Sarah Line Skovbakke

2-Deoxy d-Glucose Prevents Cell Surface Expression of NKG2D Ligands through Inhibition of N-Linked Glycosylation

NKG2D ligand surface expression is important for immune recognition of stressed and neotransformed cells. In this study, we show that surface expression of MICA/B and other NKG2D ligands is dependent on N-linked glycosylation. The inhibitor of glycolysis and N-linked glycosylation, 2-deoxy-d-glucose (2DG), potently inhibited surface expression of MICA/B after histone deacetylase inhibitor treatment; the inhibition occurred posttranscriptionally without affecting MICA promoter activity. Transient overexpression of MICA surface expression was also inhibited by 2DG. 2DG blocks N-linked glycosylation of MICA/B by a reversible mechanism that can be alleviated by addition of d-mannose; this does not, however, affect the inhibition of glycolysis. Addition of d-mannose restored MICA/B surface expression after 2DG treatment. In addition, specific pharmacological or small interfering RNA-mediated targeting of glycolytic enzymes did not affect MICA/B surface expression, strongly suggesting that N-linked glycosylation, and not glycolysis, is essential for MICA/B surface expression. Corroborating this, tunicamycin, a selective inhibitor of N-linked glycosylation, abolished MICA/B surface expression without compromising activation of MICA promoter activity. NK cell-mediated killing assay and staining with a recombinant NKG2D–Fc fusion protein showed that all functional NKG2D ligands induced by histone deacetylase inhibitor treatment were abolished by 2DG treatment and fully reconstituted by further addition of d-mannose. Our data suggest that posttranslational N-linked glycosylation is strictly required for NKG2D ligand surface expression. Cancer and infection often result in aberrant glycosylation, which could likely be involved in modulation of NKG2D ligand expression. Our data further imply that chemotherapeutic use of 2DG may restrict NKG2D ligand surface expression and inhibit secretion of immunoinhibitory soluble NKG2D ligands.

Structural changes of the ligand and of the receptor alters the receptor preference for neutrophil activating peptides starting with a formylmethionyl group

Pathogenic Staphylococcus aureus strains produce N-formylmethionyl containing peptides, of which the tetrapeptide fMIFL is a potent activator of the neutrophil formyl peptide receptor 1 (FPR1) and the PSMα2 peptide is a potent activator of the closely related FPR2. Variants derived from these two peptide activators were used to disclose the structural determinants for receptor interaction. Removal of five amino acids from the C-terminus of PSMα2 gave rise to a peptide that had lost the receptor-independent neutrophil permeabilizing effect, whereas neutrophil activation capacity as well as its preference for FPR2 was retained. Shorter peptides, PSMα21-10 and PSMα21-5, activate neutrophils, but the receptor preference for these peptides was switched to FPR1. The fMIFL-PSM5-16 peptide, in which the N-terminus of PSMα21-16 was replaced by the sequence fMIFL, was a dual agonist for FPR1/FPR2, whereas fMIFL-PSM5-10 preferred FPR1 to FPR2. Further, an Ile residue was identified as a key determinant for interaction with FPR2. A chimeric receptor in which the cytoplasmic tail of FPR1 was replaced by the corresponding part of FPR2 lost the ability to recognize FPR1 agonists, but gained function in relation to FPR2 agonists. Taken together, our data demonstrate that the C-terminus of the PSMα2 peptide plays a critical role for its cytotoxicity, but is not essential for the receptor-mediated pro-inflammatory activity. More importantly, we show that the amino acids present in the C-terminus, which are not supposed to occupy the agonist-binding pocket in the FPRs, are of importance for the choice of receptor.

The proteolytically stable peptidomimetic Pam-(Lys-βNSpe)6-NH2 selectively inhibits human neutrophil activation via formyl peptide receptor 2.

Immunomodulatory host defense peptides (HDPs) are considered to be lead compounds for novel anti-sepsis and anti-inflammatory agents. However, development of drugs based on HDPs has been hampered by problems with toxicity and low bioavailability due to in vivo proteolysis. Here, a subclass of proteolytically stable HDP mimics consisting of lipidated α-peptide/β-peptoid oligomers was investigated for their effect on neutrophil function. The most promising compound, Pam-(Lys-βNSpe)6-NH2, was shown to inhibit formyl peptide receptor 2 (FPR2) agonist-induced neutrophil granule mobilization and release of reactive oxygen species. The potency of Pam-(Lys-βNSpe)6-NH2 was comparable to that of PBP10, the most potent FPR2-selective inhibitor known. The immunomodulatory effects of structural analogs of Pam-(Lys-βNSpe)6-NH2 emphasized the importance of both the lipid and peptidomimetic parts. By using imaging flow cytometry in primary neutrophils and FPR-transfected cell lines, we found that a fluorescently labeled analog of Pam-(Lys-βNSpe)6-NH2 interacted selectively with FPR2. Furthermore, the interaction between Pam-(Lys-βNSpe)6-NH2 and FPR2 was found to prevent binding of the FPR2-specific activating peptide agonist Cy5-WKYMWM, while the binding of an FPR1-selective agonist was not inhibited. To our knowledge, Pam-(Lys-βNSpe)6-NH2 is the first HDP mimic found to inhibit activation of human neutrophils via direct interaction with FPR2. Hence, we consider Pam-(Lys-βNSpe)6-NH2 to be a convenient tool in the further dissection of the role of FPR2 in inflammation and homeostasis as well as for investigation of the importance of neutrophil stimulation in anti-infective therapy involving HDPs.

N-glycosylation of asparagine 8 regulates surface expression of major histocompatibility complex class I chain-related protein A (MICA) alleles dependent on threonine 24.

Background: Immune activation through surface expression of the human NKG2D ligand MICA is important in clearance of virus-infected or cancerous cells. Results: Molecular characterization of N-glycosylation in regulation of MICA cell surface expression. Conclusion: Surface expression of MICA alleles vary in dependence for N-glycosylation. Significance: We identify N-glycosylation as an allele-specific regulatory mechanism of MICA and pinpoint the essential residues. NKG2D is an activating receptor expressed on several types of human lymphocytes. NKG2D ligands can be induced upon cell stress and are frequently targeted post-translationally in infected or transformed cells to avoid immune recognition. Virus infection and inflammation alter protein N-glycosylation, and we have previously shown that changes in cellular N-glycosylation are involved in regulation of NKG2D ligand surface expression. The specific mode of regulation through N-glycosylation is, however, unknown. Here we investigated whether direct N-glycosylation of the NKG2D ligand MICA itself is critical for cell surface expression and sought to identify the essential residues. We found that a single N-glycosylation site (Asn8) was important for MICA018 surface expression. The frequently expressed MICA allele 008, with an altered transmembrane and intracellular domain, was not affected by mutation of this N-glycosylation site. Mutational analysis revealed that a single amino acid (Thr24) in the extracellular domain of MICA018 was essential for the N-glycosylation dependence, whereas the intracellular domain was not involved. The HHV7 immunoevasin, U21, was found to inhibit MICA018 surface expression by affecting N-glycosylation, and the retention was rescued by T24A substitution. Our study reveals N-glycosylation as an allele-specific regulatory mechanism important for regulation of surface expression of MICA018, and we pinpoint the residues essential for this N-glycosylation dependence. In addition, we show that this regulatory mechanism of MICA surface expression is likely targeted during different pathological conditions.

A Pepducin Derived from the Third Intracellular Loop of FPR2 Is a Partial Agonist for Direct Activation of This Receptor in Neutrophils But a Full Agonist for Cross-Talk Triggered Reactivation of FPR2

We recently described a novel receptor cross-talk mechanism in neutrophils, unique in that the signals generated by the PAF receptor (PAFR) and the ATP receptor (P2Y2R) transfer formyl peptide receptor 1 (FPR1) from a desensitized (non-signaling) state back to an actively signaling state (Forsman H et al., PLoS One, 8:e60169, 2013; Önnheim K, et al., Exp Cell Res, 323∶209, 2014). In addition to the G-protein coupled FPR1, neutrophils also express the closely related receptor FPR2. In this study we used an FPR2 specific pepducin, proposed to work as an allosteric modulator at the cytosolic signaling interface, to determine whether the cross-talk pathway is utilized also by FPR2. The pepducin used contains a fatty acid linked to a peptide sequence derived from the third intracellular loop of FPR2, and it activates as well as desensensitizes this receptor. We now show that neutrophils desensitized with the FPR2-specific pepducin display increased cellular responses to stimulation with PAF or ATP. The secondary PAF/ATP induced response was sensitive to FPR2-specific inhibitors, disclosing a receptor cross-talk mechanism underlying FPR2 reactivation. The pepducin induced an activity in naïve cells similar to that of a conventional FPR2 agonist, but with lower potency (partial efficacy), meaning that the pepducin is a partial agonist. The PAF- or ATP-induced reactivation was, however, much more pronounced when neutrophils had been desensitized to the pepducin as compared to cells desensitized to conventional agonists. The pepducin should thus in this respect be classified as a full agonist. In summary, we demonstrate that desensitized FPR2 can be transferred back to an actively signaling state by receptor cross-talk signals generated through PAFR and P2Y2R, and the difference in agonist potency with respect to pepducin-induced direct receptor activation and cross-talk reactivation of FPR2 puts the concept of functional selectivity in focus.

Lipidated α-peptide/β-peptoid hybrids with potent anti-inflammatory activity.

In this study, we investigated, optimized, and characterized a novel subclass of host defense peptide (HDP) mimics based on α-peptide/β-peptoid hybrid oligomers with an alternating cationic/hydrophobic design with respect to their ability to modulate the pro-inflammatory response by human primary leukocytes upon exposure to bacterial components. Structure-activity studies revealed that certain lipidated α-peptide/β-peptoid hybrid oligomers possess anti-inflammatory activities in the submicromolar range with low cytotoxicity, and that the anti-inflammatory activity of the HDP mimics is dependent on the length and position of the lipid element(s). The resulting lead compound, Pam-(Lys-βNSpe)6-NH2, blocks LPS-induced cytokine secretion with a potency comparable to that of polymyxin B. The mode of action of this HDP mimic appears not to involve direct LPS interaction since it, in contrast to polymyxin B, displayed only minor activity in the Limulus amebocyte lysate assay. Flow cytometry data showed specific interaction of a fluorophore-labeled lipidated α-peptide/β-peptoid hybrid with monocytes and granulocytes indicating a cellular target expressed by these leukocyte subsets.

The Lipidated Peptidomimetic Lau-((S)-Aoc)-(Lys-βNphe)6-NH2 Is a Novel Formyl Peptide Receptor 2 Agonist That Activates Both Human and Mouse Neutrophil NADPH Oxidase.

Neutrophils expressing formyl peptide receptor 2 (FPR2) play key roles in host defense, immune regulation, and resolution of inflammation. Consequently, the search for FPR2-specific modulators has attracted much attention due to its therapeutic potential. Earlier described agonists for this receptor display potent activity for the human receptor (FPR2) but low activity for the mouse receptor orthologue (Fpr2), rendering them inapplicable in murine models of human disease. Here we describe a novel FPR2 agonist, the proteolytically stable α-peptide/β-peptoid hybrid Lau-((S)-Aoc)-(Lys-βNphe)6-NH2 (F2M2), showing comparable potency in activating human and mouse neutrophils by inducing a rise in intracellular Ca2+ concentration and assembly of the superoxide-generating NADPH oxidase. This FPR2/Fpr2 agonist contains a headgroup consisting of a 2-aminooctanoic acid (Aoc) residue acylated with lauric acid (C12 fatty acid), which is linked to a peptide/peptoid repeat ((Lys-βNphe)6-NH2). Both the fatty acid moiety and the (S)-Aoc residue were required for FPR2/Fpr2 activation. This type of proteolytically stable FPR2-specific peptidomimetics may serve as valuable tools for future analysis of FPR2 signaling as well as for development of prophylactic immunomodulatory therapy. This novel class of cross-species FPR2/Fpr2 agonists should enable translation of results obtained with mouse neutrophils (and disease models) into enhanced understanding of human inflammatory and immune diseases.

The peptidomimetic Lau-(Lys-βNSpe)6-NH2 antagonizes formyl peptide receptor 2 expressed in mouse neutrophils

The formyl peptide receptor (FPR) gene family has a complex evolutionary history and comprises eight murine members but only three human representatives. To enable translation of results obtained in mouse models of human diseases, more comprehensive knowledge of the pharmacological similarities/differences between the human and murine FPR family members is required. Compared to FPR1 and FPR2 expressed by human neutrophils, very little is known about agonist/antagonist recognition patterns for their murine orthologues, but now we have identified two potent and selective formylated peptide agonists (fMIFL and PSMα2) for Fpr1 and Fpr2, respectively. These peptides were used to determine the inhibition profile of a set of antagonists with known specificities for the two FPRs in relation to the corresponding murine receptors. Some of the most potent and selective antagonists for the human receptors proved to be devoid of effect on their murine orthologues as determined by their inability to inhibit superoxide release from murine neutrophils upon stimulation with receptor-specific agonists. The Boc-FLFLF peptide was found to be a selective antagonist for Fpr1, whereas the lipidated peptidomimetic Lau-(Lys-βNSpe)6-NH2 and the hexapeptide WRW4 were identified as Fpr2-selective antagonists.

The Role of Formyl Peptide Receptors for Immunomodulatory Activities of Antimicrobial Peptides and Peptidomimetics

In recent years, the therapeutic potential of antimicrobial peptides (AMPs) as immunomodulators has become generally accepted. Nevertheless, only very few AMP-based compounds have progressed into clinical trials. This paradox may be explained by the fact, that some of the intrinsic properties of natural peptides, such as proteolytic and oxidative instability, render them inconvenient as therapeutics. Therefore, substantial research efforts have been dedicated to mimic the physico-chemical properties as well as biological activities of AMPs by designing and identifying more stable peptidomimetics displaying analogous immunomodulatory activity profiles. Neutrophils play key roles in host defense as major effector cells in clearance of pathogens by phagocytosis and by regulating other processes of innate immunity as well as by promoting resolution of inflammation. Several aspects of these effects are correlated to their expression of formyl peptide receptors (FPRs) that have been shown to be targets of both natural and synthetic antimicrobial peptides. In the present review recent findings highlighting the role of FPRs in mediating immunomodulatory activities of natural and synthetic AMPs as well as of stabilized peptidomimetics are discussed, and prospects for future development of immunomodulatory therapeutics are presented.

Anti-inflammatory Properties of Antimicrobial Peptides and Peptidomimetics: LPS and LTA Neutralization

Lipopolysaccharide (LPS) and lipoteichoic acid (LTA) neutralization constitute potential non-antibiotic treatment strategies for sepsis - a systemic infection-induced inflammatory response. Studies on LPS- and LTA-neutralizing compounds are abundant in literature, and a number of peptides and peptidomimetics appear to display promising activity. However, in this ongoing search for potential antisepsis drug leads, it will be preferable that the assays used by different research groups lead to readily comparable data for the most efficient compounds. Here, we propose and describe standardized methods to be used for testing of novel compounds for their LPS- and LTA-neutralizing capacity with a focus on functional suppression of pro-inflammatory responses in cell-based systems. To best mimic the human in vivo conditions, we suggest the use of freshly isolated human leukocytes combined with an appropriate method for the chosen cytokine (e.g., IL-6 or TNF-α). The described protocols comprise isolation, stimulation, and viability test of the human leukocytes.