ADP-ribosyltransferase-specific modification of human neutrophil peptide-1

Epithelial cells lining human airways and cells recruited to airways participate in the innate immune response in part by releasing human neutrophil peptides (HNP). Arginine-specific ADP-ribosyltransferases (ART) on the surface of these cells can catalyze the transfer of ADP-ribose from NAD to proteins. We reported that ART1, a mammalian ADP-ribosyltransferase, present in epithelial cells lining the human airway, modified HNP-1, altering its function. ADP-ribosylated HNP-1 was identified in bronchoalveolar lavage fluid (BALF) from patients with asthma, idiopathic pulmonary fibrosis, or a history of smoking (and having two common polymorphic forms of ART1 that differ in activity), but not in normal volunteers or patients with lymphangioleiomyomatosis. Modified HNP-1 was not found in the sputum of cystic fibrosis patients or in leukocyte granules of normal volunteers. The finding of ADP-ribosyl-HNP-1 in BALF but not in leukocyte granules suggests that the modification occurred in the airway. Most of the HNP-1 in the BALF from individuals with a history of smoking was, in fact, mono- or di-ADP-ribosylated. ART1 synthesized in Escherichia coli, glycosylphosphatidylinositol-anchored ART1 released with phosphatidylinositol-specific phospholipase C from transfected NMU cells, or ART1 expressed endogenously on C2C12 myotubes modified arginine 14 on HNP-1 with a secondary site on arginine 24. ADP-ribosylation of HNP-1 by ART1 was substantially greater than that by ART3, ART4, ART5, Pseudomonas aeruginosa exoenzyme S, or cholera toxin A subunit. Mouse ART2, which is an NAD:arginine ADP-ribosyltransferase, was able to modify HNP-1, but to a lesser extent than ART1. Although HNP-1 was not modified to a significant degree by ART5, it inhibited ART5 as well as ART1 activities. Human beta-defensin-1 (HBD1) was a poor transferase substrate. Reduction of the cysteine-rich defensins enhanced their ability to serve as ADP-ribose acceptors. We conclude that ADP-ribosylation of HNP-1 appears to be primarily an activity of ART1 and occurs in inflammatory conditions and disease.     

A Molecular Dynamics Study of Human Defensins HBD-1 and HNP-3 in Water

Abstract

Mammalian defensins are crucial components of the innate immune system. They are characterized by three disulfide bridges and exhibit broad spectrum antibacterial activity. The spacing between the cysteines and disulfide connectivities in the two classes of defensins, the α- and β-forms, are different. The structural motif of 3 β-strands appears to be conserved in α and β-defensins despite differences in disulfide connectivities and spacing between cysteines. In this study, Molecular Dynamics Simulations (MDS) have been carried out to study the conformational behavior of α- and β-defensins with and without disulfide bridges. Our results indicate that β-strands in the C-terminal region of HBD-1 and HNP-3 do not unfold during the course of MDS. The segment adopting α-helix in HBD-1 unfolds early during the simulations. The backbone hydrogen bonds in HBD-1 and HNP-3 are broken during MDS. When the disulfide bonds are absent, the N-terminal β-strand unfolds by 20 ns but β-strands are observed in the C-terminal region of HNP-3. HBD-1, without disulfide bridges, unfolds to a greater extent during the course of the MDS. Examination of distances between sulfur atoms of cysteines without disulfide bridges during the simulations indicate that there is no specific preference for native disulfide bridges, which could be the reason for the experimental observation of non-native disulfide bridge formation during chemical synthesis of human α- and β-defensins. Since defensins with non-native disulfide bridges are biologically active, the exact three dimensional structures observed for native HBD-1 and HNP-3 does not appear to be essential for exhibiting antibacterial activity.     

Human alpha- and beta-defensins block multiple steps in herpes simplex virus infection

This study examined the ability of nine human defensins (HD) to protect against herpes simplex virus infection. Noncytotoxic concentrations of all six alpha-defensins (HNP1-4, HD5, and HD6) and human beta-defensin (hBD) 3 inhibited HSV infection. Two other beta-defensins, hBD1 and 2, lacked this protective activity. Synchronized assays revealed that HNP-4, HD6, and hBD3 acted primarily by preventing binding and entry, whereas HNP1-3 and HD5 also inhibited postentry events. Even when added several hours after entry, substantial reduction in viral gene expression ensued. Human cervical epithelial cells incubated with HNP-1 or HD5 accumulated the peptides intracellularly. Surface plasmon resonance studies revealed that HNPs 1, 2, 3, and HD5 bound HSV glycoprotein B (gB) with high affinity, but showed minimal binding to heparan sulfate, the receptor for attachment. In contrast, HNP-4 and HD6 bound heparan sulfate, but not gB. HBD3 bound both gB and heparan sulfate, but hBD1 and hBD2 bound neither. Admixture of HD5 with hydroxyethylcellulose significantly protected mice from a viral challenge lethal to controls receiving an inactive peptide or hydroxyethylcellulose alone. These findings demonstrate that HDs act at multiple steps in the HSV life cycle and support the development of defensins or defensin-like peptides as microbicides.     

Purification and characterization of human neutrophil peptide 4, a novel member of the defensin family

Primary (azurophil) granules of neutrophils contain proteins which play a major role in the killing and digestion of bacteria in the phagolysosome. We have isolated and characterized a novel antimicrobial peptide from the azurophil granule fraction of discontinuous Percoll gradients. We have named this peptide human neutrophil peptide 4 (HNP-4) based on its structural similarity to a group of antimicrobial polypeptides known as defensins (HNP 1-3). Using size exclusion and reverse-phase high performance liquid chromatography, HNP-4 was purified to homogeneity as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and amino-terminal sequence analysis. The amino acid sequence determined from isolated HNP-4 and from tryptic fragments of reduced and alkylated peptide is: NH2-Val-Cys-Ser-Cys-Arg-Leu-Val-Phe-Cys-Arg-Arg-Thr-Glu- Leu-Arg-Val-Gly-Asn-Cys-Leu-Ile-Gly-Gly-Val-Ser-Phe-Thr-Tyr-Cys-Cys-Thr- Arg-Val - COOH. Based on this sequence, HNP-4 has a calculated molecular weight of 3715 and a theoretical pI of 8.61. HNP-4 shows structural similarity to the family of three human defensins. HNP-4 and the defensins have identical cysteine backbones and, like the defensins, HNP-4 is rich in arginine (15.2 mol %). However, the amino acids at 22 of the 33 positions differ between HNP-4 and human defensins. Further, HNP-4 is significantly more hydrophobic than the defensins, as determined by its retention time on reverse-phase high performance liquid chromatography. In vitro, purified HNP-4 was shown to kill Escherichia coli, Streptococcus faecalis, and Candida albicans. Compared to a mixture of the other human defensins, HNP-4 was found to be approximately 100 times more potent against E. coli and four times more potent against both S. faecalis and C. albicans.     

Antiviral Mechanisms of Human Defensins

Defensins are an effector component of the innate immune system with broad antimicrobial activity. Humans express two types of defensins, α- and β-defensins, which have antiviral activity against both enveloped and non-enveloped viruses. The diversity of defensin-sensitive viral species reflects a multitude of antiviral mechanisms. These include direct defensin targeting of viral envelopes, glycoproteins, and capsids in addition to inhibition of viral fusion and post-entry neutralization. Binding and modulation of host cell surface receptors and disruption of intracellular signaling by defensins can also inhibit viral replication. In addition, defensins can function as chemokines to augment and alter adaptive immune responses, revealing an indirect antiviral mechanism. Nonetheless, many questions regarding the antiviral activities of defensins remain. Although significant mechanistic data are known for α-defensins, molecular details for β-defensin inhibition are mostly lacking. Importantly, the role of defensin antiviral activity in vivo has not been addressed due to the lack of a complete defensin knockout model. Overall, the antiviral activity of defensins is well established as are the variety of mechanisms by which defensins achieve this inhibition; however, additional research is needed to fully understand the role of defensins in viral pathogenesis.     

Direct inactivation of viruses by human granulocyte defensins.

Human neutrophils contain a family of microbicidal peptides known as defensins. One of these defensins, human neutrophil peptide (HNP)-1, was purified, and its ability to directly inactivate several viruses was extensively tested. Herpes simplex virus (HSV) types 1 and 2, cytomegalovirus, vesicular stomatitis virus, and influenza virus A/WSN were inactivated by incubation with HNP-1. Two nonenveloped viruses, echovirus type 11 and reovirus type 3, were resistant to inactivation. Purified homologous peptides HNP-2 and HNP-3 were found to have HSV-1-neutralizing activities approximately equal to that of HNP-1. Inactivation of HSV-1 by HNP-1 depended on the time, temperature, and pH of incubation. Antiviral activity was abrogated by low temperature or prior reduction and alkylation of the defensins. Addition of serum or serum albumin to the incubation mixtures inhibited neutralization of HSV-1 by HNP-1. We used density gradient sedimentation techniques to demonstrate that HNP-1 bound to HSV-1 in a temperature-dependent manner. We speculate that binding of defensin peptides to certain viruses may impair their ability to infect cells.     

Structure-activity determinants in antifungal plant defensins MsDef1 and MtDef4 with different modes of action against Fusarium graminearum

Plant defensins are small cysteine-rich antimicrobial proteins. Their three-dimensional structures are similar in that they consist of an α-helix and three anti-parallel β-strands stabilized by four disulfide bonds. Plant defensins MsDef1 and MtDef4 are potent inhibitors of the growth of several filamentous fungi including Fusarium graminearum. However, they differ markedly in their antifungal properties as well as modes of antifungal action. MsDef1 induces prolific hyperbranching of fungal hyphae, whereas MtDef4 does not. Both defensins contain a highly conserved γ-core motif (GXCX(3-9)C), a hallmark signature present in the disulfide-stabilized antimicrobial peptides, composed of β2 and β3 strands and the interposed loop. The γ-core motifs of these two defensins differ significantly in their primary amino acid sequences and in their net charge. In this study, we have found that the major determinants of the antifungal activity and morphogenicity of these defensins reside in their γ-core motifs. The MsDef1-γ4 variant in which the γ-core motif of MsDef1 was replaced by that of MtDef4 was almost as potent as MtDef4 and also failed to induce hyperbranching of fungal hyphae. Importantly, the γ-core motif of MtDef4 alone was capable of inhibiting fungal growth, but that of MsDef1 was not. The analysis of synthetic γ-core variants of MtDef4 indicated that the cationic and hydrophobic amino acids were important for antifungal activity. Both MsDef1 and MtDef4 induced plasma membrane permeabilization; however, kinetic studies revealed that MtDef4 was more efficient in permeabilizing fungal plasma membrane than MsDef1. Furthermore, the in vitro antifungal activity of MsDef1, MsDef1-γ4, MtDef4 and peptides derived from the γ-core motif of each defensin was not solely dependent on their ability to permeabilize the fungal plasma membrane. The data reported here indicate that the γ-core motif defines the unique antifungal properties of each defensin and may facilitate de novo design of more potent antifungal peptides.     

Antimicrobial potentials and structural disorder of human and animal defensins

Defensins are moonlighting peptides which are broadly distributed throughout all the living kingdoms. They play a multitude of important roles in human health and disease, possessing several immunoregulatory functions and manifesting broad antimicrobial activities against viruses, bacteria, and fungi. Based on their patterns of intramolecular disulfide bridges, these small cysteine-rich cationic proteins are divided into three major types, α-, β-, and θ-defensins, with the α- and β-defensins being further subdivided into a number of subtypes. The various roles played by the defensins in the innate (especially mucosal) and adoptive immunities place these polypeptides at the frontiers of the defense against the microbial invasions. Current work analyzes the antimicrobial activities of human and animal defensins in light of their intrinsic disorder propensities.     

Defensins enable macrophages to inhibit the intracellular proliferation of Listeria monocytogenes

Listeria monocytogenes is a facultative intracellular pathogen that infects a large diversity of host cells, including macrophages. To avoid the phagosome microbicidal environment, L. monocytogenes secretes a pore-forming toxin (listeriolysin O, LLO) that releases the bacterium into the cytoplasm. We hypothesized that the α-defensins (HNPs) and/or humanized θ-defensin (RC-1) peptides produced by human and non-human primate neutrophils, respectively, cooperate with macrophages to control L. monocytogenes infection. Our results establish that HNP-1 and RC-1 enable macrophages to control L. monocytogenes intracellular growth by inhibiting phagosomal escape, as a consequence, bacteria remain trapped in a LAMP-1-positive phagosome. Importantly, HNP-1 interaction with macrophages and RC-1 interaction with bacteria are required to prevent macrophage infection. In accordance with these results, RC-1 is a more potent anti-listerial peptide than HNP-1 and HNP-1 is acquired by macrophages and trafficked to the phagocytosed bacteria. Finally, HNP-1 and RC-1 antimicrobial activity is complemented by their ability to prevent LLO function through two mechanisms, blocking LLO-dependent perforation of macrophage membranes and the release of LLO from the bacteria. In conclusion, at the site of infection the cooperation between antimicrobial peptides, such as HNP-1, and macrophages likely plays a critical role in the innate immune defence against L. monocytogenes.     

Cyclic and Acyclic Defensins Inhibit Human Immunodeficiency Virus Type-1 Replication by Different Mechanisms

Defensins are antimicrobial peptides expressed by plants and animals. In mammals there are three subfamilies of defensins, distinguished by structural features: α, β and θ. Alpha and β-defensins are linear peptides with broad anti-microbial activity that are expressed by many mammals including humans. In contrast, θ-defensins are cyclic anti-microbial peptides made by several non-human primates but not humans. All three defensin types have anti-HIV-1 activity, but their mechanisms of action differ. We studied the anti-HIV-1 activity of one defensin from each group, HNP-1 (α), HBD-2 (β) and RTD-1 (θ). We examined how each defensin affected HIV-1 infection and demonstrated that the cyclic defensin RTD-1 inhibited HIV-1 entry, while acyclic HNP-1 and HBD-2 inhibited HIV-1 replication even when added 12 hours post-infection and blocked viral replication after HIV-1 cDNA formation. We further found that all three defensins downmodulated CXCR4. Moreover, RTD-1 inactivated X4 HIV-1, while HNP-1 and HBD-2 inactivated both X4 and R5 HIV-1. The data presented here show that acyclic and cyclic defensins block HIV-1 replication by shared and diverse mechanisms. Moreover, we found that HNP-1 and RTD-1 directly inhibited firefly luciferase enzymatic activity, which may affect the interpretation of previously published data.     

Functional interaction of human neutrophil peptide-1 with the cell wall precursor lipid II

Defensins constitute a major class of cationic antimicrobial peptides in mammals and vertebrates, acting as effectors of innate immunity against infectious microorganisms. It is generally accepted that defensins are bactericidal by disrupting the anionic microbial membrane. Here, we provide evidence that membrane activity of human α-defensins does not correlate with antibacterial killing. We further show that the α-defensin human neutrophil peptide-1 (HNP1) binds to the cell wall precursor lipid II and that reduction of lipid II levels in the bacterial membrane significantly reduces bacterial killing. The interaction between defensins and lipid II suggests the inhibition of cell wall synthesis as a novel antibacterial mechanism of this important class of host defense peptides.     

Insect antimicrobial peptides and their applications

Insects are one of the major sources of antimicrobial peptides/proteins (AMPs). Since observation of antimicrobial activity in the hemolymph of pupae from the giant silk moths Samia Cynthia and Hyalophora cecropia in 1974 and purification of first insect AMP (cecropin) from H. cecropia pupae in 1980, over 150 insect AMPs have been purified or identified. Most insect AMPs are small and cationic, and they show activities against bacteria and/or fungi, as well as some parasites and viruses. Insect AMPs can be classified into four families based on their structures or unique sequences: the α-helical peptides (cecropin and moricin), cysteine-rich peptides (insect defensin and drosomycin), proline-rich peptides (apidaecin, drosocin, and lebocin), and glycine-rich peptides/proteins (attacin and gloverin). Among insect AMPs, defensins, cecropins, proline-rich peptides, and attacins are common, while gloverins and moricins have been identified only in Lepidoptera. Most active AMPs are small peptides of 20–50 residues, which are generated from larger inactive precursor proteins or pro-proteins, but gloverins (~14 kDa) and attacins (~20 kDa) are large antimicrobial proteins. In this mini-review, we will discuss current knowledge and recent progress in several classes of insect AMPs, including insect defensins, cecropins, attacins, lebocins and other proline-rich peptides, gloverins, and moricins, with a focus on structural-functional relationships and their potential applications.     

The membrane-bound structure and topology of a human α-defensin indicate a dimer pore mechanism for membrane disruption

Defensins are cationic and disulfide-bonded host defense proteins of many animals that target microbial cell membranes. Elucidating the three-dimensional structure, dynamics, and topology of these proteins in phospholipid bilayers is important for understanding their mechanisms of action. Using solid-state nuclear magnetic resonance spectroscopy, we have now determined the conformation, dynamics, oligomeric state, and topology of a human α-defensin, HNP-1, in DMPC/DMPG bilayers. Two-dimensional correlation spectra show that membrane-bound HNP-1 exhibits a conformation similar to that of the water-soluble state, except for the turn connecting strands β2 and β3, whose side chains exhibit immobilization and conformational perturbation upon membrane binding. At high protein/lipid ratios, rapid (1)H spin diffusion from the lipid chains to the protein was observed, indicating that HNP-1 was well inserted into the hydrocarbon core of the bilayer. Arg Cζ-lipid (31)P distances indicate that only one of the four Arg residues forms tight hydrogen-bonded guanidinium-phosphate complexes. The protein is predominantly dimerized at high protein/lipid molar ratios, as shown by (19)F spin diffusion experiments. The presence of a small fraction of monomers and the shallower insertion at lower protein concentrations suggest that HNP-1 adopts concentration-dependent oligomerization and membrane-bound structure. These data strongly support a "dimer pore" topology of HNP-1 in which the polar top of the dimer lines an aqueous pore while the hydrophobic bottom faces the lipid chains. In this structure, R25 lies closest to the membrane surface among the four Arg residues. The pore does not have a high degree of lipid disorder, in contrast to the toroidal pores formed by protegrin-1, a two-stranded β-hairpin antimicrobial peptide. These results provide the first glimpse into the membrane-bound structure and mechanism of action of human α-defensins.     

Engineering of a linear inactive analog of human β‐defensin 4 to generate peptides with potent antimicrobial activity

Human β‐defensins (HBDs) are cationic antimicrobial peptides constrained by three disulfide bridges. They have diverse range of functions in the innate immune response. It is of interest to investigate whether linear analogs of defensins can be generated, which possess antimicrobial activity. In this study, we have designed linear peptides with potent antimicrobial activity from an inactive peptide spanning the N‐terminus of HBD4. Our results show that l ‐arginine to d ‐arginine substitution imparts considerable antimicrobial activity against both bacteria and Candida albicans. Increase in hydrophobicity by fatty acylation of the peptides with myristic acid further enhances their potency. In the presence of high concentrations of salt, antimicrobial activity of the myristoylated peptide with l ‐arginine is attenuated relatively to a lesser extent as compared with the linear active peptide with d ‐arginine. Substitution of cysteine with the hydrophobic helix‐promoting amino acid α‐aminoisobutyric acid favors candidacidal activity but not antibacterial activity. The mechanism of killing by d ‐arginine substituted unacylated analog involves transient interaction with the bacterial membrane followed by translocation into the cytoplasm without membrane permeabilization. Accumulation of peptides in the cytoplasm can affect various cellular processes that lead to cell death. However, the peptide causes membrane permeabilization in case of C. albicans. Myristoylation results in greater interaction of the peptide chain with the microbial cell surface and causes membrane permeabilization. Results described in the study demonstrate that it is possible to generate highly active linear analogs of defensins by selective introduction of d ‐amino acids and fatty acids, which could be attractive candidates for development as therapeutic agents. Copyright © 2015 European Peptide Society and John Wiley & Sons, Ltd.     

Proteinase 3 carries small unusual carbohydrates and associates with αlpha-defensins

The neutrophil granulocyte is an important first line of defense against intruding pathogens and it contains a range of granules armed with antibacterial peptides and proteins. Proteinase 3 (PR3) is one among several serine proteases of the azurophilic granules in neutrophil granulocytes. Here, we characterize the glycosylation of PR3 and its association with antimicrobial human neutrophil peptides (HNPs, α-defensins) and the effect of these on the mechanism of inhibition of the major plasma inhibitor of PR3, α1-antitrypsin. The glycosylation of purified, mature PR3 showed some heterogeneity with carbohydrates at Asn 102 and 147 carrying unusual small moieties indicating heavy processing. Mass spectrometric analysis and immuno blotting revealed strong association of highly purified PR3 with α-defensins and oligomers hereof. Irreversible inhibition of PR3 by α1-antitrypsin did not affect its association with defensins. Other proteins from neutrophil granules were also found to be associated with defensins, whereas purified plasma proteins did not carry defensins. These results point to a role of defensins in controlling and targeting the activity of neutrophil granule proteins.     

Analysis and separation of residues important for the chemoattractant and antimicrobial activities of beta-defensin 3

beta-Defensins are important in mammalian immunity displaying both antimicrobial and chemoattractant activities. Three canonical disulfide intramolecular bonds are believed to be dispensable for antimicrobial activity but essential for chemoattractant ability. However, here we show that HBD3 (human beta-defensin 3) alkylated with iodoactemide and devoid of any disulfide bonds is still a potent chemoattractant. Furthermore, when the canonical six cysteine residues are replaced with alanine, the peptide is no longer active as a chemoattractant. These findings are replicated by the murine ortholog Defb14. We restore the chemoattractant activity of Defb14 and HBD3 by introduction of a single cysteine in the fifth position (Cys V) of the beta-defensin six cysteine motif. In contrast, a peptide with a single cysteine at the first position (Cys I) is inactive. Moreover, a range of overlapping linear fragments of Defb14 do not act as chemoattractants, suggesting that the chemotactic activity of this peptide is not dependent solely on an epitope surrounding Cys V. Full-length peptides either with alkylated cysteine residues or with cysteine residues replaced with alanine are still strongly antimicrobial. Defb14 peptide fragments were also tested for antimicrobial activity, and peptides derived from the N-terminal region display potent antimicrobial activity. Thus, the chemoattractant and antimicrobial activities of beta-defensins can be separated, and both of these functions are independent of intramolecular disulfide bonds. These findings are important for further understanding of the mechanism of action of defensins and for therapeutic design.     

NMR studies of defensin antimicrobial peptides. 1. Resonance assignment and secondary structure determination of rabbit NP-2 and human HNP-1.

Two-dimensional nuclear magnetic resonance spectroscopy has been used to make resonance assignments of the proton spectra of two defensin antimicrobial peptides, human neutrophil peptide HNP-1 and rabbit neutrophil peptide NP-2. The secondary structures of these peptides were determined from analysis of the proton-proton NOEs and from the positions of slowly exchanging amide protons. Both peptides contain a long stretch of a double-stranded antiparallel beta-sheet in a hairpin conformation that contains a beta-bulge, a short region of triple-stranded beta-sheet, and several tight turns. The NMR results clearly show that HNP-1 forms a dimer or higher order aggregate in solution and that Pro8 exists as a cis peptide bond. The NMR data on these peptides are compared with NMR data for a homologous peptide NP-5 [Bach, A. C., Selsted, M. E., & Pardi, A. (1987) Biochemistry 26, 4389-4397]. Analysis of the conformation-dependent proton chemical shifts shows that it is not possible to confidently judge the structural similarity of the three defensins from chemical shift data alone. However, comparison of the 3JHN alpha coupling constants in NP-2 and NP-5 indicates that the backbone conformations for these peptides are very similar. A more detailed comparison of the solution conformations of the defensins peptides is made in the following paper in this issue where the NMR data are used as input for distance geometry and molecular dynamics calculations to determine the three-dimensional structures of HNP-1 and NP-2.     

Antimicrobial activity of human β-defensin 4 analogs: Insights into the role of disulfide linkages in modulating activity

Human β-defensins (HBDs) are cationic antimicrobial peptides that are components of the innate immune system. They are characterized by three disulfide bridges. However, the number of cationic residues as well as the presence of lysine and arginine residues vary. In HBD4, the cationic residues occur predominantly in the N-terminal segment, unlike in HBD1–3. We have examined the antimicrobial activity of peptides spanning the N- and C-terminal segments of HBD4. We have introduced one, two and three disulfide bridges in the peptides corresponding to the N-terminal segments. Peptides corresponding to the N-terminal segment had identical sequences and variation was only in the number and spacing of cysteines and disulfide bridges. Antimicrobial activity to varying extents was observed for all the peptides. When two disulfide bridges were present, decrease in antimicrobial potency as well as sensitivity of activity to salt was observed. Enhanced antimicrobial activity was observed when three disulfide bridges were present. The antimicrobial potency was similar to HBD4 except against Escherichia coli and was attenuated in the presence of salt. While the presence of three disulfide bridges did not constrain the peptide to a rigid β-sheet, the activity was considerably more as compared to the peptides with one or two disulfide bridges. The peptides enter bacterial and fungal cells rapidly without membrane permeabilization and appear to exert their activity inside the cells rather than at the membrane.     

The Neisseria meningitidis ADP-Ribosyltransferase NarE Enters Human Epithelial Cells and Disrupts Epithelial Monolayer Integrity

Many pathogenic bacteria utilize ADP-ribosylating toxins to modify and impair essential functions of eukaryotic cells. It has been previously reported that Neisseria meningitidis possesses an ADP-ribosyltransferase enzyme, NarE, retaining the capacity to hydrolyse NAD and to transfer ADP-ribose moiety to arginine residues in target acceptor proteins. Here we show that upon internalization into human epithelial cells, NarE gains access to the cytoplasm and, through its ADP-ribosylating activity, targets host cell proteins. Notably, we observed that these events trigger the disruption of the epithelial monolayer integrity and the activation of the apoptotic pathway. Overall, our findings provide, for the first time, evidence for a biological activity of NarE on host cells, suggesting its possible involvement in Neisseria pathogenesis.