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

Formyl Met-Leu-Phe-Stimulated FPR1 Phosphorylation in Plate-Adherent Human Neutrophils: Enhanced Proteolysis but Lack of Inhibition by Platelet-Activating Factor

N-formyl-Met-Leu-Phe (fMLF) is a model PAMP/DAMP driving human PMN to sites of injury/infection utilizing the GPCR, FPR1. We examined a microtiter plate format for measurement of FPR1 phosphorylation in adherent PMN at high densities and found that a new phosphosensitive FPR1 fragment, 25K-FPR1, accumulates in SDS-PAGE extracts. 25K-FPR1 is fully inhibited by diisopropylfluorophosphate PMN pretreatment but is not physiologic, as its formation failed to be significantly perturbed by ATP depletion, time and temperature of adherence, or adherence mechanism. 25K-FPR1 was minimized by extracting fMLF-exposed PMN in lithium dodecylsulfate at 4°C prior to reduction/alkylation. After exposure of adherent PMN to a 5 log range of PAF before or after fMLF, unlike in suspension PMN, no inhibition of fMLF-induced FPR1 phosphorylation was observed. However, PAF induced the release of 40% of PMN lactate dehydrogenase, implying significant cell lysis. We infer that PAF-induced inhibition of fMLF-dependent FPR1 phosphorylation observed in suspension PMN does not occur in the unlysed adherent PMN. We speculate that although the conditions of the assay may induce PAF-stimulated necrosis, the cell densities on the plates may approach levels observed in inflamed tissues and provide for an explanation of PAF’s divergent effects on FPR1 phosphorylation as well as PMN function

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

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

Some Peptide H-HPLN particles co-localize with LAMP2 after prolonged incubation, suggesting delivery into the lysosomes.

<p>CHO cells expressing human CD177 were incubated for 3 h and 17 h at 37°C with 75 μg/ml Peptide H-HPLN particles. Cells were fixed and permeabilized, and the lysosomes were stained with a mouse anti-hamster LAMP2 antibody and an Alexa Fluor 488 goat anti-mouse secondary antibody. The experiment was carried out once. The selected micrographs are representative of more than 100 cells with similar labeling patterns. Scale bar, 50 μm.</p

Quantitative phage peptide binding assay.

<p>Quantitative phage peptide binding assay.</p

HPLNs displaying Peptide H bind human CD177 on cells.

<p>CHO cells expressing human CD177 were incubated for 3 h at 37°C with 75 μg/ml Peptide H-HPLNs. Cells were fixed and permeabilized, and CD177 was detected using a mouse anti-human CD177 antibody and an Alexa Fluor 488 goat anti-mouse secondary antibody. The HPLN particles are fluorescent red. As a negative control, HPLN particles displaying a scrambled peptide were used. The experiment was carried out twice, with at least 100 cells examined in each experiment. A direct correlation between the expression level of CD177 (based on fluorescence intensity) and the Peptide H-HPLN signal could be observed. The cell in the lower panel shows a typical cell distribution of CD177 and Peptide H-HPLNs after 3 h incubation. In contrast, the two cells shown in the upper panel are characteristic of the CD177 staining in the absence of Peptide H-HPLNs and in the presence of Scrambled Peptide H-HPLNs. Scale bar, 50 μm.</p