Primary and Secondary Binding of Exenatide to Liposomes

Interactions between Hyaluronic Acid and Chitosan by Isothermal Titration Calorimetry: The Effect of Ionic Strength, pH, and Polymer Molecular Weight

Hyaluronic acid (HA)/chitosan (CHI) complex coacervates have recently gained interest due to the pH-dependent ionization and semiflexibility of the polymers as well as their applicability in tissue engineering. Here, we apply isothermal titration calorimetry (ITC) to understand the apparent thermodynamics of coacervation for HA/CHI as a function of the pH, ionic strength, and chain length. We couple these ITC experiments with the knowledge of the charge states of HA and CHI from potentiometric titration to understand the mechanistic aspects of complex formation. Our data demonstrate that the driving force for the complex coacervation of HA and CHI is entropic in nature and this driving force decreased with increasing ionic strength. We also observed a decrease in the stoichiometry for ion-pairing with increasing ionic strength, which we suggest is a consequence of the changing degree of ionization for HA at higher ionic strengths. An increase in the strength of interactions with pH was hypothesized to also be a result of changes in the degree of ionization of HA, though stronger interactions were observed at the lowest pH tested, likely due to contributions from hydrogen bonding between HA and CHI.

Microscale Thermophoresis (MST) to Study Rapid Alkalinization Factor (RALF)-Receptor Interactions

Microscale thermophoresis (MST) is a simple but powerful tool to study the in vitro interaction among biomolecules, and to quantify binding affinities. MST curves describe the change in the fluorescence level of a fluorescent target as a result of an IR-laser-induced temperature change. The degree and nature of the change in fluorescence signal depends on the size, charge, and solvation shell of the molecules, properties that change in function of the binding of a ligand to the fluorescent target.We used MST to describe the interaction between components of a regulatory module involved in plant cell wall integrity control. This module comprises the secreted peptide Rapid Alkalinization Factor 23 (RALF23) and its receptor complex consisting of the GPI-anchored receptor Lorelei-Like Glycoprotein 1 (LLG1) and a receptor kinase of the CrRLK1L family, FERONIA. Here we show how MST can also be used to study three-partner interactions.

Amyloidogenicity of peptides targeting diabetes and obesity

Since the discovery of insulin, a century ago, the repertoire of therapeutic polypeptides targeting diabetes - and now also obesity - have increased substantially. The focus on quality has shifted from impure and unstable preparations of animal insulin to highly pure, homologous recombinant insulin, along with other peptide-based hormones and analogs such as amylin analogs (pramlintide, davalintide, cagrilintide), glucagon and glucagon-like peptide-1 receptor agonists (GLP-1, liraglutide, exenatide, semaglutide). Proper formulation, storage, manipulation and usage by professionals and patients are required in order to avoid agglomeration into high molecular weight products (HMWP), either amorphous or amyloid, which could result in potential loss of biological activity and short- or long-term immune reaction and silent inactivation. In this narrative review, we present perspective of the aggregation of therapeutic polypeptides used in diabetes and other metabolic diseases, covering the nature and mechanisms, analytical techniques, physical and chemical stability, strategies aimed to hamper the formation of HMWP, and perspectives on future biopharmaceutical developments.

Lipid Scrambling Induced by Membrane-Active Substances

The functional roles of the lipid asymmetry of biomembranes are attracting increasing attention. This study characterizes the activity of surfactants to induce transmembrane flip-flop of lipids and thus "scramble" this asymmetry. Detergent-induced lipid scrambling of liposomes mimicking the charge asymmetry of bacterial membranes with 20 mol % of 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-rac-glycerol in the outer leaflet only was quantified by ζ-potential measurements for octaethylene glycol dodecyl ether (C₁₂EO₈), octyl glucoside (OG), and dodecyl maltoside. Membrane leakage was separately measured by the fluorescence lifetime-based calcein leakage assay and the onset of the membrane-to-micelle transition by isothermal titration calorimetry. Partition coefficients and partial molar areas were obtained as well. For the quickly membrane-permeant C₁₂EO₈ and OG, leakage proceeds at a rather sharp threshold content in the membrane, which is well below the onset of solubilization and little dependent on incubation time; it is accompanied by fast lipid scrambling. However, unlike leakage, flip-flop is a relaxation process that speeds up gradually from taking weeks in the detergent-free membrane to minutes or less in the leaking membrane. Hence, after 24 h of incubation, 10 mol % of C₁₂EO₈ or 50 mol % of OG in the membrane suffice for virtually complete lipid scrambling, whereas leakage remains below 10% for up to 14 mol % of C₁₂EO₈ and 88 mol % of OG. There is thus a concentration window in which lipid scrambling proceeds without leakage. This implies that lipid scrambling must be considered a possible mode of action of antimicrobial peptides and other membrane-active drugs or biomolecules. A related, detergent-based protocol for scrambling the lipid asymmetry of liposomes and maybe cells without compromising their overall integrity would be a very valuable tool to study functions of lipid asymmetry.

Revealing the Mode of Action of Halictine Antimicrobial Peptides: A Comprehensive Study with Model Membranes

Antimicrobial peptides are innate host defense molecules with the ability to kill pathogens. They have been widely studied for their membrane lytic activity and their potential to overcome the ever-increasing threat of antimicrobial resistance against conventional antibiotics. Here, we focus on two halictines, antimicrobial peptides first obtained from the venom of the eusocial bee Halictus sexcinctus. The peptides, HAL-1 and HAL-2, are cationic (with +3 and +4 charges, respectively) and amphipathic, have 12 amino acid residues, and exhibit high biological activity. For this study, the mechanism of action of HAL-1 and HAL-2 was studied in detail using large and giant unilamellar vesicles composed of pure palmitoyl oleoyl phosphatidyl choline (POPC) and a mixture of POPC and the anionic lipid palmitoyl oleoyl phosphatidyl glycerol (POPG) as biomimetic models of the membranes of eukaryotes and microorganisms, respectively. A set of complementary techniques was put forward: carboxyfluorescein leakage assay, phase contrast optical microscopy, ζ-potential, static and dynamic light scattering, fluorescence and circular dichroism spectroscopies, and isothermal titration calorimetry. The results show that both halictines are able to interact strongly with anionic membranes: The interaction is exothermic and accompanied by structuring of the peptides as an α-helix and deep insertion into the membrane causing substantial membrane permeabilization at very low peptide/lipid molar ratios. Extensive vesicle aggregation was detected only at a high peptide concentration. On the other hand, the interaction of the halictines with POPC is significantly milder. Yet, the peptides were able to permeabilize the POPC membranes to some extent. Comparing both peptides, HAL-1 showed a somewhat stronger effect on model membranes. Fits to the data revealed apparent binding constants on the order of 10³-10⁴ M^-1 for anionic membranes and 1 order of magnitude lower for zwitterionic bilayers. When lytic activity results were compared at the same bound peptide/lipid ratio, the halictines exhibited a higher activity toward zwitterionic membranes. As novel peptides, small and with powerful activity, these halictines are potential candidates for becoming antimicrobial agents.