Cell-Penetrating Cross-β Peptide Assemblies with Controlled Biodegradable Properties

Challenge to overcome current limitations of cell-penetrating peptides

The penetration of biological membranes is a prime obstacle for the delivery of pharmaceutical drugs. Cell-penetrating peptide (CPP) is an efficient vehicle that can deliver various cargos across the biological membranes. Since the discovery, CPPs have been rigorously studied to unveil the underlying penetrating mechanism as well as to exploit CPPs for various biomedical applications. This review will focus on the various strategies to overcome current limitations regarding stability, selectivity, and efficacy of CPPs.

Dissolution of a fibrous peptide by terahertz free electron laser

Fibrous peptides such as amyloid fibrils have various roles in biological system, e.g., as causal factor of serious amyloidosis in human and as functional regulator of cell formation in bacteria and eukaryotes. In addition, the fiber-type format is promising as biocompatible scaffold. Therefore, the dissolution method of peptide fibril is potentially useful at many scenes in medical and material fields: as reductive way of pathogenic amyloid, as modification technique of cell structure, and as fabrication tool of biomaterials. However, the fibril structure is generally difficult to be dissociated due to its rigid stacked conformation. Here, we propose a physical engineering technology using terahertz free electron laser (FEL) at far-infrared wavelengths from 70 to 80 μm. Infrared microscopy analysis of the irradiated fibril of calcitonin peptide as a model showed that β-sheet was decreased, and α-helix, turn, and others were increased, compared to those of the fibril before the FEL irradiation. Interestingly, the dissociative effect by the far-infrared laser was remarkable than that by the mid-infrared laser tuned to 6.1 μm that corresponds to amide I. In addition, simple heating at 363 K deformed the fibril state but increased the amount of β-sheet, which was contrast with the action by the FEL, and scanning-electron microscopy and Congo-red staining revealed that the fibril was collapsed power-dependently within a range from 25 to 900 mJ energies supplied with the FEL at 74 μm. It can be considered that irradiation of intense terahertz wave can dissociate fibrous conformation of peptide with little influence of thermal effect.

Synthesis and purification of self-assembling peptide-oligonucleotide conjugates by solid-phase peptide fragment condensation

Peptide-oligonucleotide conjugates (POCs) are interesting molecules as they covalently combine 2 of the most important biomacromolecules. Sometimes, the synthesis of POCs involves unexpected difficulties; however, POCs with self-assembling propensity are even harder to synthesize and purify. Here, we show that solid-phase peptide fragment condensation combined with thiol-maleimide or copper-catalyzed azide-alkyne cycloaddition click chemistries is useful for the syntheses of self-assembling POCs. We describe guidelines for the selection of reactive functional groups and their placement during the conjugation reaction and consider the cost-effectiveness of the reaction. Purification is another important challenge during the preparation of POCs. Our results show that polyacrylamide gel electrophoresis under denaturing conditions is most suitable to recover a high yield of self-assembling POCs. This report provides the first comprehensive study of the preparation of self-assembling POCs, which will lay a foundation for the development of elegant and sophisticated molecular assemblies.

Vesicles mimicking normal and cancer cell membranes exhibit differential responses to the cell-penetrating peptide Pep-1

The cell-penetrating peptide (CPP) Pep-1 presents a great potential in drug delivery due to its intrinsic property to cross plasma membrane. However, its mechanism of entry into the cell remains unresolved. In this study, we compare the selectivity of Pep-1 towards vesicles mimicking normal and cancer cell membranes. The interaction was performed in a wide range of peptide-to-lipid molar ratios using infrared (IR), fluorescence, scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques. At low peptide concentration, fluorescence experiments show that lipid-phosphatidylserine (PS) seems to enable Pep-1 translocation into cancer cell membrane as evidenced by the blue shift of its maximal emission wavelength. DSC data show that Pep-1 induces segregation of lipids. At high peptide concentration, IR data indicate that the interaction of Pep-1 is relatively stronger with normal cell membrane than with cancer cell membrane through the phosphate groups, while the interaction is weaker with normal cell membrane than with cancer cell membrane through the carbonyl groups. TGA and DSC data reveal that vesicles of normal cell membrane are thermally more stable than vesicles of cancer cell membrane. This suggests that the additional lipid PS included in cancer cell membrane has a destabilizing effect on the membrane structure. SEM images reveal that Pep-1 form superstructures including spherical particles and fibrils in the presence of both model membranes. PS seems to enhance peptide transport across cellular membranes. The biophysical techniques in this study provide valuable insights into the properties of CPPs in drug delivery systems.

Balancing hydrophobicity and sequence pattern to influence self-assembly of amphipathic peptides

Amphipathic peptides with alternating polar and nonpolar amino acid sequences efficiently self-assemble into functional β-sheet fibrils as long as the nonpolar residues have sufficient hydrophobicity. For example, the Ac-(FKFE)2 -NH2 peptide rapidly self-assembles into β-sheet bilayer nanoribbons, while Ac-(AKAE)2 -NH2 fails to self-assemble under similar conditions due to the significantly reduced hydrophobicity and β-sheet propensity of Ala relative to Phe. Herein, we systematically explore the effect of substituting only two of the four Ala residues at various positions in the Ac-(AKAE)2 -NH2 peptide with amino acids of increasing hydrophobicity, β-sheet potential, and surface area (including Phe, 1-naphthylalanine (1-Nal), 2-naphthylalanine (2-Nal), cyclohexylalanine (Cha), and pentafluorophenylalanine (F5 -Phe)) on the self-assembly propensity of the resulting sequences. It was found that double Phe variants, regardless of the position of substitution, failed to self-assemble under the conditions used in this study. In contrast, all double 1-Nal and 2-Nal variants readily self-assembled, albeit at differing rates depending on the substitution patterns. To determine whether this was due to hydrophobicity or side chain surface area, we also prepared double Cha and F5 -Phe variant peptides (both side chain groups are more hydrophobic than Phe). Each of these variants also underwent effective self-assembly, with the aromatic F5 -Phe peptides doing so with greater efficiency. These findings provide insight into the role of amino acid hydrophobicity and sequence pattern on self-assembly proclivity of amphipathic peptides and on how targeted substitutions of nonpolar residues in these sequences can be exploited to tune the characteristics of the resulting self-assembled materials.

Calorimetric and Spectroscopic Studies of the Effects of the Cell Penetrating Peptide Pep-1 and the Antimicrobial Peptide Combi-2 on Vesicles Mimicking Escherichia coli Membrane

The objective of this study is to measure and compare the effects of the cell penetrating peptide (CPP) Pep-1 and the antimicrobial peptide (AMP) combi-2 on vesicles of membranes mimicking Escherichia coli (E. coli). To characterize the effects of Pep-1 and combi-2 on E. coli membrane vesicles, a combination of five biophysical techniques was employed: fluorescence, infrared, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) techniques. Upon addition of E. coli membranes, tryptophan fluorescence intensity of Pep-1 showed a sudden blue-shift and decreased in a nonconcentration-dependent manner while the intensity of combi-2 decreased in a concentration-dependent manner, most significantly for a very low peptide-to-lipid ratio of 1:40. Complexes of Pep-1 and combi-2 with E. coli membrane mimicking vesicles having shown a significant blue-shift in fluorescence intensity were then prepared and studied in freeze-dried states. IR results indicate that Pep-1 and combi-2 adopt a major 3₁₀-helix structure in the presence of E. coli membrane mimicking vesicles at low peptide concentration. Pep-1 and combi-2 have a similar effect on E. coli membrane mimicking vesicles at low concentration even though combi-2 is in the interfacial region of the bilayer while Pep-1 is located between the interfacial region and the hydrophobic region. Combi-2 at low concentration acts as a CPP. TGA and DSC results reveal that combi-2 has a stabilizing effect on E. coli at any concentration while Pep-1 stabilizes the E. coli membrane only at high concentration. Both peptides show a preferential interaction with one of the anionic lipids leading to clustering in E. coli membrane. SEM images reveal that Pep-1 and combi-2 form superstructures including fibrils in the presence of E. coli membrane mimicking vesicles. Calorimetric and spectroscopic techniques may be used in a complementary way with imaging techniques to gain more insights into peptide-lipid interactions.