The 120-kDa complex was absent in human neutrophils if the cells were treated with glutaraldehyde prior to cross-linking of 125I-hexapeptide to its receptor. Likewise, the larger complex was absent if neutrophils were exposed to heterologous ligand (C5a) prior to glutaraldehyde treatment and cross-linking of 125I-hexapeptide to its specific receptor. These results demonstrate that the rat neutrophils possess a functional high-affinity receptor for N-formylated chemotactic peptides and that the size of the monomeric receptor is 85-kDa and about 23-kDa larger than that of the human receptor. Upon homologous ligand binding the receptor seems to form a larger complex.