Temperature- and composition-dependent conformational transitions of amphipathic peptide-phospholipid nanodiscs

Calcium ions do not influence the Aβ(25-35) triggered morphological changes of lipid membranes

We have studied the effect of calcium ions (Ca2+) at various concentrations on the structure of lipid vesicles in the presence of amyloid-beta peptide Aβ(25-35). In particular, we have investigated the influence of calcium ions on the formation of recently documented bicelle-like structures (BLSs) emerged as a result of Aβ(25-35) triggered membrane disintegration. First, we have shown by using small-angle X-ray and neutron scattering that peptide molecules rigidify the lipid bilayer of gel phase DPPC unilamellar vesicles (ULVs), while addition of the calcium ions to the system hinders this effect of Aβ(25-35). Secondly, the Aβ(25-35) demonstrates a critical peptide concentration at which the BLSs reorganize from ULVs due to heating and cooling the samples through the lipid main phase transition temperature (Tm). However, addition of calcium ions does not affect noticeably the Aβ-induced formation of BLSs and their structural parameters, though the changes in peptide's secondary structure, e.g. the increased α-helix fraction, has been registered by circular dichroism spectroscopy. Finally, according to 31P nuclear magnetic resonance (NMR) measurements, calcium ions do not affect the lipid-peptide arrangement in BLSs and their ability to align in the magnetic field of NMR spectrometer. The influences of various concentrations of calcium ions on the lipid-peptide interactions may prove biologically important because their local concentrations vary widely in in-vivo conditions. In the present work, calcium ions were investigated as a possible tool aimed at regulating the lipid-peptide interactions that demonstrated the disruptive effect of Aβ(25-35) on lipid membranes.

Arrangement of lipid vesicles and bicelle-like structures formed in the presence of Aβ(25-35) peptide

Our complementary experimental data and molecular dynamics (MD) simulations results reveal the structure of previously observed lipid bicelle-like structures (BLSs) formed in the presence of amyloid-beta peptide Aβ(25-35) below the main phase transition temperature (Tm) of saturated phosphatidylcholine lipids and small unilamellar vesicles (SUVs) above this temperature. First, we show by using solid-state 31P nuclear magnetic resonance (NMR) spectroscopy that our BLSs being in the lipid gel phase demonstrate magnetic alignment along the magnetic field of NMR spectrometer and undergo a transition to SUVs in the lipid fluid phase when heated through the Tm. Secondly, thanks to the BLS alignment we present their lipid structure. Lipids are found located not only in the flat bilayered part but also around its perimeter, which is corroborated by the results of coarse-grained (CG) MD simulations. Finally, peptides appear to mix randomly with lipids in SUVs while assuming predominantly unordered secondary structures revealed by circular dichroism (CD), Raman spectroscopy, and all-atom MD simulations. Importantly, the former is changing little when the system undergoes morphological transitions between BLSs and SUVs. Our structural results then offer a platform for studying and understanding mechanisms of morphological transformations caused by the disruptive effect of amyloid-beta peptides on the lipid bilayer.

Characterization of nanodisc-forming peptides for membrane protein studies

Lipid-bilayer nanodiscs provide a stable, native-like membrane environment for the functional and structural studies of membrane proteins and other membrane-binding molecules. Peptide-based nanodiscs having unique properties are developed for membrane protein studies and other biological applications. While the self-assembly process rendering the formation of peptide-nanodiscs is attractive, it is important to understand the stability and suitability of these nanodisc systems for membrane protein studies. In this study, we investigated the nanodiscs formation by the anti-inflammatory and tumor-suppressing peptide AEM28. AEM28 is a chimeric peptide containing a cationic-rich heparan sulfate proteoglycan- (HSPG)-binding domain from human apolipoprotein E (hapoE) (141-150) followed by the 18A peptide's amino acid sequence. AEM28-based nanodiscs made with different types of lipids were characterized using various biophysical techniques and compared with the nanodiscs formed using 2F or 4F peptides. Variable temperature dynamic light-scattering and 31P NMR experiments indicated the fusion and size heterogeneity of nanodiscs at high temperatures. The suitability of AEM28 and Ac-18A-NH2- (2F-) based nanodiscs for studying membrane proteins is demonstrated by reconstituting and characterizing a drug-metabolizing enzyme, cytochrome-P450 (CYP450), or the redox complex CYP450-CYP450 reductase. AEM28 and 2F were also tested for their efficacies in solubilizing E. coli membranes to understand the possibility of using them for detergent-free membrane protein isolation. Our experimental results suggest that AEM28 nanodiscs are suitable for studying membrane proteins with a net positive charge, whereas 2F-based nanodiscs are compatible with any membrane proteins and their complexes irrespective of their charge. Furthermore, both peptides solubilized E. coli cell membranes, indicating their use in membrane protein isolation and other applications related to membrane solubilization.

Peptide-containing nanoformulations: Skin barrier penetration and activity contribution

Transdermal drug delivery presents a less invasive pathway, circumventing the need to pass through the gastrointestinal tract and liver, thereby reducing drug breakdown, initial metabolism, and gastrointestinal discomfort. Nevertheless, the unique composition and dense structure of the stratum corneum present a significant barrier to transdermal delivery. This article presents an overview of the current developments in peptides and nanotechnology to address this challenge. Initially, we sum up peptide-containing nanoformulations for transdermal drug delivery, examining them through the lenses of both inorganic and organic materials. Particular emphasis is placed on the diverse roles that peptides play within these nanoformulations, including conferring functionality upon nanocarriers and enhancing the biological efficacy of drugs. Subsequently, we summarize innovative strategies for enhancing skin penetration, categorizing them into passive and active approaches. Lastly, we discuss the therapeutic potential of peptide-containing nanoformulations in addressing a range of diseases, drawing insights from the biological activities and functions of peptides. Furthermore, the challenges hindering clinical translation are also discussed, providing valuable insights for future advancements in transdermal drug delivery.

Enhancing the stability and homogeneity of non-ionic polymer nanodiscs by tuning electrostatic interactions

The nanodisc technology is increasingly used for structural studies on membrane proteins and drug delivery. The development of synthetic polymer nanodiscs and the recent discovery of non-ionic inulin-based polymers have significantly broadened the scope of nanodiscs. While the lipid exchange and size flexibility properties of the self-assembled polymer-based nanodiscs are valuable for various applications, the non-ionic polymer nanodiscs are remarkably unique in that they enable the reconstitution of any protein, protein-protein complexes, or drugs irrespective of their charge. However, the non-ionic nature of the belt could influence the stability and size homogeneity of inulin-based polymer nanodiscs. In this study, we investigate the size stability and homogeneity of nanodiscs formed by non-ionic lipid-solubilizing polymers using different biophysical methods. Polymer nanodiscs containing zwitterionic DMPC and different ratios of DMPC:DMPG lipids were made using anionic SMA-EA or non-ionic pentyl-inulin polymers. Non-ionic polymer nanodiscs made using zwitterionic DMPC lipids produced a very broad elution profile on SEC due to their instability in the column, thus affecting sample monodispersity which was confirmed by DLS experiments that showed multiple peaks. However, the inclusion of anionic DMPG lipids improved the stability as observed from SEC and DLS profiles, which was further confirmed by TEM images. Whereas, anionic SMA-EA-based DMPC-nanodiscs showed excellent stability and size homogeneity when solubilizing zwitterionic lipids. The stability of DMPC:DMPG non-ionic polymer nanodiscs is attributed to the inter-nanodisc repulsion by the anionic-DMPG that prevents the uncontrolled collision and fusion of nanodiscs. Thus, the reported results demonstrate the use of electrostatic interactions to tune the solubility, stability, and size homogeneity of non-ionic polymer nanodiscs which are important features for enabling functional and atomic-resolution structural studies of membrane proteins, other lipid-binding molecules, and water-soluble biomolecules including cytosolic proteins, nucleic acids and metabolites.

Synthesis of Erythrocyte Nanodiscs for Bacterial Toxin Neutralization

Nanodiscs are a compelling nanomedicine platform due to their ultrasmall size and distinct disc shape. Current nanodisc formulations are made primarily with synthetic lipid bilayers and proteins. Here, we report a cellular nanodisc made with human red blood cell (RBC) membrane (denoted "RBC-ND") and show its effective neutralization against bacterial toxins. In vitro, RBC-ND neutralizes the hemolytic activity and cytotoxicity caused by purified α-toxin or complex whole secreted proteins (wSP) from methicillin-resistant Staphylococcus aureus bacteria. In vivo, RBC-ND confers significant survival benefits for mice intoxicated with α-toxin or wSP in both therapeutic and prevention regimens. Moreover, RBC-ND shows good biocompatibility and biosafety in vivo. Overall, RBC-ND distinguishes itself by inheriting the biological functions of the source cell membrane for bioactivity. The design strategy of RBC-ND can be generalized to other types of cell membranes for broad applications.

Amphipathic peptide-phospholipid nanofibers: Kinetics of fiber formation and molecular transfer between assemblies

Understanding the kinetics of nano-assembly formation is important to elucidate the biological processes involved and develop novel nanomaterials with biological functions. In the present study, we report the kinetic mechanisms of nanofiber formation from a mixture of phospholipids and the amphipathic peptide 18A[A11C], carrying cysteine substitution of the apolipoprotein A-I-derived peptide 18A at residue 11. 18A[A11C] with acetylated N-terminus and amidated C-terminus can associate with phosphatidylcholine to form fibrous aggregates at neutral pH and lipid-to-peptide molar ratio of ∼1, although the reaction pathways of self-assembly remain unclear. Here, the peptide was added to giant 1-palmitoyl-2-oleoyl phosphatidylcholine vesicles to monitor nanofiber formation under fluorescence microscopy. The peptide initially solubilized the lipid vesicles into particles smaller than the resolution of optical microscope, and fibrous aggregates appeared subsequently. Transmission electron microscopy and dynamic light scattering analyses revealed that the vesicle-solubilized particles were spherical or circular, measuring ∼10-20 nm in diameter. The rate of nanofiber formation of 18A with 1,2-dipalmitoyl phosphatidylcholine from the particles was proportional to the square of lipid-peptide concentration in the system, suggesting that the association of particles, accompanied by conformational changes, was the rate-limiting step. Moreover, molecules in the nanofibers could be transferred between aggregates faster than those in the lipid vesicles. These findings provide useful information for the development and control of nano-assembling structures using peptides and phospholipids.

A comparative characterisation of commercially available lipid-polymer nanoparticles formed from model membranes

From the discovery of the first membrane-interacting polymer, styrene maleic-acid (SMA), there has been a rapid development of membrane solubilising polymers. These new polymers can solubilise membranes under a wide range of conditions and produce varied sizes of nanoparticles, yet there has been a lack of broad comparison between the common polymer types and solubilising conditions. Here, we present a comparative study on the three most common commercial polymers: SMA 3:1, SMA 2:1, and DIBMA. Additionally, this work presents, for the first time, a comparative characterisation of polymethacrylate copolymer (PMA). Absorbance and dynamic light scattering measurements were used to evaluate solubilisation across key buffer conditions in a simple, adaptable assay format that looked at pH, salinity, and divalent cation concentration. Lipid-polymer nanoparticles formed from SMA variants were found to be the most susceptible to buffer effects, with nanoparticles from either zwitterionic DMPC or POPC:POPG (3:1) bilayers only forming in low to moderate salinity (< 600 mM NaCl) and above pH 6. DIBMA-lipid nanoparticles could be formed above a pH of 5 and were stable in up to 4 M NaCl. Similarly, PMA-lipid nanoparticles were stable in all NaCl concentrations tested (up to 4 M) and a broad pH range (3-10). However, for both DIBMA and PMA nanoparticles there is a severe penalty observed for bilayer solubilisation in non-optimal conditions or when using a charged membrane. Additionally, lipid fluidity of the DMPC-polymer nanoparticles was analysed through cw-EPR, showing no cooperative gel-fluid transition as would be expected for native-like lipid membranes.

Polymer-Nanodiscs as a Novel Alignment Medium for High-Resolution NMR-Based Structural Studies of Nucleic Acids

Residual dipolar couplings (RDCs) are increasingly used for high-throughput NMR-based structural studies and to provide long-range angular constraints to validate and refine structures of various molecules determined by X-ray crystallography and NMR spectroscopy. RDCs of a given molecule can be measured in an anisotropic environment that aligns in an external magnetic field. Here, we demonstrate the first application of polymer-based nanodiscs for the measurement of RDCs from nucleic acids. Polymer-based nanodiscs prepared using negatively charged SMA-EA polymer and zwitterionic DMPC lipids were characterized by size-exclusion chromatography, 1H NMR, dynamic light-scattering, and 2H NMR. The magnetically aligned polymer-nanodiscs were used as an alignment medium to measure RDCs from a 13C/15N-labeled fluoride riboswitch aptamer using 2D ARTSY-HSQC NMR experiments. The results showed that the alignment of nanodiscs is stable for nucleic acids and nanodisc-induced RDCs fit well with the previously determined solution structure of the riboswitch. These results demonstrate that SMA-EA-based lipid-nanodiscs can be used as a stable alignment medium for high-resolution structural and dynamical studies of nucleic acids, and they can also be applicable to study various other biomolecules and small molecules in general.

End-Sealing of Peptide Nanotubes by Cationic Amphiphilic Polypeptides and Their Salt-Responsive Accordion-like Opening and Closing Behavior

One strategy to prepare phase-separated co-assembly is to use the existing assembly as a platform to architect structures. For this purpose, the edge of a sheet or tube-shaped molecular assembly, which is less hydrophilic than the bulk region can become a starting point to build assembly units to realize more complex structures. In this study, we succeeded in preparing rod-shaped nanocapsules with previously unachieved sealing efficiency (>99%) by fine-tuning the properties of cationic amphiphilic polypeptides to seal the ends of neutral charge nanotubes. In addition, we demonstrated the nanocapsule's reversible responsiveness to salt. In high salt concentrations, a decrease in electrostatic repulsion between cationic polypeptides caused tearing and shrinking of the nanocapsule's sealing dome, which resulted in an opened nanotube. On the other hand, when salt was removed, the electrostatic repulsion among the cationic peptides localizing on the edge of opened nanocapsules was recovered, and the sealing membrane swelled up like an accordion to create a distance between the peptides, resulting in the restoration of the seal.

Improvement of Thermal Stability of Amphipathic Peptide-Phospholipid Nanodiscs via Lateral Association of α-Helices by Disulfide Cross-Linking

Amphipathic α-helical peptides have been reported to form discoidal particles or nanodiscs with phospholipids, in which a lipid bilayer patch is encircled by peptides. Peptide-based nanodiscs have broad applicability because of their ease of preparation, size flexibility, and structural plasticity. We previously revealed that the nanodiscs formed by apolipoprotein-A-I-derived peptide 18A showed temperature-dependent structural destabilization above the gel-to-liquid-crystalline phase transition temperature of the lipid bilayer. It has been suggested that this destabilization is due to the migration of peptides bound to the edge of the discs to the bilayer surface. In this study, we designed a peptide that could stabilize nanodisc structures against the phase transition of lipid bilayers by disulfide cross-linking of peptides. An 18A-dimer cross-linked by a proline residue, 37pA (Ac-18A-P-18A-CONH₂), also showed thermal destabilization of nanodiscs like 18A. However, cross-linking the sides of the two α-helices of the cysteine-substituted analogue 37pA-C2 with disulfide bonds led to the formation of nanodiscs that were more stable to temperature changes. This stabilizing effect was mainly due to the formation of a cyclic 37pA-C2 monomer by intramolecular disulfide cross-linking. These results suggest that the lateral association of two α-helices, which is the basis of the double-belt structure, is an important factor for the implementation of stable nanodiscs. The results of this study will help in development of more stable nanoparticles with membrane proteins in the future.

Low Drug Loading Hampers the Clinical Translation of Peptide Drugs-Containing Metered-Dose Inhalers

Peptide-based drugs have attracted extensive attention from the medical and pharmaceutical industry because of their relatively high safety and efficacy. However, most of the peptide drugs approved are administrated by injection, which can easily cause poor patient compliance. In this circumstance, pulmonary administration as an alternative to injection administration can not only avoid the above issue but also accelerate the absorption rate of peptide drugs and improve bioavailability. Among the pulmonary delivery systems available on the market, metered-dose inhalers (MDIs) have emerged as appealing candidates for pulmonary delivery systems with clinical translational value, owing to their many merits, including portable, easy-to-operate, and cost-effective properties. Nevertheless, the industrialization of peptide drugs-containing MDIs encounters a bottleneck of low drug loading, owing to the incompatibility between the propellant and the peptide drugs, which cannot be effectively overcome by the current carrier particle encapsulation strategy. Herein, we put forward the following strategies: (1) To screen amphiphilic materials with high surface activity and strong interaction with peptide drugs; (2) To construct a chemical connection between peptide drugs and amphiphilic substances; (3) To optimize the cosolvent for dispersing peptide drugs. We suppose these strategies have the potential to defeat the bottleneck problem and provide a new idea for the industrialization of peptide drugs-containing MDIs.

Lipid Nanodiscs for High-Resolution NMR Studies of Membrane Proteins

Membrane proteins (MPs) play essential roles in numerous cellular processes. Because around 70% of the currently marketed drugs target MPs, a detailed understanding of their structure, binding properties, and functional dynamics in a physiologically relevant environment is crucial for a more detailed understanding of this important protein class. We here summarize the benefits of using lipid nanodiscs for NMR structural investigations and provide a detailed overview of the currently used lipid nanodisc systems as well as their applications in solution-state NMR. Despite the increasing use of other structural methods for the structure determination of MPs in lipid nanodiscs, solution NMR turns out to be a versatile tool to probe a wide range of MP features, ranging from the structure determination of small to medium-sized MPs to probing ligand and partner protein binding as well as functionally relevant dynamical signatures in a lipid nanodisc setting. We will expand on these topics by discussing recent NMR studies with lipid nanodiscs and work out a key workflow for optimizing the nanodisc incorporation of an MP for subsequent NMR investigations. With this, we hope to provide a comprehensive background to enable an informed assessment of the applicability of lipid nanodiscs for NMR studies of a particular MP of interest.