A novel method for purifying recombinant human host defense cathelicidin LL-37 by utilizing its inherent property of aggregation

Oligomer Dynamics of LL-37 Truncated Fragments Probed by α-Hemolysin Pore and Molecular Simulations

Oligomerization of antimicrobial peptides (AMPs) is critical in their effects on pathogens. LL-37 and its truncated fragments are widely investigated regarding their structures, antimicrobial activities, and application, such as developing new antibiotics. Due to the small size and weak intermolecular interactions of LL-37 fragments, it is still elusive to establish the relationship between oligomeric states and antimicrobial activities. Here, an α-hemolysin nanopore, mass spectrometry (MS), and molecular dynamic (MD) simulations are used to characterize the oligomeric states of two LL-37 fragments. Nanopore studies provide evidence of trapping events related to the oligomer formation and provide further details on their stabilities, which are confirmed by MS and MD simulations. Furthermore, simulation results reveal the molecular basis of oligomer dynamics and states of LL-37 fragments. This work provides unique insights into the relationship between the oligomer dynamics of AMPs and their antimicrobial activities at the single-molecule level. The study demonstrates how integrating methods allows deciphering single molecule level understanding from nanopore sensing approaches.

Design, synthesis, and characterization of non-hemolytic antimicrobial peptides related to human cathelicidin LL-37

We designed and synthesised the N-terminally labeled cationic and hydrophobic peptides, i.e., FFKKSKEKIGKEFKKIVQKI (P1) and FRRSRERIGREFRRIVQRI (P2) related to the human cathelicidin LL-37 peptide. The integrity and molecular weight of the peptides were confirmed by mass spectrometry. The purity and homogeneity of peptides P1 and P2 were determined by comparing LCMS or analytical HPLC chromatograms. The circular dichroism spectroscopy reveals the conformational transitions upon interaction with membranes. Predictably, peptides P1 and P2 showed a random coil structure in the buffer and formed α-helix secondary structure in TFE and SDS micelles. This assessment was further confirmed by 2D NMR spectroscopic methods. The analytical HPLC binding assay measurements revealed that peptides P1 and P2 display preferential interactions with the anionic lipid bilayer (POPC:POPG) moderately than zwitterionic (POPC). The efficacies of the peptides were tested against Gram-positive and Gram-negative bacteria. It is imperative to note here that the arginine-rich P2 exerted higher activity against all the test organisms as compared with that shown by the lysine-rich peptide P1. To test the toxicity of these peptides, a hemolytic assay was performed. P1 and P2 showed very little to no toxicity for a hemolytic assay, which is significant for P1 and P2 to be used as potential therapeutic agents in practical applications. Both peptides P1 and P2 were non-hemolytic and appeared to be more promising as they demonstrated wide-spectrum antimicrobial activity.

The Small Metal-Binding Protein SmbP Simplifies the Recombinant Expression and Purification of the Antimicrobial Peptide LL-37

(1) Background: The cathelicidin peptide LL-37 is a prominent molecule with many biological activities, including antimicrobial. Due to its importance, here, we describe the production of LL-37 tagged with SmbP, a relatively new carrier protein that improves the production of recombinant proteins and peptides in Escherichia coli. We present an alternative method for the rapid expression, purification, and antimicrobial evaluation of LL-37, that involves only one purification step. (2) Methods: A DNA construct of SmbP_LL-37 was transformed into E. coli BL21(DE3); after overnight expression, the protein was purified directly from the cell lysate using immobilized metal-affinity chromatography. SmbP_LL-37 was treated with Enterokinase to obtain the free LL-37 peptide. The antimicrobial activity of both SmbP_LL-37 and free LL-37 was determined using the colony forming unit assay method. (3) Results: SmbP_LL-37 was observed in the soluble fraction of the cell lysate; after purification with IMAC, protein gel electrophoresis, and analysis by ImageJ, it showed 90% purity. A total of 3.6 mg of SmbP_LL-37 was produced from one liter of cell culture. SmbP_LL-37 and free LL-37 both showed inhibition activity against Staphylococcus aureus and Escherichia coli. (4) Conclusions: The SmbP fusion protein is a valuable tool for producing biologically-active LL-37 peptide. The production method described here should be of interest for the expression and purification of additional cationic peptides, since it cuts the purification time considerably prior to determination of antimicrobial activity.

Long-Lasting Stable Expression of Human LL-37 Antimicrobial Peptide in Transgenic Barley Plants

Antimicrobial peptides play a crucial role in the innate immune system of multicellular organisms. LL-37 is the only known member of the human cathelicidin family. As well as possessing antibacterial properties, it is actively involved in various physiological responses in eukaryotic cells. Accordingly, there is considerable interest in large-scale, low-cost, and microbial endotoxin-free production of LL-37 recombinant peptides for pharmaceutical applications. As a heterologous expression biofactory, we have previously obtained homologous barley (Hordeum vulgare L.) as an attractive vehicle for producing recombinant human LL-37 in the grain storage compartment, endosperm. The long-term stability of expression and inheritance of transgenes is necessary for the successful commercialization of recombinant proteins. Here, we report the stable inheritance and expression of the LL-37 gene in barley after six generations, including two consecutive seasons of experimental field cultivation. The transgenic plants showed normal growth and remained fertile. Based on the bacteria viability test, the produced peptide LL-37 retained high antibacterial activity.

Antimicrobial and Amyloidogenic Activity of Peptides. Can Antimicrobial Peptides Be Used against SARS-CoV-2?

At present, much attention is paid to the use of antimicrobial peptides (AMPs) of natural and artificial origin to combat pathogens. AMPs have several points that determine their biological activity. We analyzed the structural properties of AMPs, as well as described their mechanism of action and impact on pathogenic bacteria and viruses. Recently published data on the development of new AMP drugs based on a combination of molecular design and genetic engineering approaches are presented. In this article, we have focused on information on the amyloidogenic properties of AMP. This review examines AMP development strategies from the perspective of the current high prevalence of antibiotic-resistant bacteria, and the potential prospects and challenges of using AMPs against infection caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

Cooperative Function of LL-37 and HNP1 Protects Mammalian Cell Membranes from Lysis

LL-37, cleaved from human cathelicidin, and human neutrophil peptide-1 (HNP1) from the defensin family are antimicrobial peptides that are occasionally co-released from neutrophils, which synergistically kill bacteria. We report that this couple presents another type of cooperativity against host eukaryotic cells, in which they antagonistically minimize cytotoxicity by protecting membranes from lysis. Our results describe the potential of the LL-37/HNP1 cooperativity that switches from membrane-destructive to membrane-protective functions, depending on whether the target is an enemy or a host.

Design of Antimicrobial Peptides: Progress Made with Human Cathelicidin LL-37

The incorporation of the innate immune system into humans is essential for survival and health due to the rapid replication of invading microbes and the delayed action of the adaptive immune system. Antimicrobial peptides are important components of human innate immunity. Over 100 such peptides have been identified in various human tissues. Human cathelicidin LL-37 is best studied, and there has been a growing interest in designing new peptides based on LL-37. This chapter describes the alternative processing of the human cathelicidin precursor, protease digestion, and lab cutting of LL-37. Both a synthetic peptide library and structure-based design are utilized to identify the active regions. Although challenging, the determination of the 3D structure of LL-37 enabled the identification of the core antimicrobial region. The minimal region of LL-37 can be function-dependent. We discuss the design and potential applications of LL-37 into antibacterial, antibiofilm, antiviral, antifungal, immune modulating, and anticancer peptides. LL-37 has been engineered into 17BIPHE2, a stable, selective, and potent antimicrobial, antibiofilm, and anticancer peptide. Both 17BIPHE2 and SAAP-148 can eliminate the ESKAPE pathogens and show topical in vivo antibiofilm efficacy. Also discussed are other application strategies, including peptide formulation, antimicrobial implants, and peptide-inducing factors such as vitamin D and sunlight. Finally, we summarize what we learned from peptide design based on human LL-37.

Expression in Escherichia coli of novel recombinant hybrid antimicrobial peptide AL32-P113 with enhanced antimicrobial activity in vitro

Antibiotic-resistant pathogens have become a major public health problem worldwide. New discoveries and strategies as regards antibiotic drug development are urgently in need for curing infected patients. Antimicrobial peptides (AMPs) are short cationic peptides that play important roles in innate immune system with a broad spectrum of antimicrobial activity. Recently, hybrid AMPs have been reported to increase antimicrobial activity, stability, and in vivo half-life. In the present study, a gene encoding for AL32-P113 hybrid peptide consisting of two truncated active forms of human LL-37 and histatin-5 (Hst-5) was commercially constructed, cloned into pTXB-1 commercial plasmid, and expressed in E. coli BL21 (DE3). To increase the yield of target protein expression, IPTG concentration, time and temperature were optimized. The results indicate that AL32-P113-intein fusion protein with 33.7 kDa was expressed mostly in inclusion form and estimated to be 20% of the total protein. After chitin affinity purification, 5.7-kDa of AL32-P113 peptide was separated with an average concentration of 12.1 mg per litre of bacterial culture and over 86% purity. The minimum inhibitory concentration (MIC) was evaluated for antimicrobial activity determination of recombinant AL32-P113 compared to synthetic peptides, LL-37, Hst-5, and L31-P113. The results implied that both hybrid peptides exhibited potent antimicrobial activity against gram-negative bacteria and yeast cells whereas the L31-P113 peptide possessed approximately four times greater antimicrobial activity in gram-positive bacteria than parent LL-37. An increasing of undesired hemolysis of these hybrid peptides toward human red cells was also observed when red blood cell hemolytic assay was performed. Several factors including charge and secondary structure predicted by public software were utilized for explanation of the antimicrobial potency of both hybrid peptides. This study proved that hybrid peptides show broader and more potent antimicrobial ability against pathogens and they could be applied as a therapeutic approach for topical treatment of microbial infection in the future.

A QCM-D study of the concentration- and time-dependent interactions of human LL37 with model mammalian lipid bilayers

The human antimicrobial peptide LL37 is promising as an alternative to antibiotics due to its biophysical interactions with charged bacterial lipids. However, its clinical potential is limited due to its interactions with zwitterionic mammalian lipids leading to cytotoxicity. Mechanistic insight into the LL37 interactions with mammalian lipids may enable rational design of less toxic LL37-based therapeutics. To this end, we studied concentration- and time-dependent interactions of LL37 with zwitterionic model phosphatidylcholine (PC) bilayers with quartz crystal microbalance with dissipation (QCM-D). LL37 mass adsorption and PC bilayer viscoelasticity changes were monitored by measuring changes in frequency (Δf) and dissipation (ΔD), respectively. The Voigt-Kelvin viscoelastic model was applied to Δf and ΔD to study changes in bilayer thickness and density with LL37 concentration. At low concentrations (0.10-1.00 μM), LL37 adsorbed onto bilayers in a concentration-dependent manner. Further analyses of Δf, ΔD and thickness revealed that peptide saturation on the bilayers was a threshold for interactions observed above 2.00 μM, interactions that were rapid, multi-step, and reached equilibrium in a concentration- and time-dependent manner. Based on these data, we proposed a model of stable transmembrane pore formation at 2.00-10.0 μM, or transition from a primarily lipid to a primarily protein film with a transmembrane pore formation intermediate state at concentrations of LL37 > 10 μM. The concentration-dependent interactions between LL37 and PC bilayers correlated with the observed concentration-dependent biological activities of LL37 (antimicrobial, immunomodulatory and non-cytotoxic at 0.1-1.0 μM, hemolytic and some cytotoxicity at 2.0-13 μM and cytotoxic at >13 μM).

Yeast-Based Synthetic Biology Platform for Antimicrobial Peptide Production

Antibiotic resistance is one of the most challenging global health threats in our society. Antimicrobial peptides (AMPs) represent promising alternatives to conventional antibiotics for the treatment of drug-resistant infections. However, they are limited by their high manufacturing cost. Engineering living organisms represents a promising approach to produce such molecules in an inexpensive manner. Here, we genetically modified the yeast Pichia pastoris to produce the prototypical AMP apidaecin Ia using a fusion protein approach that leverages the beneficial properties ( e.g., stability) of human serum albumin. The peptide was successfully isolated from the fusion protein construct, purified, and demonstrated to have bioactivity against Escherichia coli. To demonstrate this approach as a manufacturing solution to AMPs, we scaled-up production in bioreactors to generate high AMP yields. We envision that this system could lead to improved AMP biomanufacturing platforms.

Aggregation and Its Influence on the Immunomodulatory Activity of Synthetic Innate Defense Regulator Peptides

There is increasing interest in developing cationic host defense peptides (HDPs) and their synthetic derivatives as antimicrobial, immunomodulatory, and anti-biofilm agents. These activities are often evaluated without considering biologically relevant concentrations of salts or serum; furthermore certain HDPs have been shown to aggregate in vitro. Here we examined the effect of aggregation on the immunomodulatory activity of a synthetic innate defense regulator peptide, 1018 (VRLIVAVRIWRR-NH₂). A variety of salts and solutes were screened to determine their influence on 1018 aggregation, revealing that this peptide "salts out" of solution in an anion-specific and concentration-dependent manner. Furthermore, the immunomodulatory activity of 1018 was found to be inhibited under aggregation-promoting conditions. A series of 1018 derivatives were synthesized with the goal of disrupting this self-assembly process. Indeed, some derivatives exhibited reduced aggregation while maintaining certain immunomodulatory functions, demonstrating that it is possible to engineer optimized synthetic HDPs to avoid unwanted peptide aggregation.

Green fluorescent protein as a scaffold for high efficiency production of functional bacteriotoxic proteins in Escherichia coli

The availability of simple, robust, and cost-effective methods for the large-scale production of bacteriotoxic peptides such as antimicrobial peptides (AMPs) is essential for basic and pharmaceutical research. However, the production of bacteriotoxic proteins has been difficult due to a high degree of toxicity in bacteria and proteolytic degradation. In this study, we inserted AMPs into the Green fluorescent protein (GFP) in a loop region and expressed them as insoluble proteins in high yield, circumventing the inherent toxicity of AMP production in Escherichia coli. The AMPs inserted were released by cyanogen bromide and purified by chromatography. We showed that highly potent AMPs such as Protegrin-1, PMAP-36, Buforin-2, and Bactridin-1 are produced in high yields and produced AMPs showed similar activities compared to chemically synthesized AMPs. We increased the yield more than two-fold by inserting three copies of Protegrin-1 in the GFP scaffold. The immunogold electron micrographs showed that the expressed Protegrin-1 in the GFP scaffold forms large and small size aggregates in the core region of the inclusion body and become entirely nonfunctional, therefore not influencing the proliferation of E. coli. Our novel method will be applicable for diverse bacteriotoxic peptides which can be exploited in biomedical and pharmaceutical researches.

Rational Design of a Carrier Protein for the Production of Recombinant Toxic Peptides in Escherichia coli

Commercial uses of bioactive peptides require low cost, effective methods for their production. We developed a new carrier protein for high yield production of recombinant peptides in Escherichia coli very well suited for the production of toxic peptides like antimicrobial peptides. GKY20, a short antimicrobial peptide derived from the C-terminus of human thrombin, was fused to the C-terminus of Onconase, a small ribonuclease (104 amino acids), which efficiently drove the peptide into inclusion bodies with very high expression levels (about 200-250 mg/L). After purification of the fusion protein by immobilized metal ion affinity chromatography, peptide was obtained by chemical cleavage in diluted acetic acid of an acid labile Asp-Pro sequence with more than 95% efficiency. To improve peptide purification, Onconase was mutated to eliminate all acid labile sequences thus reducing the release of unwanted peptides during the acid cleavage. Mutations were chosen to preserve the differential solubility of Onconase as function of pH, which allows its selective precipitation at neutral pH after the cleavage. The improved carrier allowed the production of 15-18 mg of recombinant peptide per liter of culture with 96-98% purity without the need of further chromatographic steps after the acid cleavage. The antimicrobial activity of the recombinant peptide, with an additional proline at the N-terminus, was tested on Gram-negative and Gram-positive strains and was found to be identical to that measured for synthetic GKY20. This finding suggests that N-terminal proline residue does not change the antimicrobial properties of recombinant (P)GKY20. The improved carrier, which does not contain cysteine and methionine residues, Asp-Pro and Asn-Gly sequences, is well suited for the production of peptides using any of the most popular chemical cleavage methods.

Unique features of human cathelicidin LL-37

Cathelicidins are antimicrobial peptides produced by humans and animals in response to various pathogenic microbes. This review intends to provide a brief overview of the expression, structure, properties and function of human cathelicidin LL-37 which may be a therapeutic agent against a variety of bacterial and viral diseases, cancers, and hard-to-heal wounds. Cathelicidins act as a primary defense against bacteria and other pathogens in the case of inflammation. They are able to kill bacteria and fungi, inhibit and destroy bacterial biofilms, and possess antiviral and antiparasitics properties. They can also play a role in angiogenesis, wound healing, and the regulation of apoptosis. The host defense peptide LL-37 has emerged as a novel modulator of tumor growth and metastasis in carcinogenesis of various types of cancers. LL-37 is an antimicrobial peptide able of inducing various effects. It acts as an anti- and pro- inflammatory factor. Cathelicidins are able to directly and selectively destroy membranes of various microbes and cancer cells, but they do not attack normal cells. The role of cathelicidins in cancer is double-sided. They play an important role in killing cancer cells and may provide a new possibility for the development of cancer therapeutics. However, they also can participate in carcinogenesis. Due to its activity spectrum LL-37 could be applied in pharmacotherapy. Cathelicidin peptides could serve as a template for the development of modern anti-microbial and anti-viral drugs. LL-37 is an excellent candidate to develop into therapeutics for infected wounds.

Development of an Analytical Method for the Rapid Quantitation of Peptides Used in Microbicide Formulations

Recently, a growing number of macromolecules such as peptides and proteins have been formulated into various microbicide formulations for the prevention of sexually transmitted infections. However, a fast and reliable high-throughput method for quantitating peptide/protein in polymer-based microbicide formulations is still lacking. As a result, we developed and validated a reversed-phase high-performance liquid chromatography method for the quantitation of gp120 fragment and LL-37 simultaneously in various microbicide gel formulations. This method was capable of detecting a limit of linearity (regression coefficient of 0.999) for gp120 fragment and LL-37 within a range of 0.625-80 and 1.25-80 µg mL^-1, respectively. The lower limit of quantification for gp120 fragment and LL-37 was 1.14 and 0.31 µg mL^-1, respectively. Method validation demonstrated acceptable intra- and inter-day RSD % (<5 %) and accuracy (95.67-100.5 %). Formulating both peptides into polymeric pharmaceutical gel formulations showed high extraction efficiency (in the range of 95.90 ± 3.03 to 111.45 ± 2.51 %). Using this method, we were able to separate and identify the forced degraded products from both peptides simultaneously without affecting the quantitation of both peptides in the polymeric dosage forms. Furthermore, this method was able to detect and separate degradants that were unable to be revealed using gel eletrophoresis.

Native oligomerization determines the mode of action and biological activities of human cathelicidin LL-37

LL-37 is a multifunctional component of innate immunity, with a membrane-directed antimicrobial activity and receptor-mediated pleiotropic effects on host cells. Sequence variations in its primate orthologues suggest that two types of functional features have evolved; human LL-37-like peptides form amphipathic helical structures and self-assemble under physiological conditions, whereas rhesus RL-37-like peptides only adopt this structure in the presence of bacterial membranes. The first type of peptide has a lower and more medium-sensitive antimicrobial activity than the second type, but an increased capacity to stimulate host cells. Oligomerization strongly affects the mode of interaction with biological membranes and, consequently, both cytotoxicity and receptor-mediated activities. In the present study we explored the effects of LL-37 self-association by using obligate disulfide-linked dimers with either parallel or antiparallel orientations. These had an increased propensity to form stacked helices in bulk solution and when in contact with either anionic or neutral model membranes. The antimicrobial activity against Gram-positive or Gram-negative bacteria, as well as the cytotoxic effects on host cells, strongly depended on the type of dimerization. To investigate the extent of native oligomerization we replaced Phe5 with the photoactive residue Bpa (p-benzoyl-L-phenylalanine), which, upon UV irradiation, enabled covalent cross-linking and allowed us to assess the extent of oligomerization in both physiological solution and in model membranes.

High-quality 3D structures shine light on antibacterial, anti-biofilm and antiviral activities of human cathelicidin LL-37 and its fragments

Host defense antimicrobial peptides are key components of human innate immunity that plays an indispensible role in human health. While there are multiple copies of cathelicidin genes in horses, cattle, pigs, and sheep, only one cathelicidin gene is found in humans. Interestingly, this single cathelicidin gene can be processed into different forms of antimicrobial peptides. LL-37, the most commonly studied form, is not only antimicrobial but also possesses other functional roles such as chemotaxis, apoptosis, wound healing, immune modulation, and cancer metastasis. This article reviews recent advances made in structural and biophysical studies of human LL-37 and its fragments, which serve as a basis to understand their antibacterial, anti-biofilm and antiviral activities. High-quality structures were made possible by using improved 2D NMR methods for peptide fragments and 3D NMR spectroscopy for intact LL-37. The two hydrophobic domains in the long amphipathic helix (residues 2-31) of LL-37 separated by a hydrophilic residue serine 9 explain its cooperative binding to bacterial lipopolysaccharides (LPS). Both aromatic rings (F5, F6, F17, and F27) and interfacial basic amino acids of LL-37 directly interact with anionic phosphatidylglycerols (PG). Although the peptide sequences reported in the literature vary slightly, there is a consensus that the central helix of LL-37 is essential for disrupting superbugs (e.g., MRSA), bacterial biofilms, and viruses such as human immunodeficiency virus 1 (HIV-1) and respiratory syncytial virus (RSV). In the central helix, the central arginine R23 is of particular importance in binding to bacterial membranes or DNA. Mapping the functional roles of the cationic amino acids of the major antimicrobial region of LL-37 provides a basis for designing antimicrobial peptides with desired properties. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.

Recombinant expression of novel protegrin-1 dimer and LL-37-linker-histatin-5 hybrid peptide mediated biotin carboxyl carrier protein fusion partner

Antimicrobial peptides (AMPs) hold great promise as potential therapeutic approach for curing of infectious diseases. Prokaryotic protein expression renders high scalability with an effective purification of several heterogeneous proteins. However, it might be inappropriate for recombinant AMPs expression thereby its antimicrobial activity against the host cells. Several fusion partners demonstrated antimicrobial activity neutralization of AMPs expression and purification in Escherichia coli. In order to improve the antimicrobial effect, several hybrid AMPs have been designed and developed. As expected to increase the antimicrobial activity, a dimeric form of porcine protegrin-1 (PG-1) and human LL-37-linker-histatin-5 (LL-37-linker-Hst-5) hybrid peptide were alternatively constructed in this study. Hydroxylamine hydrochloride and thrombin cleavage sites were designed for releasing of hybrid peptide and PG-1 dimer from biotin carboxyl carrier protein (BCCP) fusion partner. The full-length AMPs gene was connected down-stream of BCCP gene using the overlap extension-PCR, cloned into pET-28a vector and expressed in E. coli BL21(DE3)pLysS. After IPTG induction, approximately 20% of BCCP-AMPs was expressed as intracytoplasmic inclusion bodies with an expected molecular weight of 24.5kDa. The mean of purified and refolded BCCP-AMPs was 1.5mg/L with 76% purity. The presence of expressed protein was subsequently determined by Western blotting analysis. Finally, radial diffusion assay supported that these peptides displayed functional antimicrobial activity against E. coli and Staphylococcus aureus standard strains. Two novel AMPs established in this study would be potentially developed as extensive intervention for treating of infectious diseases.

Database-Guided Discovery of Potent Peptides to Combat HIV-1 or Superbugs

Antimicrobial peptides (AMPs), small host defense proteins, are indispensable for the protection of multicellular organisms such as plants and animals from infection. The number of AMPs discovered per year increased steadily since the 1980s. Over 2,000 natural AMPs from bacteria, protozoa, fungi, plants, and animals have been registered into the antimicrobial peptide database (APD). The majority of these AMPs (>86%) possess 11–50 amino acids with a net charge from 0 to +7 and hydrophobic percentages between 31–70%. This article summarizes peptide discovery on the basis of the APD. The major methods are the linguistic model, database screening, de novo design, and template-based design. Using these methods, we identified various potent peptides against human immunodeficiency virus type 1 (HIV-1) or methicillin-resistant Staphylococcus aureus (MRSA). While the stepwise designed anti-HIV peptide is disulfide-linked and rich in arginines, the ab initio designed anti-MRSA peptide is linear and rich in leucines. Thus, there are different requirements for antiviral and antibacterial peptides, which could kill pathogens via different molecular targets. The biased amino acid composition in the database-designed peptides, or natural peptides such as θ-defensins, requires the use of the improved two-dimensional NMR method for structural determination to avoid the publication of misleading structure and dynamics. In the case of human cathelicidin LL-37, structural determination requires 3D NMR techniques. The high-quality structure of LL-37 provides a solid basis for understanding its interactions with membranes of bacteria and other pathogens. In conclusion, the APD database is a comprehensive platform for storing, classifying, searching, predicting, and designing potent peptides against pathogenic bacteria, viruses, fungi, parasites, and cancer cells.

Human antimicrobial peptide LL-37 inhibits adhesion of Candida albicans by interacting with yeast cell-wall carbohydrates

Candida albicans is the major fungal pathogen of humans. Fungal adhesion to host cells is the first step of mucosal infiltration. Antimicrobial peptides play important roles in the initial mucosal defense against C. albicans infection. LL-37 is the only member of the human cathelicidin family of antimicrobial peptides and is commonly expressed in various tissues and cells, including epithelial cells of both the oral cavity and urogenital tract. We found that, at sufficiently low concentrations that do not kill the fungus, LL-37 was still able to reduce C. albicans infectivity by inhibiting C. albicans adhesion to plastic surfaces, oral epidermoid OECM-1 cells, and urinary bladders of female BALB/c mice. Moreover, LL-37-treated C. albicans floating cells that did not adhere to the underlying substratum aggregated as a consequence of LL-37 bound to the cell surfaces. According to the results of a competition assay, the inhibitory effects of LL-37 on cell adhesion and aggregation were mediated by its preferential binding to mannan, the main component of the C. albicans cell wall, and partially by its ability to bind chitin or glucan, which underlie the mannan layer. Therefore, targeting of cell-wall carbohydrates by LL-37 provides a new strategy to prevent C. albicans infection, and LL-37 is a useful, new tool to screen for other C. albicans components involved in adhesion.

Spotlight on Human LL-37, an Immunomodulatory Peptide with Promising Cell-Penetrating Properties

Cationic antimicrobial peptides are major components of innate immunity and help control the initial steps of the infectious process. They are expressed not only by immunocytes, but also by epithelial cells. They share an amphipathic secondary structure with a polar cationic site, which explains their tropism for prokaryote membranes and their hydrophobic site contributing to the destructuration of these membranes. LL-37 is the only cationic antimicrobial peptide derived from human cathelicidin. LL-37 can also cross the plasma membrane of eukaryotic cells, probably through special domains of this membrane called lipid rafts. This transfer could be beneficial in the context of vaccination: the activation of intracellular toll-like receptors by a complex formed between CpG oligonucleotides and LL-37 could conceivably play a major role in the building of a cellular immunity involving NK cells.

Antimicrobial and membrane disrupting activities of a peptide derived from the human cathelicidin antimicrobial peptide LL37

A 21-residue peptide segment, LL7-27 (RKSKEKIGKEFKRIVQRIKDF), corresponding to residues 7-27 of the only human cathelicidin antimicrobial peptide, LL37, is shown to exhibit potent activity against microbes (particularly Gram-positive bacteria) but not against erythrocytes. The structure, membrane orientation, and target membrane selectivity of LL7-27 are characterized by differential scanning calorimetry, fluorescence, circular dichroism, and NMR experiments. An anilinonaphthalene-8-sulfonic acid uptake assay reveals two distinct modes of Escherichia coli outer membrane perturbation elicited by LL37 and LL7-27. The circular dichroism results show that conformational transitions are mediated by lipid-specific interactions in the case of LL7-27, unlike LL37. It folds into an alpha-helical conformation upon binding to anionic (but not zwitterionic) vesicles, and also does not induce dye leakage from zwitterionic lipid vesicles. Differential scanning calorimetry thermograms show that LL7-27 is completely integrated with DMPC/DMPG (3:1) liposomes, but induces peptide-rich and peptide-poor domains in DMPC liposomes. (15)N NMR experiments on mechanically aligned lipid bilayers suggest that, like the full-length peptide LL37, the peptide LL7-27 is oriented close to the bilayer surface, indicating a carpet-type mechanism of action for the peptide. (31)P NMR spectra obtained from POPC/POPG (3:1) bilayers containing LL7-27 show substantial disruption of the lipid bilayer structure and agree with the peptide's ability to induce dye leakage from POPC/POPG (3:1) vesicles. Cholesterol is shown to suppress peptide-induced disorder in the lipid bilayer structure. These results explain the susceptibility of bacteria and the resistance of erythrocytes to LL7-27, and may have implications for the design of membrane-selective therapeutic agents.

The chemistry and biology of LL-37

LL-37 is a human host defence peptide that has a wide range of biological functions, including antimicrobial and immunomodulatory properties. This review summarises how molecular structure influences the balance between the immunomodulatory and antimicrobial functions of LL-37.

Carrier proteins for fusion expression of antimicrobial peptides in Escherichia coli

Antimicrobial peptides are an essential component of innate immunity and play an important role in host defence against microbial pathogens. They have received increasing attention recently as potential novel pharmaceutical agents. To meet the requirement for necessary basic science studies and clinical trials, large quantities of these peptides are needed. In general, isolation from natural sources and chemical synthesis are not cost-effective. The relatively low cost and easy scale-up of the recombinant approach renders it the most attractive means for large-scale production of antimicrobial peptides. Among the many systems available for protein expression, Escherichia coli remains the most widely used host. Antimicrobial peptides produced in E. coli are often expressed as fusion proteins, which effectively masks these peptides' potential lethal effect towards the bacterial host and protects the peptides from proteolytic degradation. Although some carriers confer peptide solubility, others promote the formation of inclusion bodies. The present minireview considers the most commonly used carrier proteins for fusion expression of antimicrobial peptides in E. coli. The favourable properties of SUMO (small ubiquitin-related modifier) as a novel fusion partner are also discussed.

Expression of antimicrobial peptide LH multimers in Escherichia coli C43(DE3)

The tandem repeats of LFB15(W4,10)-HP(4-16) (LH) gene were cloned into vector pET32a(+) for recombinant expression in Escherichia coli. The E. coli C43(DE3) was successfully used as the expression host to avoid the cell death during induction in E. coli BL21(DE3). Fusion LH dimer was expressed as inclusion body at a portion of 35% of total cell protein and could be well purified by Ni(2+)-chelating chromatography. The recombinant LH was released by the cleavage of 50% formic acid, and its yield reached 11.3 mg/l with purity of 95%. The MIC(50) of 3.6 and 1.9 microM of recombinant LH against E. coli CMCC 44102 and Bacillus subtilis ATCC 6633 were determined, respectively. The results demonstrated that expression of tandem LH gene in E. coli C43(DE3) and formic acid cleavage would provide a potent efficient platform for the production of interested peptides.

RAPD: a database of recombinantly-produced antimicrobial peptides

We have developed the Recombinantly-produced Antimicrobial Peptides Database (RAPD) to house relevant information on recombinant approaches to generate antimicrobial peptides. Key information stored in the database, which is extracted from published experiments, includes expression host, fusion strategy, release method and yield for individual peptides. Bibliographic data directly related to each particular case are also available. RAPD allows easy comparison of the relative popularity and efficiency of different strategies, and can thus be used as a guideline for future production of similar peptides. The database is freely available at http://faculty.ist.unomaha.edu/chen/rapd/index.php.

Structures of human host defense cathelicidin LL-37 and its smallest antimicrobial peptide KR-12 in lipid micelles

As a key component of the innate immunity system, human cathelicidin LL-37 plays an essential role in protecting humans against infectious diseases. To elucidate the structural basis for its targeting bacterial membrane, we have determined the high quality structure of (13)C,(15)N-labeled LL-37 by three-dimensional triple-resonance NMR spectroscopy, because two-dimensional (1)H NMR did not provide sufficient spectral resolution. The structure of LL-37 in SDS micelles is composed of a curved amphipathic helix-bend-helix motif spanning residues 2-31 followed by a disordered C-terminal tail. The helical bend is located between residues Gly-14 and Glu-16. Similar chemical shifts and (15)N nuclear Overhauser effect (NOE) patterns of the peptide in complex with dioctanoylphosphatidylglycerol (D8PG) micelles indicate a similar structure. The aromatic rings of Phe-5, Phe-6, Phe-17, and Phe-27 of LL-37, as well as arginines, showed intermolecular NOE cross-peaks with D8PG, providing direct evidence for the association of the entire amphipathic helix with anionic lipid micelles. The structure of LL-37 serves as a model for understanding the structure and function relationship of homologous primate cathelicidins. Using synthetic peptides, we also identified the smallest antibacterial peptide KR-12 corresponding to residues 18-29 of LL-37. Importantly, KR-12 displayed a selective toxic effect on bacteria but not human cells. NMR structural analysis revealed a short three-turn amphipathic helix rich in positively charged side chains, allowing for effective competition for anionic phosphatidylglycerols in bacterial membranes. KR-12 may be a useful peptide template for developing novel antimicrobial agents of therapeutic use.

Ovarian cancers overexpress the antimicrobial protein hCAP-18 and its derivative LL-37 increases ovarian cancer cell proliferation and invasion

The role of the pro-inflammatory peptide, LL-37, and its pro-form, human cationic antimicrobial protein 18 (hCAP-18), in cancer development and progression is poorly understood. In damaged and inflamed tissue, LL-37 functions as a chemoattractant, mitogen and pro-angiogenic factor suggesting that the peptide may potentiate tumor progression. The aim of this study was to characterize the distribution of hCAP-18/LL-37 in normal and cancerous ovarian tissue and to examine the effects of LL-37 on ovarian cancer cells. Expression of hCAP-18/LL-37 was localized to immune and granulosa cells of normal ovarian tissue. By contrast, ovarian tumors displayed significantly higher levels of hCAP-18/LL-37 where expression was observed in tumor and stromal cells. Protein expression was statistically compared to the degree of immune cell infiltration and microvessel density in epithelial-derived ovarian tumors and a significant correlation was observed for both. It was demonstrated that ovarian tumor tissue lysates and ovarian cancer cell lines express hCAP-18/LL-37. Treatment of ovarian cancer cell lines with recombinant LL-37 stimulated proliferation, chemotaxis, invasion and matrix metalloproteinase expression. These data demonstrate for the first time that hCAP-18/LL-37 is significantly overexpressed in ovarian tumors and suggest LL-37 may contribute to ovarian tumorigenesis through direct stimulation of tumor cells, initiation of angiogenesis and recruitment of immune cells. These data provide further evidence of the existing relationship between pro-inflammatory molecules and ovarian cancer progression.

On-resin cleavage of bacterially expressed fusion proteins for purification of active recombinant peptides SK-29, KR-20, LL-29, and LL-23 from human sweat or skin

Post-translational processing of host defense cathelicidin peptide LL-37 in human sweat and skin generates new antimicrobial peptides. To understand structure and mechanism of action of these LL-37 derivatives, this article presents the cloning and expression of SK-29, KR-20, LL-29, and LL-23. We also provide a two-step chromatographic purification protocol of general use. First, resin-bound fusion proteins were directly subject to efficient upstream thrombin cleavage to release peptide-containing fragments. The resin, resin-bound carrier, and residual uncut fusion proteins were subsequently removed by centrifugation. Second, the peptide-containing fragments were digested with formic acid to release the required peptides followed by reverse-phase HPLC purification. We obtained 1.7 mg of recombinant SK-29, 0.7 mg KR-20, 2.1mg LL-29, and 5.4 mg LL-23, each from one liter of rich medium culture. Analytical HPLC, MS, and NMR indicated high quality of all the purified peptides. Antibacterial assays revealed the minimum inhibitory concentrations (MIC) for SK-29, KR-20, LL-29, and LL-23 are 80, 60, 40, and >150 microM, respectively. The poorest toxicity of LL-23 to Escherichia coli K12 correlates with its higher level of bacterial expression, reduced aggregation tendency, and loss of binding to a yet-to-be-characterized molecular target. Thus, on-resin protein cleavage facilitates the evaluation of peptide aggregation ability and may allow the identification of potential new bacterial targets of antimicrobial peptides. On-resin cleavage may be applied to the release of membrane proteins expressed as fusions.