The host-defense peptide piscidin P1 reorganizes lipid domains in membranes and decreases activation energies in mechanosensitive ion channels.

The host-defense peptide (HDP) piscidin 1 (P1), isolated from the mast cells of striped bass, has potent activities against bacteria, viruses, fungi, and cancer cells and can also modulate the activity of membrane receptors. Given its broad pharmacological potential, here we used several approaches to better understand its interactions with multicomponent bilayers representing models of bacterial (phosphatidylethanolamine (PE)/phosphatidylglycerol) and mammalian (phosphatidylcholine/cholesterol (PC/Chol)) membranes. Using solid-state NMR, we solved the structure of P1 bound to PC/Chol and compared it with that of P3, a less potent homolog. The comparison disclosed that although both peptides are interfacially bound and α-helical, they differ in bilayer orientations and depths of insertion, and these differences depend on bilayer composition. Although Chol is thought to make mammalian membranes less susceptible to HDP-mediated destabilization, we found that Chol does not affect the permeabilization effects of P1. X-ray diffraction experiments revealed that both piscidins produce a demixing effect in PC/Chol membranes by increasing the fraction of the Chol-depleted phase. Furthermore, P1 increased the temperature required for the lamellar-to-hexagonal phase transition in PE bilayers, suggesting that it imposes positive membrane curvature. Patch-clamp measurements on the inner Escherichia coli membrane showed that P1 and P3, at concentrations sufficient for antimicrobial activity, substantially decrease the activating tension for bacterial mechanosensitive channels. This indicated that piscidins can cause lipid redistribution and restructuring in the microenvironment near proteins. We conclude that the mechanism of piscidin's antimicrobial activity extends beyond simple membrane destabilization, helping to rationalize its broader spectrum of pharmacological effects.

The so-called critical condition for polyelectrolyte-colloid complex formation.

Complexes formed between oppositely charged polyelectrolytes (PE's) and either biological or abiotic colloid particles play a central role in such remarkably diverse areas as enzyme immobilization, protein purification, growth factor delivery, personal care products, food formulations and as precursors to coacervates and multilayers. Unlike PE adsorption on oppositely charged planar surfaces-also driven by electrostatics-PE-colloid complexes are often equilibrium states exhibiting reversible formation at a well-defined "critical" colloid surface charge density. We consider how the experimentally observed breadth of this transition, for three polyelectrolyte-colloid systems, is broadened-compared to theoretical expectations-due to (1) colloid (protein) charge anisotropy, (2) colloid (micelle) polydispersity, and (3) colloid (micelle) instability.

Polysaccharide zeta-potentials and protein-affinity.

The ζ-potential, a parameter typically obtained by model-dependent transformation of the measured electrophoretic mobility, is frequently used to understand polysaccharide-protein complexation. We tested the hypothesis that two anionic polysaccharides with identical ζ-potentials would show equal binding affinity to the protein ß-lactoglobulin (BLG). We selected two polysaccharide polyelectrolytes (PE) with very different structures: hyaluronic acid (HA) and tragacanthin (TG). Highly precise (±0.1%) turbidimetric titrations were performed to determine critical pH values of complex formation; and PE ζ-potentials were measured for different ionic strengths I at those critical pH values. While phase boundaries (pHcvs. I) showed that HA binds to BLG more strongly (e.g. at a lower pH, for fixed I), comparisons made at fixed ζ-potential indicated that TG binds more strongly. The source of this contradiction is the effect of the bulky side chains of TG on its friction coefficient which diminishes its mobility and hence the resultant ζ-potential; while having a distinctly separate effect on the interaction between BLG and the carboxylated backbone of TG. Thus, unless the locus of the bound protein coincides with the shear plane, the ζ-potential does not directly contribute to the electrostatic PE-protein interaction.

Coacervation and precipitation in polysaccharide-protein systems.

Precipitation poses a consistent problem for the growing applications of biopolymer coacervation, but the relationship between the two types of phase separation is not well understood. To clarify this relationship, we studied phase separation as a function of pH and ionic strength, in three systems of proteins with anionic polysaccharides: ß-lactoglobulin (BLG)/hyaluronic acid (HA); BLG/tragacanthin (TG); and monoclonal antibody (mAb)/HA. We found that coacervation and precipitation are intrinsically different phenomena, responsive to different factors, but their simultaneity (for example with changing pH) may be confused with transitions from one state to another. We propose that coacervate does not literally turn into precipitate, but rather that both coacervate and precipitate are in equilibrium with free protein and polyanion, so that dissolution of one and formation of the other can overlap in time. While protein-polyanion complexes must achieve neutrality for coacervation, precipitation only requires tight binding which leads to the expulsion of counterions and water molecules. The pH-dependence of phase separation, considered in terms of protein and polyion charge, revealed that the electrostatic magnitude of the protein's polymer-binding site ("charge patch") plays a key role in the strength of interaction. These findings were supported by the inhibition of precipitation, seen when the bulky side chains of TG impede close protein-polymer interactions.

Rheology and Gelation of Hyaluronic Acid/Chitosan Coacervates.

Hyaluronic acid (HA) and chitosan (CHI) are biopolyelectrolytes which are interesting for both the medical and polymer physics communities due to their biocompatibility and semi-flexibility, respectively. In this work, we demonstrate by rheology experiments that the linear viscoelasticity of HA/CHI coacervates depends strongly on the molecular weight of the polymers. Moduli for coacervates were found significantly higher than those of individual HA and CHI physical gels. A remarkable 1.5-fold increase in moduli was noted when catechol-conjugated HA and CHI were used instead. This was attributed to the conversion of coacervates to chemical gels by oxidation of 3,4-dihydroxyphenylalanine (DOPA) groups in HA and CHI to di-DOPA crosslinks. These rheological results put HA/CHI coacervates in the category of strong candidates as injectable tissue scaffolds or medical adhesives.

Copper-binding anticancer peptides from the piscidin family: an expanded mechanism that encompasses physical and chemical bilayer disruption.

In the search for novel broad-spectrum therapeutics to fight chronic infections, inflammation, and cancer, host defense peptides (HDPs) have garnered increasing interest. Characterizing their biologically-active conformations and minimum motifs for function represents a requisite step to developing them into efficacious and safe therapeutics. Here, we demonstrate that metallating HDPs with Cu2+ is an effective chemical strategy to improve their cytotoxicity on cancer cells. Mechanistically, we find that prepared as Cu2+-complexes, the peptides not only physically but also chemically damage lipid membranes. Our testing ground features piscidins 1 and 3 (P1/3), two amphipathic, histidine-rich, membrane-interacting, and cell-penetrating HDPs that are α-helical bound to membranes. To investigate their membrane location, permeabilization effects, and lipid-oxidation capability, we employ neutron reflectometry, impedance spectroscopy, neutron diffraction, and UV spectroscopy. While P1-apo is more potent than P3-apo, metallation boosts their cytotoxicities by up to two- and seven-fold, respectively. Remarkably, P3-Cu2+ is particularly effective at inserting in bilayers, causing water crevices in the hydrocarbon region and placing Cu2+ near the double bonds of the acyl chains, as needed to oxidize them. This study points at a new paradigm where complexing HDPs with Cu2+ to expand their mechanistic reach could be explored to design more potent peptide-based anticancer therapeutics.

A Potent Host Defense Peptide Triggers DNA Damage and Is Active against Multidrug-Resistant Gram-Negative Pathogens.

Gram-negative bacteria are some of the biggest threats to public health due to a large prevalence of antibiotic resistance. The difficulty in treating bacterial infections, stemming from their double membrane structure combined with efflux pumps in the outer membrane, has resulted in a much greater need for antimicrobials with activity against these pathogens. Tunicate host defense peptide (HDP), Clavanin A, is capable of not only inhibiting Gram-negative growth but also potentiating activity in the presence of Zn(II). Here, we provide evidence that the improvements of Clavanin A activity in the presence of Zn(II) are due to its novel mechanism of action. We employed E. coli TD172 (ΔrecA::kan) and the terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay to show in cellulae that DNA damage occurs upon treatment with Clavanin A. In vitro assays demonstrated that Zn(II) ions are required for the nuclease activity of the peptide. The quantum mechanics/molecular mechanics (QM/MM) calculations were used to investigate the mechanism of DNA damage. In the rate-determining step of the proposed mechanism, due to its Lewis acidity, the Zn(II) ion activates the scissile P-O bond of DNA and creates a hydroxyl nucleophile from a water molecule. A subsequent attack by this group to the electrophilic phosphorus cleaves the scissile phosphoester bond. Additionally, we utilized bacterial cytological profiling (BCP), circular dichroism (CD) spectroscopy in the presence of lipid vesicles, and surface plasmon resonance combined with electrical impedance spectroscopy in order to address the apparent discrepancies between our results and the previous studies regarding the mechanism of action of Clavanin A. Finally, our approach may lead to the identification of additional Clavanin A like HDPs and promote the development of antimicrobial peptide based therapeutics.

Liquid-liquid and liquid-solid phase separation in protein-polyelectrolyte systems.

The coacervation of systems containing colloids (e.g. proteins or micelles) and polyelectrolytes (notably ionic polysaccharides) is often accompanied by precipitation. This can introduce inhomogeneity, irreversibility and irreproducible kinetics in applications in food science and bioengineering, with negative impact on texture and stability of food products, and unpredictable delivery of active "payloads." The relationship between coacervation and precipitation is obscure in that coacervates might be intermediates in the formation of precipitates, or else the two phenomena might proceed by different but possibly simultaneous mechanisms. This review will summarize the recent literature on coacervation/precipitation in protein-polyelectrolyte systems for which reports are most abundant, particularly in the context of food science. We present current findings and opinions about the relationship between the two types of phase separation. Results vary considerably depending not only on the protein-polyelectrolyte pairs chosen, but also on conditions including macromolecular concentrations and ionic strength. Nevertheless, we offer some general approaches that could explain a variety of observations.

Precipitate-Coacervate Transformation in Polyelectrolyte-Mixed Micelle Systems.

The polycation/anionic-nonionic mixed micelle, poly(diallyldimethylammonium chloride)-sodium dodecyl sulfate/Triton X-100 (PDADMAC-SDS/TX100), is a model polyelectrolyte-colloid system in that the micellar mole fraction of SDS (Y) controls the micelle surface charge density, thus modulating the polyelectrolyte-colloid interaction. The exquisite temperature dependence of this system provides an important additional variable, controlling both liquid-liquid (L-L) and liquid-solid (L-S) phase separation, both of which are driven by the entropy of small ion release. In order to elucidate these transitions, we applied high-precision turbidimetry (±0.1 %), isothermal titration calorimetry, and epifluorescence microscopy which demonstrates preservation of micelle structure under all conditions. The L-S region at large Y including precipitation displays a remarkable linear, inverse Y-dependence of the L-S transition temperature Ts. In sharp contrast, the critical temperature for L-L coacervation Tφ, shows nearly symmetrical effects of positive and negative deviations in Y from the point of soluble complex neutrality, which is controlled in solution by the micelle charge and the number of micelles bound per polymer chain n (Zcomplex = Zpolymer + nZmicelle). In solid-like states, n no longer signifies the number of micelles bound per polymer chain, since the proximity of micelles inverts the host-guest relationship with each micelle binding multiple PE chains. This intimate binding goes hand-in-hand with the entropy of release of micelle-localized charge-compensating ions whose concentration depends on Y. These ions need not be released in L-L coacervation, but during L-S transition their displacement by PE accounts for the inverse dependence of Ts on micelle charge, Y.