Clavanins, alpha-helical antimicrobial peptides from tunicate hemocytes

Hemocytes from the invertebrate Styela clava, a solitary tunicate, contained a family of four alpha-helical antimicrobial peptides that were purified, sequenced and named clavanins A, B, C and D. Each clavanin contained 23 amino acid residues and was C-terminally amidated. The tunicate peptides resembled magainins in size, primary sequence and antibacterial activity. Synthetic clavanin A was prepared and displayed comparable antimicrobial activity to magainins and cecropins. The presence of alpha-helical antimicrobial peptides in the hemocytes of a urochordate suggests that such peptides are primeval effectors of innate immunity in the vertebrate lineage.

Antimicrobial peptides of leukocytes

Leukocytes use an array of antimicrobial peptides and proteins to help them destroy invading microorganisms. These endogenous antibiotic molecules are remarkable for their structural variety, rapid evolutionary divergence, intraspecies variation, and complex yet subtle interactions with their targets. This arsenal has been studied most extensively in neutrophils, where its members include lactoferrin, secretory phospholipase A2, lysozyme, and the cathelicidins in the secretory granule compartment; defensins, bactericidal permeability inducing protein, serprocidins, and (again) lysozyme in the azurophil granule compartment; and calprotectin in the cytosolic compartment. That antimicrobial peptides and proteins are not limited to neutrophils and other phagocytes was made clear by the recent discovery of a microbicidal protein, NK lysin, in porcine natural killer cells and cytotoxic T lymphocytes. The structural homology of NK-lysin to amebapore, an antimicrobial cytolytic peptide of the parasitic protozoan Entamoeba histolytica, provides remarkable support for a long-suspected evolutionary connection between the leukocytes of higher animals and their unicellular, protozoan ancestors.

Secretion of type II phospholipase A2 and cryptdin by rat small intestinal Paneth cells

We examined the secretion of antimicrobial proteins and peptides into surgically isolated and continuously perfused segments of rat small intestine. Up to nine discrete antimicrobial molecules appeared in the intestinal perfusates following intravenous administration of bethanechol, a cholinergic agonist, or intralumenal instillation of lipopolysaccharide (LPS). Among them were three markers of Paneth cell secretion: lysozyme; type II (secretory) phospholipase A2; and at least one intestinal defensin, RIP-3, that appeared to be an alternatively processed variant of the rat neutrophil defensin RatNP-3. Both bethanechol- and LPS-stimulated intestinal lumenal perfusates (washings) contained molecules that killed Escherichia coli, Salmonella typhimurium, and Listeria monocytogenes in vitro. These molecules were more active against the avirulent S. typhimurium strain 7953S (phoP) than against its virulent parent, S. typhimurium 14028S. These data demonstrate that small intestinal Paneth cells secrete antimicrobial peptides in vivo, that this secretion is regulated by the autonomic (parasympathetic) cholinergic nervous system, and that the release of antimicrobial molecules can be triggered by the presence of bacterial LPS in the intestinal lumen.

Widespread expression of beta-defensin hBD-1 in human secretory glands and epithelial cells

We compared the expression of human alpha- and beta-defensins by various human tissues. mRNA for alpha-defensins HNP1-3, abundant in bone marrow, was detected in peripheral blood leukocytes, spleen and thymus by RT-PCR, which revealed alpha-defensins HD5 and HD6 only in the small intestine. In contrast, the pancreas and kidney expressed high levels of hBD-1 and lower levels of this beta-defensin were found in many organs by RT-PCR (salivary gland > trachea > prostate and placenta > thymus, testis, small intestine). hBD-1 mRNA was produced constitutively by cultured normal human epithelial cells derived from the trachea, bronchi, small airways and the mammary gland. These largely non-overlapping tissue distributions of human alpha- and beta-defensins suggest that hBD-1 may be positioned to defend epithelial cells and mucosae from infection, whereas expression of HNP1-3 in neutrophils and HD5 and HD6 in Paneth cells allows these alpha-defensins to participate in systemic and small intestinal host defenses, respectively.

Protegrins: structural requirements for inactivating elementary bodies of Chlamydia trachomatis

We tested 20 protegrins against Chlamydia trachomatis serovar L2 (L2/434/Bu). Five of the protegrins had native structures; the others included nonamidated, enantiomeric, and truncated variants and peptides with <2 disulfide bonds. Antichlamydial activity resided principally in residues 5 to 15 of native protegrin PG-1, and optimal activity required both intramolecular disulfide bonds.

Endogenous vertebrate antibiotics. Defensins, protegrins, and other cysteine-rich antimicrobial peptides

Although newly recognized, endogenous cystine-stabilized beta-sheet antimicrobial peptides have ancient origins. These peptides can arm circulating phagocytes and cells of the gastrointestinal, respiratory, and genitourinary tracts to resist invasion by bacteria, mycobacteria, fungi, and enveloped viruses. Defensins and protegrin-like peptides are likely to play a considerable role in innate immunity and may provide molecular templates that can be used to generate novel antibiotics for topical and systemic use.

Intramolecular disulfide bonds enhance the antimicrobial and lytic activities of protegrins at physiological sodium chloride concentrations

Protegrins are 2-kDa antimicrobial peptides that contain 16-18 amino acid residues and two intramolecular disulfide bonds. We studied the contribution of these disulfide bonds to the bactericidal activity of protegrins in physiological concentrations of NaCl by comparing protegrin PG-1 with variants that lacked one or both cysteine disulfides. Whereas the bactericidal and liposome-lytic properties of protegrin PG-1 were enhanced by adding 100 mM NaCl to the phosphate-buffered medium, NaCl addition strongly inhibited the effects of its linearized, disulfide-free variant, [A6, A8, A13, A15]protegrin-1. Whereas protegrin PG-1 manifested beta-sheet structure by CD (circular dichroism) and ATR-FTIR (attenuated-total-reflectance-Fourier-transform-infrared) spectroscopy in buffer or membrane-mimetic environments, [A6, A8, A13, A15]protegrin-1 manifested disordered structure in phosphate buffer and alpha-helical characteristics in membrane-mimetic environments. Both single-disulfide protegrin variants, [A8, A13]protegrin-1 and [A6, A15]protegrin-1, assumed beta-sheet conformations with liposomes that simulated bacterial membranes, and both retained substantial bactericidal activity when 100 mM NaCl was present. These findings demonstrate that the intramolecular disulfide bonds of protegrins are required for their antiparallel beta-sheet conformation in membrane-mimetic environments and for their potent antimicrobial activity in media containing NaCl concentrations comparable to those found in serum and extracellular fluids.

Solution structure of protegrin-1, a broad-spectrum antimicrobial peptide from porcine leukocytes

BACKGROUND

The protegrins are a family of arginine- and cysteine-rich cationic peptides found in porcine leukocytes that exhibit a broad range of antimicrobial and antiviral activities. They are composed of 16-18 amino-acid residues including four cysteines, which form two disulfide linkages. To begin to understand the mechanism of action of these peptides, we set out to determine the structure of protegrin-1 (PG-1).

RESULTS

We used two-dimensional homonuclear nuclear magnetic resonance spectroscopy to study the conformation of both natural and synthetic PG-1 under several conditions. A refined three-dimensional structure of synthetic PG-1 is presented.

CONCLUSIONS

Both synthetic and natural protegrin-1 form a well-defined structure in solution composed primarily of a two-stranded antiparallel beta sheet, with strands connected by a beta turn. The structure of PG-1 suggests ways in which the peptide may interact with itself or other molecules to form the membrane pores and the large membrane-associated assemblages observed in protegrin-treated, gram-negative bacteria.

Susceptibility of Neisseria gonorrhoeae to protegrins

We developed a sensitive and quantitative radial diffusion method to ascertain the susceptibility of six strains of Neisseria gonorrhoeae to antimicrobial peptides derived from mammalian leukocytes. The test organisms included the well-characterized serum-resistant FA19 and serum-sensitive F62 strains plus four antibiotic-resistant clinical isolates. Although each N. gonorrhoeae strain was resistant to human neutrophil defensins, all six were exquisitely sensitive to protegrins, a family of small beta-sheet antimicrobial peptides recently identified in porcine leukocytes. Protegrin-treated N. gonorrhoeae became vacuolated and had striking membrane changes when viewed by transmission and scanning electron microscopy. Because low concentrations of protegrins can also inactivate Chlamydia trachomatis and human immunodeficiency virus, they show promise for development as topical agents to avert sexually transmitted diseases.

Susceptibility of Chlamydia trachomatis to protegrins and defensins

We compared the susceptibilities of Chlamydia trachomatis elementary bodies (EBs) to human defensin HNP-2 and porcine protegrin PG-1, cysteine-rich beta-sheet antimicrobial peptides produced by mammalian leukocytes. Although both peptides protected McCoy cell monolayers from infection by chlamydial EBs, protegrins were especially potent. Protegrin-mediated inactivation of chlamydiae occurred rapidly, was relatively independent of the presence of serum, and was effective against serovars L2, D, and H. Protegrin-treated EBs showed striking morphological changes, with obvious damage to their limiting membranes and loss of their cytoplasmic contents and nucleoid. Their effectiveness against chlamydial EBs and other sexually transmitted pathogens combined with their relative lack of cytotoxicity suggests that protegrins and related molecules could serve as prototypes for topical agents to prevent sexually transmitted chlamydial infection.

In vitro activity of the antimicrobial peptides human and rabbit defensins and porcine leukocyte protegrin against Mycobacterium tuberculosis

Three independent assay methods were used to investigate the activities of antimicrobial peptides (human and rabbit defensins and protegrin from porcine leukocytes) against Mycobacterium tuberculosis in vitro. M. tuberculosis H37Ra was cultured in the presence of human neutrophil peptide 1, synthetic rabbit neutrophil peptide 1, or porcine protegrin 1 at 37 degrees C for 6 to 48 h, and antimycobacterial activity was measured by CFU assay. These peptides at a concentration of 50 microg/ml showed significant antibacterial effects on M. tuberculosis after 24 and 48 h of incubation (85.9 to 97.5% at 24 h and 91.6 to 99.4% at 48 h). A radiometric method and a radial diffusion assay confirmed these observations. Antibacterial activity against M. tuberculosis was independent of calcium (1.0 mM) or magnesium (1.0 mM) and not inhibited by sodium chloride (100 mM). The optimal pH for antibacterial activity against M. tuberculosis was greater than 4.0. Three clinical isolates of M. tuberculosis were also studied, and these peptides showed 86.3 to 99.0% reduction in CFU of these organisms. Morphological studies using scanning electron microscopy showed that defensins caused lesions on the surface of H37Ra. These observations suggest that antimicrobial peptides such as defensins and protegrins may represent an important component of the host defense mechanism against M. tuberculosis and offer a potential new approach to therapy.

Structures of genes for two cathelin-associated antimicrobial peptides: prophenin-2 and PR-39

We characterized genes for prophenin (PF)-2 and PR-39, two cathelin-associated antimicrobial peptides found in porcine leukocytes. Both contained 4 exons and 3 introns and were compact, contiguous and highly homologous. Exons I-III encoded most of their cathelin domains. Exon IV specified the final few cathelin residues, including its conserved C-terminal valine, followed by the mature PR-39 peptide or a PF-2 precursor. The highly conserved 5' flanking sequences of this gene family contained NF-kappa B, IL-6, GM-CSF and NF-1 binding motifs and the introns were unusually conserved. These data suggest that the panoply of porcine cathelin-associated antimicrobial peptides arose relatively recently via gene reduplications and exon shuffling, and that in vivo expression of cathelin-associated antimicrobial peptides may respond to mediators generated early during infection.

Defensins and other endogenous peptide antibiotics of vertebrates

Gene-encoded peptide antibiotics are ubiquitous components of host defenses in mammals, birds, amphibia, insects, and plants. Their de novo synthesis or release from storage sites can be induced rapidly, which makes them particularly important in the initial phases of resistance to microbial invasion. The endogenous antimicrobial peptides of animals are products of single genes and are synthesized as preproproteins. Multistep processing yields the mature peptide, which generally acts by inducing microbial membrane permeabilization. Several families of antimicrobial peptides have been identified that differ with respect to the presence of disulfide linkages, amino acid composition, structural conformation, and spectrum of activity. The arginine-rich three disulfide-containing beta-sheet defensins are remarkably abundant and widely distributed in animals and plants. The antibiotic peptides of higher eukaryotes merit further study for their role in natural immunity and their potential as novel therapeutic compounds.

The structure of porcine protegrin genes

We cloned the genes of three protegrins, a family of cathelin-associated antimicrobial peptides originally isolated from porcine leukocytes. Each gene comprised 4 exons and 3 introns, wherein Exon I encoded the signal sequence and the first 37 amino acids of cathelin, Exons II and III contained 36 and 24 additional cathelin residues and Exon IV contained the final two cathelin residues followed by the protegrin sequence. This quadripartite gene structure helps explain how structurally diverse antimicrobial peptides can be expressed on common, cathelin-containing precursors. Southern blot probed with an oligonucleotide specific for protegrin genes suggested that several identical or nearly identical protegrin genes were densely clustered in the pig chromosome.

Antibacterial proteins of granulocytes differ in interaction with endotoxin. Comparison of bactericidal/permeability-increasing protein, p15s, and defensins

Bactericidal/permeability-increasing protein (BPI), antibacterial 15-kDa protein isoforms (p15s), and defensins (neutrophil peptides or NPs) are granule-associated antibacterial proteins of polymorphonuclear leukocytes (PMN) that have both direct and synergistic growth inhibitory activity against Gram-negative bacteria. In this study, we have compared in vitro the abilities of these antibacterial proteins, alone and in combination, to inhibit the endotoxic activity of isolated LPS and whole bacteria. All three proteins blocked endotoxin activity in: 1) the Limulus amoebocyte lysate assay, 2) priming of PMN for enhanced arachidonate release, and 3) stimulating leukocyte oxidase activity in 1% blood. However, the proteins differ markedly in both relative potency (BPI &gt; p15s = NP1) in the presence of the plasma LPS-binding protein and in the range of LPS chemotypes that can be inhibited. BPI potently neutralizes LPS of any chemotype, but p15s and defensins are less active against long-chain (S-type) LPS. In whole blood ex vivo, the p15s and NP1 are approximately 1000-fold less potent than BPI, but at subinhibitory doses act in synergy with BPI to inhibit the TNF-inducing activity of a serum-resistant encapsulated strain of Escherichia coli (K1/r). The anti-endotoxic effects of p15 and NP1 against E. coli K1/r in whole blood appear secondary to growth arrest, because, in marked contrast to BPI, they are not evident against nonviable bacteria (pretreated with antibiotic) nor isolated LPS. Thus, BPI stands out for its ability to inhibit isolated or bacterial LPS under physiologic conditions. However, p15s and defensins may also contribute to suppression of endotoxic signaling by Gram-negative bacteria via synergistic (with BPI) growth inhibition upon extracellular release of these proteins from PMN during inflammation.

Antibacterial proteins of granulocytes differ in interaction with endotoxin. Comparison of bactericidal/permeability-increasing protein, p15s, and defensins

Bactericidal/permeability-increasing protein (BPI), antibacterial 15-kDa protein isoforms (p15s), and defensins (neutrophil peptides or NPs) are granule-associated antibacterial proteins of polymorphonuclear leukocytes (PMN) that have both direct and synergistic growth inhibitory activity against Gram-negative bacteria. In this study, we have compared in vitro the abilities of these antibacterial proteins, alone and in combination, to inhibit the endotoxic activity of isolated LPS and whole bacteria. All three proteins blocked endotoxin activity in: 1) the Limulus amoebocyte lysate assay, 2) priming of PMN for enhanced arachidonate release, and 3) stimulating leukocyte oxidase activity in 1% blood. However, the proteins differ markedly in both relative potency (BPI >> p15s = NP1) in the presence of the plasma LPS-binding protein and in the range of LPS chemotypes that can be inhibited. BPI potently neutralizes LPS of any chemotype, but p15s and defensins are less active against long-chain (S-type) LPS. In whole blood ex vivo, the p15s and NP1 are approximately 1000-fold less potent than BPI, but at subinhibitory doses act in synergy with BPI to inhibit the TNF-inducing activity of a serum-resistant encapsulated strain of Escherichia coli (K1/r). The anti-endotoxic effects of p15 and NP1 against E. coli K1/r in whole blood appear secondary to growth arrest, because, in marked contrast to BPI, they are not evident against nonviable bacteria (pretreated with antibiotic) nor isolated LPS. Thus, BPI stands out for its ability to inhibit isolated or bacterial LPS under physiologic conditions. However, p15s and defensins may also contribute to suppression of endotoxic signaling by Gram-negative bacteria via synergistic (with BPI) growth inhibition upon extracellular release of these proteins from PMN during inflammation.

Determination of disulphide bridges in PG-2, an antimicrobial peptide from porcine leukocytes

We determined the cysteine connectivity of protegrin PG-2, a leukocyte-derived antimicrobial peptide, by performing sequential enzyme digestions with chymotrypsin and thermolysin, and monitoring each digest by direct liquid chromatography-electrospray mass spectrometric analysis. This approach resolved the disulphide pairing pattern unambiguously with only picomolar amounts of PG-2. The inferred cysteine connectivity was confirmed by traditional amino acid composition analyses using nanomolar amounts of the protegrin. The results suggest that protegrins will assume a tachyplesin-like, disulphide-stabilized anti-parallel beta-sheet configuration in solution.

Defensins

Defensins are a family of small cationic, antibiotic peptides that contain six cysteines in disulfide linkage. The peptides are abundant in phagocytes and small intestinal mucosa of humans and other mammals and in the hemolymph of insects. They contribute to host defense against microbes and may participate in tissue inflammation and endocrine regulation during infection. Bioengineered defensins are potentially useful as prophylactic and therapeutic agents in infections.

Prophenin-1, an exceptionally proline-rich antimicrobial peptide from porcine leukocytes

We purified and characterized an unusual antimicrobial peptide, prophenin-1 (PF-1), from porcine leukocytes. The peptide had a mass of 8,683 and contained 79 residues, including 42 (53.2%) prolines and 15 (19.0%) phenylalanines. Its N-terminal 60 residues consisted of three perfect and three nearly perfect repeats of a decamer, FPPPNFPGPR. Prophenin-1 was encoded on a cathelin-containing precursor and showed substantially more activity against E. coli, a Gram-negative bacterium, than against Listeria monocytogenes, a Gram-positive organism, in vitro.

Identification of CG-1, a natural peptide antibiotic derived from human neutrophil cathepsin G

Cathepsin G is a neutral serine protease of the granzyme B family which is found in human PMN, cells known to be important in the defense of the periodontium against periodontal bacteria. We propose that cathepsin G serves as a "pro-antibiotic" containing peptide domains which express selective antibiotic properties. In this study, we used HPLC to separate the low-molecular-weight peptides derived from the ultrafiltrate of a granule extract from unstimulated PMN. One of the peptides exhibited intense bactericidal activity as determined by radial diffusion overlay assay (against Escherichia coli ML-35P), an amino-terminal sequence "RVSSFLPWIR...", and a 3.1-kDa molecular mass determined by electrospray ionization-mass spectrometry. The sequence and mass are consistent with the C-terminus of cathepsin G deduced by cDNA analysis. These findings support the hypothesis that antibiotic peptides derived from cathepsin G occur naturally in human PMN. Since this is the first naturally occurring antibiotic peptide derived from cathepsin G, we designate it "CG-1".

Bactericidal properties of murine intestinal phospholipase A2

We purified a molecule from the murine small intestine that killed both Escherichia coli and Listeria monocytogenes, and identified it as intestinal phospholipase A2 (iPLA2) by NH2-terminal sequencing and enzymatic measurements. The ability of iPLA2 to kill. L. monocytogenes was greatly enhanced by 5 mM calcium, inhibited by EGTA and abolished after reduction and alkylation, suggesting that enzymatic activity was required for iPLA2-mediated bactericidal activity. A mouse-avirulent phoP mutant, S. typhimurium 7953S, was 3.5-fold more susceptible to iPLA2 than its isogenic virulent parent, S. typhimurium 14028S (estimated minimal bactericidal concentrations 12.7 +/- 0.5 micrograms/ml vs. 43.9 +/- 4.5 micrograms/ml P < 0.001). Overall, these findings identify iPLA2 as part of the antimicrobial arsenal that equips Paneth cells to protect the small intestinal crypts from microbial invasion. Because iPLA2 is identical to Type 2 phospholipase A2 molecules found in other sites, including spleen, platelets and inflammatory exudate cells, this enzyme may also contribute to antibacterial defenses elsewhere in the body.

Cryptdins: endogenous antibiotic peptides of small intestinal Paneth cells

We purified three peptides ("cryptdins") from the small intestines of mice, established their primary amino acid sequences and examined their antimicrobial activity. Their primary sequences revealed approximately 50% identity to a group of antimicrobial defensins that we had previously isolated from the granules of rat neutrophils. In addition to their ability to kill Gram-positive (L. monocytogenes) and Gram-negative bacteria (E. coli and S. typhimurium) in vitro, the peptides were much more active against an avirulent (phoP) S. typhimurium strain than against its isogenic, mouse-virulent progenitor. Overall, these data suggest that endogenous antimicrobial peptides produced by Paneth cells may protect small intestinal crypts, which are critical sites of epithelial cell renewal, from invasion by autochthonous flora or by perorally acquired potential pathogens, such as Listeria and Salmonella.

Defensins

Defensins are widely distributed and abundant 3-4 kDa antimicrobial peptides that are variable cationic and contain six disulfide-paired cysteines. Three structurally distinct peptide families have been identified: 'classical' defensins, beta-defensins and insect defensins. In many animal species, defensin genes are found in clusters with substantial sequence variability outside the core disulfide-linked cysteines. Defensin peptides have been found in the granules of phagocytes and intestinal Paneth cells, on epithelial surfaces of the intestine and the trachea, and in the hemolymph of insects. They are produced from larger precursors by stepwise, tissue-specific, proteolytic processing, a production resembling that of peptide hormones. Microbes in the phagocytic vacuoles of granulocytes and certain macrophages encounter high concentrations of defensins. Increased transcription of defensin genes and stimulus-dependent release of pre-synthesized defensin-containing cytoplasmic granules contribute to the local antimicrobial response.