Design and characterization of chionodracine-derived antimicrobial peptides with enhanced activity against drug-resistant human pathogens

Antibacterial and anticancer activity of two NK-lysin-derived peptides from the Antarctic teleost Trematomus bernacchii

The NK-lysin antimicrobial peptide, first identified in mammals, possesses both antibacterial and cytotoxic activity against cancer cell lines. Homologue peptides isolated from different fish species have been examined for their functional characteristics in the last few years. In this study, a NK-lysin transcript was identified in silico from the head kidney transcriptome of the Antarctic teleost Trematomus bernacchii. The corresponding amino acid sequence, slightly longer than NK-lysins of other fish species, contains six cysteine residues that in mammalian counterparts form three disulphide bridges. Real time-PCR analysis indicated its predominant expression in T. bernacchii immune-related organs and tissues, with greatest mRNA abundance detected in gills and spleen. Instead of focusing on the full T. bernacchii derived NK-lysin mature molecule, we selected a 27 amino acid residue peptide (named NKL-WT), corresponding to the potent antibiotic NK-2 sequence found in human NK-lysin. Moreover, we designed a mutant peptide (named NKL-MUT) in which two alanine residues substitute the two cysteines found in the NKL-WT. The two peptides were obtained by solid phase organic synthesis to investigate their functional features. NKL-WT and NKL-MUT displayed antibacterial activity against the human pathogenic bacterium Enterococcus faecalis and the ESKAPE pathogen Acinetobacter baumannii, respectively. Moreover, at the determined Minimum Inhibitory Concentration and Minimum Bactericidal Concentration values against these pathogens, both peptides showed high selectivity as they did not exhibit any haemolytic activity on erythrocytes or cytotoxic activity against mammalian primary cell lines. Finally, the NKL-MUT selectively triggers the killing of the melanoma cell line B16F10 by means of a pro-apoptotic pathway at a concentration range in which no effects were found in normal mammalian cell lines. In conclusion, the two peptides could be considered as promising candidates in the fight against antibiotic resistance and tumour proliferative action, and also be used as innovative adjuvants, either to decrease chemotherapy side effects or to enhance anticancer drug activity.

Anti-Virulence Potential of a Chionodracine-Derived Peptide against Multidrug-Resistant Pseudomonas aeruginosa Clinical Isolates from Cystic Fibrosis Patients

Pseudomonas aeruginosa is an opportunistic pathogen causing several chronic infections resistant to currently available antibiotics. Its pathogenicity is related to the production of different virulence factors such as biofilm and protease secretion. Pseudomonas communities can persist in biofilms that protect bacterial cells from antibiotics. Hence, there is a need for innovative approaches that are able to counteract these virulence factors, which play a pivotal role, especially in chronic infections. In this context, antimicrobial peptides are emerging drugs showing a broad spectrum of antibacterial activity. Here, we tested the anti-virulence activity of a chionodracine-derived peptide (KHS-Cnd) on five P. aeruginosa clinical isolates from cystic fibrosis patients. We demonstrated that KHS-Cnd impaired biofilm development and caused biofilm disaggregation without affecting bacterial viability in nearly all of the tested strains. Ultrastructural morphological analysis showed that the effect of KHS-Cnd on biofilm could be related to a different compactness of the matrix. KHS-Cnd was also able to reduce adhesion to pulmonary cell lines and to impair the invasion of host cells by P. aeruginosa. A cytotoxic effect of KHS-Cnd was observed only at the highest tested concentration. This study highlights the potential of KHS-Cnd as an anti-biofilm and anti-virulence molecule against P. aeruginosa clinical strains.

COVID-19-Associated Fungal Infections: An Urgent Need for Alternative Therapeutic Approach?

Secondary fungal infections may complicate the clinical course of patients affected by viral respiratory diseases, especially those admitted to intensive care unit. Hospitalized COVID-19 patients are at increased risk of fungal co-infections exacerbating the prognosis of disease due to misdiagnosis that often result in treatment failure and high mortality rate. COVID-19-associated fungal infections caused by predominantly Aspergillus and Candida species, and fungi of the order Mucorales have been reported from several countries to become significant challenge for healthcare system. Early diagnosis and adequate antifungal therapy is essential to improve clinical outcomes, however, drug resistance shows a rising trend highlighting the need for alternative therapeutic agents. The purpose of this review is to summarize the current knowledge on COVID-19-associated mycoses, treatment strategies and the most recent advancements in antifungal drug development focusing on peptides with antifungal activity.

Design and Characterization of Myristoylated and Non-Myristoylated Peptides Effective against Candida spp. Clinical Isolates

The increasing resistance of fungi to antibiotics is a severe challenge in public health, and newly effective drugs are required. Promising potential medications are lipopeptides, linear antimicrobial peptides (AMPs) conjugated to a lipid tail, usually at the N-terminus. In this paper, we investigated the in vitro and in vivo antifungal activity of three short myristoylated and non-myristoylated peptides derived from a mutant of the AMP Chionodracine. We determined their interaction with anionic and zwitterionic membrane-mimicking vesicles and their structure during this interaction. We then investigated their cytotoxic and hemolytic activity against mammalian cells. Lipidated peptides showed a broad spectrum of activity against a relevant panel of pathogen fungi belonging to Candida spp., including the multidrug-resistant C. auris. The antifungal activity was also observed vs. biofilms of C. albicans, C. tropicalis, and C. auris. Finally, a pilot efficacy study was conducted on the in vivo model consisting of Galleria mellonella larvae. Treatment with the most-promising myristoylated peptide was effective in counteracting the infection from C. auris and C. albicans and the death of the larvae. Therefore, this myristoylated peptide is a potential candidate to develop antifungal agents against human fungal pathogens.

Attacins: A Promising Class of Insect Antimicrobial Peptides

Insects produce a large repertoire of antimicrobial peptides (AMPs) as the first line of defense against bacteria, viruses, fungi or parasites. These peptides are produced from a large precursor that contains a signal domain, which is cleaved in vivo to produce the mature protein with antimicrobial activity. At present, AMPs from insects include several families which can be classified as cecropins, ponericins, defensins, lebocins, drosocin, Metchnikowin, gloverins, diptericins and attacins according to their structure and/or function. This short review is focused on attacins, a class of glycine-rich peptides/proteins that have been first discovered in the cecropia moth (Hyalophora cecropia). They are a rather heterogeneous group of immunity-related proteins that exhibit an antimicrobial effect mainly against Gram-negative bacteria. Here, we discuss different attacin and attacin-like AMPs that have been discovered so far and analyze their structure and phylogeny. Special focus is given to the physiological importance and mechanism of action of attacins against microbial pathogens together with their potential pharmacological applications, emphasizing their roles as antimicrobials.

Emerging Strategies to Combat β-Lactamase Producing ESKAPE Pathogens

Since the discovery of penicillin by Alexander Fleming in 1929 as a therapeutic agent against staphylococci, β-lactam antibiotics (BLAs) remained the most successful antibiotic classes against the majority of bacterial strains, reaching a percentage of 65% of all medical prescriptions. Unfortunately, the emergence and diversification of β-lactamases pose indefinite health issues, limiting the clinical effectiveness of all current BLAs. One solution is to develop β-lactamase inhibitors (BLIs) capable of restoring the activity of β-lactam drugs. In this review, we will briefly present the older and new BLAs classes, their mechanisms of action, and an update of the BLIs capable of restoring the activity of β-lactam drugs against ESKAPE (Enterococcus spp., Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp.) pathogens. Subsequently, we will discuss several promising alternative approaches such as bacteriophages, antimicrobial peptides, nanoparticles, CRISPR (clustered regularly interspaced short palindromic repeats) cas technology, or vaccination developed to limit antimicrobial resistance in this endless fight against Gram-negative pathogens.

Structural Analysis and Design of Chionodracine-Derived Peptides Using Circular Dichroism and Molecular Dynamics Simulations

Antimicrobial peptides have been identified as one of the alternatives to the extensive use of common antibiotics as they show a broad spectrum of activity against human pathogens. Among these is Chionodracine (Cnd), a host-defense peptide isolated from the Antarctic icefish Chionodraco hamatus, which belongs to the family of Piscidins. Previously, we demonstrated that Cnd and its analogs display high antimicrobial activity against ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter species). Herein, we investigate the interactions with lipid membranes of Cnd and two analogs, Cnd-m3 and Cnd-m3a, showing enhanced potency. Using a combination of Circular Dichroism, fluorescence spectroscopy, and all-atom Molecular Dynamics (MD) simulations, we determined the structural basis for the different activity among these peptides. We show that all peptides are predominantly unstructured in water and fold, preferentially as α-helices, in the presence of lipid vesicles of various compositions. Through a series of MD simulations of 400 ns time scale, we show the effect of mutations on the structure and lipid interactions of Cnd and its analogs. By explaining the structural basis for the activity of these analogs, our findings provide structural templates to design minimalistic peptides for therapeutics.

Trematocine, a Novel Antimicrobial Peptide from the Antarctic Fish Trematomus bernacchii: Identification and Biological Activity

Antimicrobial peptides (AMPs) are short peptides active against a wide range of pathogens and, therefore, they are considered a useful alternative to conventional antibiotics. We have identified a new AMP in a transcriptome derived from the Antarctic fish Trematomus bernacchii. This peptide, named Trematocine, has been investigated for its expression both at the basal level and after in vivo immunization with an endemic Antarctic bacterium (Psychrobacter sp. TAD1). Results agree with the expected behavior of a fish innate immune component, therefore we decided to synthesize the putative mature sequence of Trematocine to determine the structure, the interaction with biological membranes, and the biological activity. We showed that Trematocine folds into a α-helical structure in the presence of both zwitterionic and anionic charged vesicles. We demonstrated that Trematocine has a highly specific interaction with anionic charged vesicles and that it can kill Gram-negative bacteria, possibly via a carpet like mechanism. Moreover, Trematocine showed minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values against selected Gram-positive and Gram-negative bacteria similar to other AMPs isolated from Antarctic fishes. The peptide is a possible candidate for a new drug as it does not show any haemolytic or cytotoxic activity against mammalian cells at the concentration needed to kill the tested bacteria.

Intrinsically disordered HAX-1 regulates Ca2+ cycling by interacting with lipid membranes and the phospholamban cytoplasmic region

Hematopoietic-substrate-1 associated protein X-1 (HAX-1) is a 279 amino acid protein expressed ubiquitously. In cardiac muscle, HAX-1 was found to modulate the sarcoendoplasmic reticulum calcium ATPase (SERCA) by shifting its apparent Ca2+ affinity (pCa). It has been hypothesized that HAX-1 binds phospholamban (PLN), enhancing its inhibitory function on SERCA. HAX-1 effects are reversed by cAMP-dependent protein kinase A that phosphorylates PLN at Ser16. To date, the molecular mechanisms for HAX-1 regulation of the SERCA/PLN complex are still unknown. Using enzymatic, in cell assays, circular dichroism, and NMR spectroscopy, we found that in the absence of a binding partner HAX-1 is essentially disordered and adopts a partial secondary structure upon interaction with lipid membranes. Also, HAX-1 interacts with the cytoplasmic region of monomeric and pentameric PLN as detected by NMR and in cell FRET assays, respectively. We propose that the regulation of the SERCA/PLN complex by HAX-1 is mediated by its interactions with lipid membranes, adding another layer of control in Ca2+ homeostatic balance in the heart muscle.

Degradable antimicrobial polycarbonates with unexpected activity and selectivity for treating multidrug-resistant Klebsiella pneumoniae lung infection in mice

Multidrug resistant (MDR) Klebsiella pneumoniae is a major cause of healthcare-associated infections around the world, with attendant high rates of morbidity and mortality. Progressive reduction in potency of antibiotics capable of treating MDR K. pneumoniae infections - including lung infection - as a consequence of escalating drug resistance provides the motivation to develop drug candidates targeting MDR K. pneumoniae. We recently reported degradable broad-spectrum antimicrobial guanidinium-functionalized polycarbonates with unique antimicrobial mechanism - membrane translocation followed by precipitation of cytosolic materials. These polymers exhibited high potency against bacteria with negligible toxicity. The polymer with ethyl spacer between the quanidinium group and the polymer backbone (pEt_20) showed excellent in vivo efficacy for treating MDR K. pneumoniae-caused peritonitis in mice. In this study, the structures of the polymers were optimized for the treatment of MDR Klebsiella pneumoniae lung infection. Specifically, in vitro antimicrobial activity and selectivity of guanidinium-functionalized polycarbonates containing the same number of guanidinium groups but of a shorter chain length and a structural analogue containing a thiouronium moiety as the pendent cationic group were evaluated. The polymers with optimal compositions and varying hydrophobicity were assessed against 25 clinically isolated K. pneumonia strains for antimicrobial activity and killing kinetics. The results showed that the polymers killed the bacteria more efficiently than clinically used antibiotics, and repeated use of the polymers did not cause drug resistance in K. pneumonia. Particularly, the polymer with butyl spacer (pBut_20) self-assembled into micelles at high concentrations, where the hydrophobic component was shielded in the micellar core, preventing interacting with mammalian cells. A subtle change in the hydrophobicity increased the antimicrobial activity while reducing in vivo toxicity. The in vivo efficacy studies showed that pBut_20 alleviated K. pneumonia lung infection without inducing damage to major organs. Taken together, pBut_20 is promising for treating MDR Klebsiella pneumoniae lung infection in vivo. STATEMENT OF SIGNIFICANCE: Multidrug resistant (MDR) Klebsiella pneumoniae is a major cause of healthcare-associated infections, with attendant high rates of morbidity and mortality. The progressive reduction in antibiotics capable of treating MDR K. pneumoniae infections - including lung infection - as a consequence of escalating drug resistance rates provides the motivation to develop drug candidates. In this study, we report a degradable guanidinium-functionalized polycarbonate with unexpected antimicrobial activity and selectivity towards MDR Klebsiella pneumoniae. A subtle change in polymer hydrophobicity increases antimicrobial activity while reducing in vivo toxicity due to self-assembly at high concentrations. The polymer with optimal composition alleviates Klebsiella pneumonia lung infection without inducing damage to major organs. The polymer is promising for treating MDR Klebsiella pneumoniae lung infection in vivo.

Fish-derived antimicrobial peptides: Activity of a chionodracine mutant against bacterial models and human bacterial pathogens

The increasing resistance to conventional antibiotics is an urgent problem that can be addressed by the discovery of new antimicrobial drugs such as antimicrobial peptides (AMPs). AMPs are components of innate immune system of eukaryotes and are not prone to the conventional mechanisms that are responsible of drug resistance. Fish are an important source of AMPs and, recently, we have isolated and characterized a new 22 amino acid residues peptide, the chionodracine (Cnd), from the Antarctic icefish Chionodraco hamatus. In this paper we focused on a new Cnd-derived mutant peptide, namely Cnd-m3a, designed to improve the selectivity against prokaryotic cells and the antimicrobial activity against human pathogens of the initial Cnd template. Cnd-m3a was used for immunization of rabbits, which gave rise to a polyclonal antibody able to detect the peptide. The interaction kinetic of Cnd-m3a with the Antarctic bacterium Psychrobacter sp. (TAD1) was imaged using a transmission electron microscopy (TEM) immunogold method. Initially the peptide was associated with the plasma membrane, but after 180 min of incubation, it was found in the cytoplasm interacting with a DNA target inside the bacterial cells. Using fluorescent probes we showed that the newly designed mutant can create pores in the outer membrane of the bacteria E. coli and Psychrobacter sp. (TAD1), confirming the results of TEM analysis. Moreover, in vitro assays demonstrated that Cnd-m3a is able to bind lipid vesicles of different compositions with a preference toward negatively charged ones, which mimics the prokaryotic cell. The Cnd-m3a peptide showed quite low hemolytic activity and weak cytotoxic effect against human primary and tumor cell lines, but high antimicrobial activity against selected Gram - human pathogens. These results highlighted the high potential of the Cnd-m3a peptide as a starting point for developing a new human therapeutic agent.