Role of the carboxyl terminal di-leucine in phosphorylation and internalization of C5a receptor

AbstractThe carboxyl tail of G protein-coupled receptors contains motifs that regulate receptor interactions with intracellular partners. Activation of the human neutrophil complement fragment C5a receptor (C5aR) is terminated by phosphorylation of the carboxyl tail followed by receptor internalization. In this study, we demonstrated that bulky hydrophobic residues in the membrane-proximal region of the C5aR carboxyl tail play an important role in proper structure and function of the receptor: Substitution of leucine 319 with alanine (L319A) resulted in receptor retention in the endoplasmic reticulum, whereas a L318A substitution allowed receptor transport to the cell surface, but showed slow internalization upon activation, presumably due to a defect in phosphorylation by both PKC and GRK. Normal agonist-induced activation of ERK1/2 and intracellular calcium release suggested that the L318A mutation did not affect receptor signaling. Binding of GRK2 and PKCβII to intracellular loop 3 of C5aR in vitro indicated that mutagenesis of L318 did not affect kinase binding. Limited proteolysis with trypsin revealed a conformational difference between wild type and mutant receptor. Our studies support a model in which the L318/L319 stabilizes an amphipathic helix (Q305–R320) in the membrane-proximal region of C5aR

Regulation of Human Formyl Peptide Receptor 1 Synthesis: Role of Single Nucleotide Polymorphisms, Transcription Factors, and Inflammatory Mediators

The gene encoding the human formyl peptide receptor 1 (FPR1) is heterogeneous, containing numerous single nucleotide polymorphisms (SNPs). Here, we examine the effect of these SNPs on gene transcription and protein translation. We also identify gene promoter sequences and putative FPR1 transcription factors. To test the effect of codon bias and codon pair bias on FPR1 expression, four FPR1 genetic variants were expressed in human myeloid U937 cells fused to a reporter gene encoding firefly luciferase. No significant differences in luciferase activity were detected, suggesting that the translational regulation and protein stability of FPR1 are modulated by factors other than the SNP codon bias and the variant amino acid properties. Deletion and mutagenesis analysis of the FPR1 promoter showed that a CCAAT box is not required for gene transcription. A −88/41 promoter construct resulted in the strongest transcriptional activity, whereas a −72/41 construct showed large reduction in activity. The region between −88 and −72 contains a consensus binding site for the transcription factor PU.1. Mutagenesis of this site caused significant reduction in reporter gene expression. The PU.1 binding was confirmed in vivo by chromatin immunoprecipitation, and the binding to nucleotides −84 to −76 (TTCCTATTT) was confirmed in vitro by an electrophoretic mobility shift assay. Thus, similar to many other myeloid genes, FPR1 promoter activity requires PU.1. Two single nucleotide polymorphisms at −56 and −54 did not significantly affect FPR1 gene expression, despite differences in binding of transcription factor IRF1 in vitro. Inflammatory mediators such as interferon-γ, tumor necrosis factor-α, and lipopolysaccharide did not increase FPR1 promoter activity in myeloid cells, whereas differentiation induced by DMSO and retinoic acid enhanced the activity. This implies that the expression of FPR1 in myeloid cells is developmentally regulated, and that the differentiated cells are equipped for immediate response to microbial infections

CD177-mediated nanoparticle targeting of human and mouse neutrophils

<div><p>Neutrophils are the most abundant white blood cells, with a vital role in innate immune defense against bacterial and fungal pathogens. Although mostly associated with pathological processes directly related to immune defense, they can also play a detrimental role in inflammatory conditions and have been found to have a pro-metastatic role in the spread of cancer cells. Here, we explore ways to temporarily suppress these detrimental activities. We first examined the possibility of using siRNA and antisense oligonucleotides (ASOs) for transient knockdown of the human and mouse C5a receptor, an important chemoattractant receptor involved in neutrophil-mediated injury that is associated with myocardial infarction, sepsis, and neurodegenerative diseases. We found that siRNAs and ASOs transfected into cultured cell lines can eliminate 70–90% of C5a receptor mRNA and protein within 72 h of administration, a clinically relevant time frame after a cardiovascular event. Targeted drug delivery to specific cells or tissues of interest in a mammalian host, however, remains a major challenge. Here, using phage display technology, we have identified peptides that bind specifically to CD177, a neutrophil-specific surface molecule. We have attached these peptides to fluorescent, lipid-based nanoparticles and confirmed targeting and delivery to cultured cells ectopically presenting either human or mouse CD177. In addition, we have shown peptide-nanoparticle binding specifically to neutrophils in human and mouse blood. We anticipate that these or related tagged nanoparticles may be therapeutically useful for delivery of siRNAs or ASOs to neutrophils for transient knockdown of pro-inflammatory proteins such as the C5a receptor.</p></div

Peptide M-HPLN particles bind CD177-expressing mouse neutrophils in whole blood.

<p>Peptide M-HPLN particles were incubated with whole mouse blood for 1 h at 37°C. The white blood cells were collected from the buffy coat and the red blood cells were lysed. The white blood cells were centrifuged onto glass slides, fixed in methanol and stained with rabbit anti-mouse CD177 polyclonal antibody and Alexa Fluor 488 goat anti-rabbit secondary antibody. Peptide H-HPLN particles were used as a negative control. The experiment was carried out twice. The micrographs represent the results from >100 cells. Scale bar, 10 μm.</p

Amino acid sequences of peptides that bind human and mouse CD177.

<p>Amino acid sequences of peptides that bind human and mouse CD177.</p

Mouse C5aR1 siRNA and human C5aR1 siRNA pool result in receptor knockdown.

<p>CHO cells expressing mouse C5aR1-GFP were transfected with 100 nM mouse C5aR1 ON-TARGETplus SMART siRNA–6, 100 nM GFP siRNA (positive control), or 100 nM negative control siRNA. 72 h post transfection cells were analyzed by flow cytometry to measure the relative expression of mouse C5aR1-GFP (left panel). CHO cells expressing human C5aR1-GFP were transfected with 100 nM human C5aR1 ON-TARGETplus SMARTpool siRNA, 100 nM GFP siRNA (positive control), or 100 nM negative control siRNA (right panel). Relative knockdown is based on the percentage of the cells that are to the left of the gate relative to the negative control sample. The experiment was carried out twice with similar results.</p

Flow cytometry confirms Peptide H-HPLN binding to CD177-positive neutrophils in a pool of purified human leukocytes.

<p>Leukocytes were incubated on ice in the presence or absence of anti-CD177 antibody, washed and incubated on ice in the presence or absence of Peptide H-HPLN and Alexa Fluor 488 goat anti-mouse antibody. <b>A.</b> Forward and side scatter plot of human leukocytes (and remaining red blood cells) with the neutrophil population shown in a circle. <b>B.</b> FL-1 histogram of CD177 expression. Left panel: Total cell population in the absence and presence of anti-CD177 antibody (1° Ab) and Alexa Fluor 488 goat anti-mouse antibody (2° Ab). Right panel: Gated neutrophil population in the absence or presence of anti-CD177 antibody (1° Ab) and Alexa Fluor 488 goat anti-mouse antibody (2° Ab). <b>C.</b> FL2 plot showing Peptide H-HPLN binding. Total cell population in the absence or presence of Peptide H-HPLNs (left panel) and gated neutrophil population in the absence and presence of Peptide H-HPLNs (right panel). <b>D.</b> FL1-A and FL2-A plot showing fluorescence of the total cell population (left panel) and gated neutrophil population (right panel). The experiment was carried out twice with similar results. The blood sample shown had a higher percentage of CD177-positive neutrophils than the blood sample from the other donor.</p

HPLN particles displaying Peptide M become internalized by CHO cells expressing mouse CD177-HA.

<p>CHO cells expressing mouse CD177 containing an HA-tag were incubated with 75 μg/ml Peptide M-HPLN particles for 2 h on ice to allow binding. Cells were then washed and kept on ice or warmed to 37°C for 1 h. Cells were then treated with subtilisin to remove surface-bound Peptide M-HPLN particles, or treated with buffer alone. After fixation and permeabilization, mouse CD177-HA was stained with a mouse anti-HA antibody and an Alexa Fluor 488 goat anti-mouse secondary antibody. The experiment was carried out once. Most cells out of more than 100 cells visualized on the slides had a similar staining pattern as those seen in these micrographs. Scale bar, 50 μm.</p