Cathelicidins: a novel protein family with a common proregion and a variable C-terminal antimicrobial domain

Improved activity of a synthetic indolicidin analog

A novel cationic peptide, CP-11, based on the structure of the bovine neutrophil peptide indolicidin, was designed to increase the number of positively charged residues, maintain the short length (13 amino acids), and enhance the amphipathicity relative to those of indolicidin. CP-11, and especially its carboxymethylated derivative, CP-11C, demonstrated improved activity against gram-negative bacteria and Candida albicans, while it maintained the activity of indolicidin against staphylococci and demonstrated a reduced ability to lyse erythrocytes. In Escherichia coli, CP-11 was better able than indolicidin to permeabilize both the outer membrane, as indicated by the enhancement of uptake of 1-N-phenylnaphthylamine, and the inner membrane, as determined by the unmasking of cytoplasmic beta-galactosidase, providing an explanation for its improved activity.

Porcine polymorphonuclear leukocytes generate extracellular microbicidal activity by elastase-mediated activation of secreted proprotegrins

Antimicrobial peptides of several structural classes have been found in phagocytes and epithelial cells of many animals. The broadly microbicidal protegrins (PG1, -2, and -3) were originally isolated as 16 to 18-amino-acid peptides from pig neutrophil lysates, but the corresponding cDNA sequences encoded much larger precursors that belonged to the cathelicidin family of antimicrobial peptides. We explored the storage, secretion, and microbicidal activation of protegrins in porcine neutrophils and in a model system consisting of recombinant proprotegrin 3 (pPG3) and various serine proteases and their inhibitors. Protegrins were stored in neutrophils as inactive proforms that were cleaved by neutrophil elastase to mature protegrins during the preparation of granule lysate and during phorbol myristate acetate-stimulated granule secretion from intact neutrophils. Recombinant pPG3 was efficiently cleaved by trace amounts of human neutrophil elastase or equivalent amounts of elastase activity from porcine neutrophils, but pPG3 was relatively resistant to porcine pancreatic elastase or human neutrophil cathepsin G. The recombinant pPG3 and neutrophil proprotegrins lacked microbicidal activity, but the mature protegrins generated in the elastase-mediated cleavage reaction were as active against Listeria monocytogenes as the chemically synthesized protegrin. The secretion and elastase-mediated activation of proprotegrins accounted for much of the stable microbicidal activity of porcine neutrophil secretions against L. monocytogenes. Secreted proprotegrins and trace amounts of elastase constitute a binary microbicidal system that is likely to contribute to the antimicrobial activity of porcine inflammatory fluids.

p15s (15-kD Antimicrobial Proteins) Are Stored in the Secondary Granules of Rabbit Granulocytes: Implications for Antibacterial Synergy With the Bactericidal/Permeability-Increasing Protein in Inflammatory Fluids

The bactericidal potency toward complement-resistant Escherichia coli of bactericidal/permeability-increasing protein (BPI) released from polymorphonuclear leukocytes (PMNs) in glycogen-induced inflammatory peritoneal exudates of rabbits is dependent on synergy with extracellular p15s. This synergy depends on the high molar ratio of p15s to BPI in the extracellular fluid (∼50:1), which greatly exceeds the intracellular ratio (∼5:1). To explore the possible basis of the greater accumulation of p15s in inflammatory fluid, we examined the subcellular localization of BPI and p15 in PMNs. Immunogold electron microscopy confirmed the storage of BPI in primary granules and showed that p15s are stored in secondary granules. Reverse-transcription polymerase chain reaction of density-fractionated rabbit bone marrow cells verified that p15s are expressed later than BPI during myeloid differentiation. As the inflammatory response evolves, p15 mRNA appears earlier in blood and exudate cells than mRNA for BPI, consistent with release of progressively less mature precursors from bone marrow. Finally, Ca2+-ionophore–mediated exocytosis of p15s occurs more readily than release of BPI. We therefore propose that localization of a synergistic partner of BPI (p15s) in more readily released secondary granules allows the neutrophil to mobilize potent BPI-dependent antibacterial activity extracellularly without significant depletion of intracellular BPI stores.

Biological characterization of two novel cathelicidin-derived peptides and identification of structural requirements for their antimicrobial and cell lytic activities

Cathelicidins are a family of myeloid antimicrobial peptide precursors that have been identified in several mammalian species (Zanetti, M., Gennaro, R., and Romeo, D. (1995) FEBS Lett. 374, 1-5). Two novel bovine congeners have been deduced from cDNA. Their C-terminal sequences of 27 and 28 residues correspond to putative antimicrobial peptides with a cationic N-terminal region predicted to assume an amphipathic alpha-helical conformation followed by a hydrophobic C-terminal tail. Peptides corresponding to these sequences have been chemically synthesized and shown to exert a potent antimicrobial activity against Gram-negative and Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus, and fungi. Both peptides are also cytotoxic to human erythrocytes and neutrophils, although at higher than microbicidal concentrations. The target selectivity has been improved by synthesizing truncated analogues, comprising only the 18 N-terminal residues, which show a great reduction in cytotoxic, but not in antimicrobial activity. The involvement of the C-terminal hydrophobic tail in the cytotoxic activity has been further demonstrated by inducing a major loss of activity in an analogue after replacing highly hydrophobic residues with more hydrophilic ones.

Purification and Characterization of a Proline-Rich Antibacterial Peptide, with Sequence Similarity to Bactenecin-7, from the Haemocytes of the Shore Crab, Carcinus Maenas

Antibacterial peptides are important for non-specific host defence in many animals. They have been extensively characterized from mammals, amphibians, insects and chelicerates but have not so far been found in crustaceans. Here we report the presence of several constitutive antibacterial proteins, active against both gram-positive and gram-negative bacteria, in the haemocytes of the shore crab, Carcinus maenas. These proteins have molecular masses of > 70 kDa, approximately 45 kDa, approximately 14 kDa and 6.5 kDa. The 6.5 kDa peptide has been purified to homogeneity by Sep Pak C18 extraction, gel filtration and reverse-phase HPLC. Partial N-terminal sequence analysis further shows that it is proline rich and shares more than 60% identity in a 28-amino-acid overlap with the mature form of bactenecin 7, an antimicrobial peptide from bovine neutrophils which belongs to the cathelicidin family of mammalian peptide antibiotics.

Purification and structural characterization of bovine cathelicidins, precursors of antimicrobial peptides

Cathelicidins are a novel family of antimicrobial peptide precursors from mammalian myeloid cells. They are characterized by a conserved N-terminal region while the C-terminal antimicrobial domain can vary considerably in both primary sequence and length. Four cathelicidins, proBac5, proBac7, prododecapeptide and proBMAP-28, have been concurrently purified from bovine neutrophils, using simple and rapid methodologies. The correlation of ES-MS data from the purified proteins with their cDNA-deduced sequences has revealed several common features of their primary sequence, such as the presence of N-terminal 5-oxoproline (pyroglutamate) residues and two disulfide bridges in a 1-2, 3-4 arrangement. The N-terminal domains of the cathelicidins present one or two Asp-Pro bonds, which are particularly acid-labile in proBac5 and proBac7, but stable in prododecapeptide. This suggests that the spatial organization around these bonds may vary in different cathelicidins, and favour hydrolysis in some cases. An unexpected feature of the prododecapeptide is that it exists as dimers formed by three possible combinations of its two isoforms. The isolation of a truncated, monomeric form of this protein, lacking the cysteine-containing antimicrobial dodecapeptide, indicates that dimerization occurs via disulfide bridge formation at the level of the C-terminal domain and that the dodecapeptide is likely released as a dimer from its precursor. Sequence-based secondary structure predictions and CD results indicate for cathelicidins a 30-50% content of extended conformation and <20% content of alpha-helical conformation, with the alpha-helical segment placed near the N-terminus. Finally, similarity searching and topology-based structure prediction underline a significant sequential and structural similarity between the conserved N-terminal domain of cathelicidins and cystatin-like domains, placing this family within the cystatin superfamily. When assayed against cathepsin L, unlike the potent cystatin inhibitors, three of the four cathelicidins show only a poor inhibitory activity (Ki = 0.6-3 microM).

Antibiotic proteins of polymorphonuclear leukocytes

The polymorphonuclear leukocyte (PMN) plays an essential role in the innate defense of the mammalian host against bacterial invaders. Responding chemotactically, the PMN delivers a complex antibiotic arsenal to sites of infection. Among these cytotoxic systems is an array of antimicrobial proteins and peptides that the PMN directs at microorganisms both before (i.e. extracellularly) and after sequestration into a phagocytic vacuole. In addition to their microbicidal capacity, several of these proteins bind to and neutralize the endotoxic activity of Gram-negative bacterial lipopolysaccharides (LPS). In this review the principle features of these antibiotic proteins are briefly summarized with emphasis on their possible actions in biological settings. In many instances, additional functions independent of cytotoxicity have been described raising the possibility that some of these proteins subserve multiple roles in inflammation.