Design, Synthesis, Antibacterial Potential, and Structural Characterization of N-Acylated Derivatives of the Human Autophagy 16 Polypeptide

Discovery and Optimization of Novel SaFabI Inhibitors as Specific Therapeutic Agents for MRSA Infection

As the rate-limiting enzyme in fatty acid biosynthesis, Staphylococcus aureus enoyl-acyl carrier protein reductase (SaFabI) emerges as a compelling target for combating methicillin-resistant S. aureus (MRSA) infections. Herein, compound 1, featuring a 4-(1H-benzo[d]imidazol-2-yl)pyrrolidin-2-one scaffold, was identified as a potent SaFabI inhibitor (IC50 = 976.8 nM) from an in-house library. Subsequent optimization yielded compound n31, with improved inhibitory efficacy on enzymatic activity (IC50 = 174.2 nM) and selective potency against S. aureus (MIC = 1-2 μg/mL). Mechanistically, n31 directly inhibited SaFabI in cellular contexts. Moreover, n31 exhibited favorable safety and pharmacokinetic profiles, and dose-dependently treated MRSA-induced skin infections, outperforming the approved drug, linezolid. The chiral separation of n31 resulted in (S)-n31, with superior activities (IC50 = 94.0 nM, MIC = 0.25-1 μg/mL) and in vivo therapeutic efficacy. In brief, our research proposes (S)-n31 as a promising candidate for SaFabI-targeted therapy, offering specific anti-S. aureus efficacy and potential for further development.

Antimicrobial fibres derived from aryl-diazonium conjugation of chitosan with Harakeke (Phormium tenax) and Hemp (Cannabis sativa) Hurd

Surface functionalisation of natural materials to develop sustainable and environmentally friendly antimicrobial fibres has received great research interest in recent years. Herein, chitosan covalent conjugation via aryl-diazonium based chemistry onto Phormium tenax fibres (PTF) and hemp hurds (HH) was investigated. PTF are fibres derived from Harakeke/New Zealand flax, an indigenous and abundant plant source of leaf fibres, which served as an important 19th century export commodity of New Zealand. HH are obtained as a by-product from the hemp (Cannabis sativa) industry and find applications as traditional construction material, animal bedding, chemical absorbent, insulation, fireboard etc. This study reports aryl-diazonium covalent attachment of chitosan and PD13 (6-O-(3-(2-(N,N-dimethylamino)ethylamino)-2-hydroxypropyl)chitosan), a chitosan derivative with improved antibacterial activity, on to PTF and HH. The modification was confirmed using FTIR, XPS, SEM and water contact angle studies. Comparison of aryl-diazonium versus the use of succinic anhydride bridging for chitosan attachment was also investigated, with the diazonium method giving improved results. The treated PTF and HH fibres had good antibacterial activity against Staphylococcus aureus and this study contributes to the development of sustainable antibacterial fibres using bio-based materials.

Molecular and cellular evidence of a direct interaction between the TRAF2 C-terminal domain and ganglioside GM1

TNF receptor-associated factor 2 (TRAF2) is involved in different cellular processes including signal transduction and transcription regulation. We here provide evidence of a direct interaction between the TRAF domain of TRAF2 and the monosialotetrahexosylganglioside (GM1). Previously, we showed that the TRAF domain occurs mainly in a trimeric form in solution, but it can also exist as a stable monomer when in the nanomolar concentration range. Here, we report that the quaternary structure of the TRAF domain is also affected by pH changes, since a weakly acidic pH (5.5) favors the dissociation of the trimeric TRAF domain into stable monomers, as previously observed at neutral pH (7.6) with the diluted protein. The TRAF domain-GM1 binding was similar at pH 5.5 and 7.6, suggesting that GM1 interacts with both the trimeric and monomeric forms of the protein. However, only the monomeric protein appeared to cause membrane deformation and inward vesiculation in GM1-containing giant unilamellar vesicles (GUVs). The formation of complexes between GM1 and TRAF2, or its TRAF domain, was also observed in cultured human leukemic HAP1 cells expressing either the truncated TRAF domain or the endogenous full length TRAF2. The GM1-protein complexes were observed after treatment with tunicamycin and were more concentrated in cells undergoing apoptosis, a condition which is known to cause cytoplasm acidification. These findings open the avenue for future studies aimed at deciphering the physiopathological relevance of the TRAF domain-GM1 interaction.

Conjugation of antimicrobial peptides to enhance therapeutic efficacy

The growing prevalence of antimicrobial resistance (AMR) has brought with it a continual increase in the numbers of deaths from multidrug-resistant (MDR) infections. Since the current arsenal of antibiotics has become increasingly ineffective, there exists an urgent need for discovery and development of novel antimicrobials. Antimicrobial peptides (AMPs) are considered to be a promising class of molecules due to their broad-spectrum activities and low resistance rates compared with other types of antibiotics. Since AMPs also often play major roles in elevating the host immune response, the molecules may also be called "host defense peptides." Despite the great promise of AMPs, the majority remain unsuitable for clinical use due to issues of structural instability, degradation by proteases, and/or toxicity to host cells. Moreover, AMP activities in vivo can be influenced by many factors, such as interaction with blood and serum biomolecules, physiological salt concentrations or different pH values. To overcome these limitations, structural modifications can be made to the AMP. Among several modifications, physical and chemical conjugation of AMP to other biomolecules is widely considered an effective strategy. In this review, we discuss structural modification strategies related to conjugation of AMPs and their possible effects on mode of action. The conjugation of fatty acids, glycans, antibiotics, photosensitizers, polymers, nucleic acids, nanoparticles, and immobilization to biomaterials are highlighted.

Isoamphipathic antibacterial molecules regulating activity and toxicity through positional isomerism

Peptidomimetic antimicrobials exhibit a selective interaction with bacterial cells over mammalian cells once they have achieved an optimum amphiphilic balance (hydrophobicity/hydrophilicity) in the molecular architecture. To date, hydrophobicity and cationic charge have been considered the crucial parameters to attain such amphiphilic balance. However, optimization of these properties is not enough to circumvent unwanted toxicity towards mammalian cells. Hence, herein, we report new isoamphipathic antibacterial molecules (IAMs: 1-3) where positional isomerism was introduced as one of the guiding factors for molecular design. This class of molecules displayed good (MIC = 1-8 μg mL^-1 or μM) to moderate [MIC = 32-64 μg mL^-1 (32.2-64.4 μM)] antibacterial activity against multiple Gram-positive and Gram-negative bacteria. Positional isomerism showed a strong influence on regulating antibacterial activity and toxicity for ortho [IAM-1: MIC = 1-32 μg mL^-1 (1-32.2 μM), HC₅₀ = 650 μg mL^-1 (654.6 μM)], meta [IAM-2: MIC = 1-16 μg mL^-1 (1-16.1 μM), HC₅₀ = 98 μg mL^-1 (98.7 μM)] and para [IAM-3: MIC = 1-16 μg mL^-1 (1-16.1 μM), HC₅₀ = 160 μg mL^-1 (161.1 μM)] isomers. Co-culture studies and investigation of membrane dynamics indicated that ortho isomer, IAM-1 exerted more selective activity towards bacterial over mammalian membranes, compared to meta and para isomers. Furthermore, the mechanism of action of the lead molecule (IAM-1) has been characterized through detailed molecular dynamics simulations. In addition, the lead molecule displayed substantial efficacy against dormant bacteria and mature biofilms, unlike conventional antibiotics. Importantly, IAM-1 exhibited moderate in vivo activity against MRSA wound infection in a murine model with no detectable dermal toxicity. Altogether, the report explored the design and development of isoamphipathic antibacterial molecules to establish the role of positional isomerism in achieving selective and potential antibacterial agents.

Modulatory role of copper on hIAPP aggregation and toxicity in presence of insulin

Aggregation of the human islets amyloid polypeptide, or hIAPP, is linked to β-cell death in type II diabetes mellitus (T2DM). Different pancreatic β-cell environmental variables such as pH, insulin and metal ions play a key role in controlling the hIAPP aggregation. Since insulin and hIAPP are co-secreted, it is known from numerous studies that insulin suppresses hIAPP fibrillation by preventing the initial dimerization process. On the other hand, zinc and copper each have an inhibitory impact on hIAPP fibrillation, but copper promotes the production of toxic oligomers. Interestingly, the insulin oligomeric equilibrium is controlled by the concentration of zinc ions when the effect of insulin and zinc has been tested together. Lower zinc concentrations cause the equilibrium to shift towards the monomer and dimer states of insulin, which bind to monomeric hIAPP and stop it from developing into a fibril. On the other hand, the combined effects of copper and insulin have not yet been done. In this study, we have demonstrated how the presence of copper affects hIAPP aggregation and the toxicity of the resultant conformers with or without insulin. For this purpose, we have used a set of biophysical techniques, including NMR, fluorescence, CD etc., in combination with AFM and cell cytotoxicity assay. In the presence and/or absence of insulin, copper induces hIAPP to form structurally distinct stable toxic oligomers, deterring the fibrillation process. More specifically, the oligomers generated in the presence of insulin have slightly higher toxicity than those formed in the absence of insulin. This research will increase our understanding of the combined modulatory effect of two β-cell environmental factors on hIAPP aggregation.

Synthesis and antibacterial analysis of C-6 amino-functionalised chitosan derivatives

Synthesis of 6-O-(3-alkylamino-2-hydroxypropyl) derivatives of chitosan was achieved using a four-step strategy of N-protection, O-epoxide addition, epoxide ring opening using an amine and N-deprotection. Benzaldehyde and phthalic anhydride were used for the N-protection step, producing N-benzylidene and N-phthaloyl protected derivatives, respectively, resulting in two corresponding final 6-O-(3-alkylamino-2-hydroxypropyl) derivative series, BD1-BD6 and PD1-PD14. All the compounds were characterized using FTIR, XPS and PXRD studies and tested for antibacterial efficacy. The phthalimide protection strategy was found to be easier to apply and effective in terms of the synthetic process and improvement in antibacterial activity. Amongst the newly synthesized compounds, PD13 (6-O-(3-(2-(N,N-dimethylamino)ethylamino)-2-hydroxypropyl)chitosan) was the most active with eight times greater activity compared to the unmodified chitosan and, PD7 6-O-(3-(3-(N-(3-aminopropyl)propane-1,3-diamino)propylamino)-2-hydroxypropyl)chitosan) having a four-fold activity than chitosan, was found to be the second most potent derivative. This work has produced new chitosan derivatives those are more potent than chitosan itself and show promise in antimicrobial applications.

Mechanistic insight into functionally different human islet polypeptide (hIAPP) amyloid: the intrinsic role of the C-terminal structural motifs

Targeting amyloidosis requires high-resolution insight into the underlying mechanisms of amyloid aggregation. The sequence-specific intrinsic properties of a peptide or protein largely govern the amyloidogenic propensity. Thus, it is essential to delineate the structural motifs that define the subsequent downstream amyloidogenic cascade of events. Additionally, it is important to understand the role played by extrinsic factors, such as temperature or sample agitation, in modulating the overall energy barrier that prompts divergent nucleation events. Consequently, these changes can affect the fibrillation kinetics, resulting in structurally and functionally distinct amyloidogenic conformers associated with disease pathogenesis. Here, we have focused on human Islet Polypeptide (hIAPP) amyloidogenesis for the full-length peptide along with its N- and C-terminal fragments, under different temperatures and sample agitation conditions. This helped us to gain a comprehensive understanding of the intrinsic role of specific functional epitopes in the primary structure of the peptide that regulates amyloidogenesis and subsequent cytotoxicity. Intriguingly, our study involving an array of biophysical experiments and ex vivo data suggests a direct influence of external changes on the C-terminal fibrillating sequence. Furthermore, the observations indicate a possible collaborative role of this segment in nucleating hIAPP amyloidogenesis in a physiological scenario, thus making it a potential target for future therapeutic interventions.

Natural Antimicrobial Peptides Self-assemble as α/β Chameleon Amyloids

Amyloid protein fibrils and some antimicrobial peptides (AMPs) share biophysical and structural properties. This observation suggests that ordered self-assembly can act as an AMP-regulating mechanism, and, vice versa, that human amyloids play a role in host defense against pathogens, as opposed to their common association with neurodegenerative and systemic diseases. Based on previous structural information on toxic amyloid peptides, we developed a sequence-based bioinformatics platform and, led by its predictions, experimentally identified 14 fibril-forming AMPs (ffAMPs) from living organisms, which demonstrated cross-β and cross-α amyloid properties. The results support the amyloid-antimicrobial link. The high prevalence of ffAMPs produced by amphibians and marine creatures among other species suggests that they confer unique advantageous properties in distinctive environments, potentially providing stability and adherence properties. Most of the newly identified 14 ffAMPs showed lipid-induced and/or time-dependent secondary structure transitions in the fibril form, indicating structural and functional cross-α/β chameleons. Specifically, ffAMP cytotoxicity against human cells correlated with the inherent or lipid-induced α-helical fibril structure. The findings raise hypotheses about the role of fibril secondary structure switching in regulation of processes, such as the transition between a stable storage conformation and an active state with toxicity against specific cell types.

Lipidation of Antimicrobial Peptides as a Design Strategy for Future Alternatives to Antibiotics

Multi-drug-resistant bacteria are becoming more prevalent, and treating these bacteria is becoming a global concern. One alternative approach to combat bacterial resistance is to use antimicrobial (AMPs) or host-defense peptides (HDPs) because they possess broad-spectrum activity, function in a variety of ways, and lead to minimal resistance. However, the therapeutic efficacy of HDPs is limited by a number of factors, including systemic toxicity, rapid degradation, and low bioavailability. One approach to circumvent these issues is to use lipidation, i.e., the attachment of one or more fatty acid chains to the amine groups of the N-terminus or a lysine residue of an HDP. In this review, we examined lipidated analogs of 66 different HDPs reported in the literature to determine: (i) whether there is a link between acyl chain length and antibacterial activity; (ii) whether the charge and (iii) the hydrophobicity of the HDP play a role; and (iv) whether acyl chain length and toxicity are related. Overall, the analysis suggests that lipidated HDPs with improved activity over the nonlipidated counterpart had acyl chain lengths of 8–12 carbons. Moreover, active lipidated peptides attached to short HDPs tended to have longer acyl chain lengths. Neither the charge of the parent HDP nor the percent hydrophobicity of the peptide had an apparent significant impact on the antibacterial activity. Finally, the relationship between acyl chain length and toxicity was difficult to determine due to the fact that toxicity is quantified in different ways. The impact of these trends, as well as combined strategies such as the incorporation of d- and non-natural amino acids or alternative approaches, will be discussed in light of how lipidation may play a role in the future development of antimicrobial peptide-based alternatives to current therapeutics.

Effect of Secondary Structure and Side Chain Length of Hydrophobic Amino Acid Residues on the Antimicrobial Activity and Toxicity of 14-Residue-Long de novo AMPs

Herein we report the efficacy and toxicity of three de novo designed cationic antimicrobial peptides (AMPs) LL-14, VV-14 and ββ-14, where side chains of the hydrophobic amino acids were reduced gradually. The AMPs showed broad-spectrum antimicrobial activity against three pathogens from the ESKAPE group and two fungal strains. This study showed that side chains which are either too long or too short increase toxicity and lower antimicrobial activity, respectively. VV-14 was found to be non-cytotoxic and highly potent under physiological salt concentrations against several pathogens, especially Salmonella typhi TY2. These AMPs acted via membrane deformation, depolarization, and lysis. The activity of the AMPs is related to their ability to take on amphipathic helical conformations in the presence of microbial membrane mimics. Among AMPs with the same charge, hydrophobic interactions between the side chains of the residues with cell membrane lipids determine their antimicrobial potency and cytotoxicity. Strikingly, an optimum hydrophobic interaction is the crux of generating highly potent non-cytotoxic AMPs.

The Best Peptidomimetic Strategies to Undercover Antibacterial Peptides

Health-care systems that develop rapidly and efficiently may increase the lifespan of humans. Nevertheless, the older population is more fragile, and is at an increased risk of disease development. A concurrently growing number of surgeries and transplantations have caused antibiotics to be used much more frequently, and for much longer periods of time, which in turn increases microbial resistance. In 1945, Fleming warned against the abuse of antibiotics in his Nobel lecture: "The time may come when penicillin can be bought by anyone in the shops. Then there is the danger that the ignorant man may easily underdose himself and by exposing his microbes to non-lethal quantities of the drug make them resistant". After 70 years, we are witnessing the fulfilment of Fleming's prophecy, as more than 700,000 people die each year due to drug-resistant diseases. Naturally occurring antimicrobial peptides protect all living matter against bacteria, and now different peptidomimetic strategies to engineer innovative antibiotics are being developed to defend humans against bacterial infections.

Synthesis and Antibacterial Analysis of Analogues of the Marine Alkaloid Pseudoceratidine

In an effort to gain more understanding on the structure activity relationship of pseudoceratidine 1, a di-bromo pyrrole spermidine alkaloid derived from the marine sponge Pseudoceratina purpurea that has been shown to exhibit potent biofouling, anti-fungal, antibacterial, and anti-malarial activities, a large series of 65 compounds that incorporated several aspects of structural variation has been synthesised through an efficient, divergent method that allowed for a number of analogues to be generated from common precursors. Subsequently, all analogues were assessed for their antibacterial activity against both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria. Overall, several compounds exhibited comparable or better activity than that of pseudoceratidine 1, and it was found that this class of compounds is generally more effective against Gram-positive than Gram-negative bacteria. Furthermore, altering several structural features allowed for the establishment of a comprehensive structure activity relationship (SAR), where it was concluded that several structural features are critical for potent anti-bacterial activity, including di-halogenation (preferable bromine, but chlorine is also effective) on the pyrrole ring, two pyrrolic units in the structure and with one or more secondary amines in the chain adjoining these units, with longer chains giving rise to better activities.