pH sensitive coiled coils: a strategy for enhanced liposomal drug delivery

Weakly acidic pH-responsive liposomal content release induced by histidine-modified agents

Internal stimuli-responsive controlled release from liposomal vesicles is an innovative approach for site-specific delivery of therapeutic drugs. In this study, to enhance the endosomal pH control of drug release from liposomes, a series of histidine-modified pH-sensitive Cn-His (n = 8, 12, 18) agents were designed and utilized as triggers for liposomal content release. The pH-dependent properties of Cn-His-incorporated liposomes were characterized using dynamic light scattering, ζ-potential, and fluorescence spectroscopy. The liposomes maintained a relatively uniform size across all pH conditions. However, the ζ-potential exhibited positive values at endosomal acidic pH levels and neutral or negative values at physiological pH levels. Furthermore, acidic pH-dependent release of both polar content (carboxyfluorescein) and nonpolar content (Nile red) was observed from the Cn-His-incorporated liposomes. Notably, with C12-His as the pH sensitizer, the pH dependence of liposomal content release was significantly evident. This work establishes endosomal pH-controllable liposome platforms, laying the groundwork for developing clinically applicable triggered release formulations.

Preorganized cyclic modules facilitate the self-assembly of protein nanostructures

The rational design of supramolecular assemblies aims to generate complex systems based on the simple information encoded in the chemical structure. Programmable molecules such as nucleic acids and polypeptides are particularly suitable for designing diverse assemblies and shapes not found in nature. Here, we describe a strategy for assembling modular architectures based on structurally and covalently preorganized subunits. Cyclization through spontaneous self-splicing of split intein and coiled-coil dimer-based interactions of polypeptide chains provide structural constraints, facilitating the desired assembly. We demonstrate the implementation of a strategy based on the preorganization of the subunits by designing a two-chain coiled-coil protein origami (CCPO) assembly that adopts a tetrahedral topology only when one or both subunit chains are covalently cyclized. Employing this strategy, we further design a 109 kDa trimeric CCPO assembly comprising 24 CC-forming segments. In this case, intein cyclization was crucial for the assembly of a concave octahedral scaffold, a newly designed protein fold. The study highlights the importance of preorganization of building modules to facilitate the self-assembly of higher-order supramolecular structures.

Peptide-lipid hybrid vesicles with stimuli-responsive phase separation for controlled membrane functions

A disulfide-tethered peptide-lipid conjugate self-assembled into a homogeneously distributed peptide-lipid hybrid vesicle. Upon dithiothreitol treatment, the homogeneous peptide-lipid membrane spontaneously divided into lipid-rich and peptide-rich domains, while the vesicle retained its size and shape. Membrane phase separation enhanced temperature-dependent cargo release.

Self-assembling nanocarriers from engineered proteins: Design, functionalization, and application for drug delivery

Self-assembling proteins are valuable building blocks for constructing drug nanocarriers due to their self-assembly behavior, monodispersity, biocompatibility, and biodegradability. Genetic and chemical modifications allow for modular design of protein nanocarriers with effective drug encapsulation, targetability, stimuli responsiveness, and in vivo half-life. Protein nanocarriers have been developed to deliver various therapeutic molecules including small molecules, proteins, and nucleic acids with proven in vitro and in vivo efficacy. This article reviews recent advances in protein nanocarriers that are not derived from natural protein nanostructures, such as protein cages or virus like particles. The protein nanocarriers described here are self-assembled from rationally or de novo designed recombinant proteins, as well as recombinant proteins complexed with other biomolecules, presenting properties that are unique from those of natural protein carriers. Design, functionalization, and therapeutic application of protein nanocarriers will be discussed.

Temperature-sensitive polymers to promote heat-triggered drug release from liposomes: Towards bypassing EPR

Heat-triggered drug release from temperature-sensitive nanocarriers upon the application of mild hyperthermia is a promising approach to achieve site-specific delivery of drugs. The combination of mild hyperthermia (41-42 °C) and temperature-sensitive liposomes (TSL) that undergo lipid phase-transition and drug release has been studied extensively and has shown promising therapeutic outcome in a variety of animal tumor models as well as initial indications of success in humans. Sensitization of liposomes to mild hyperthermia by means of exploiting the thermal behavior of temperature-sensitive polymers (TSP) provides novel opportunities. Recently, TSP-modified liposomes (TSPL) have shown potential for enhancing tumor-directed drug delivery, either by triggered drug release or by triggered cell interactions in response to heat. In this review, we describe different classes of TSPL, and analyze and discuss the mechanisms and kinetics of content release from TSPL in response to local heating. In addition, the impact of lipid composition, polymer and copolymer characteristics, serum components and PEGylation on the mechanism of content release and TSPL performance is addressed. This is done from the perspective of rationally designing TSPL, with the overall goal of conceiving efficient strategies to increase the efficacy of TSPL plus hyperthermia to improve the outcome of targeted anticancer therapy.

Strategies for Liposome Drug Delivery Systems to Improve Tumor Treatment Efficacy

In the advancement of tumor therapy, in addition to the search for new antitumor compounds, the development of nano-drug delivery systems has opened up new pathways for tumor treatment by addressing some of the limitations of traditional drugs. Liposomes have received much attention for their high biocompatibility, low toxicity, high inclusivity, and improved drug bioavailability. They are one of the most studied nanocarriers, changing the size and surface characteristics of liposomes to better fit the tumor environment by taking advantage of the unique pathophysiology of tumors. They can also be designed as tumor targeting drug delivery vehicles for the precise delivery of active drugs into tumor cells. This paper reviews the current development of liposome formulations, summarizes the characterization methods of liposomes, and proposes strategies to improve the effectiveness of tumor treatment. Finally, it provides an outlook on the challenges and future directions of the field. Graphical abstract.

Coiled coil-based therapeutics and drug delivery systems

Coiled coils are characterized by an arrangement of two or more α-helices into a superhelix and one of few protein motifs where the sequence-to-structure relationship to a large extent have been decoded and understood. The abundance of both natural and de novo designed coil coils provides a rich molecular toolbox for self-assembly of elaborate bespoke molecular architectures, nanostructures, and materials. Leveraging on the numerous possibilities to tune both affinities and preferences for polypeptide oligomerization, coiled coils offer unique possibilities to design modular and dynamic assemblies that can respond in a predictable manner to biomolecular interactions and subtle physicochemical cues. In this review, strategies to use coiled coils in design of novel therapeutics and advanced drug delivery systems are discussed. The applications of coiled coils for generating drug carriers and vaccines, and various aspects of using coiled coils for controlling and triggering drug release, and for improving drug targeting and drug uptake are described. The plethora of innovative coiled coil-based molecular systems provide new knowledge and techniques for improving efficacy of existing drugs and can facilitate development of novel therapeutic strategies.

Design and Construction of pH-Selective Self-Lytic Liposome System

Liposomes are well-investigated drug or gene delivery vehicles for chemotherapy, used by taking advantage of their biocompatibility and biodegradability. A central question on the construction of intracellular liposomal delivery systems is to entrap the liposomes of interest in the highly acidic and proteolytic endosomal environment. In the other words, it is essential that the liposomes release a therapeutic drug into the cytosol before they are degraded in the endosome. As a strategy to enhance the endosome escape, the self-lytic liposomes with acidic pH-selective membrane active polypeptide are considered highly effective. Here, an acidic pH-selective membrane-lytic polypeptide (LPE) and its retro isomer (rLPE) were designed, and then their membrane-lytic activities against EggPC liposomes were determined. It was noticed that the rLPE polypeptide showed an increase in activity compared with the LPE polypeptide. Furthermore, the rLPE polypeptide was conjugated to liposomes via a flexible Gly-Gly-Gly-Gly linker to facilitate the pH-selective content release. The rLPE anchoring liposomes exhibited distinctly different contents release behavior at physiological and endosomal pHs, namely typical contents release from liposomes was positively observed at acidic pH range. The overarching goal of this paper is to develop efficient pH-selective therapeutic delivery systems by using our findings.

Tuning intermediate filament mechanics by variation of pH and ion charges

The cytoskeleton is formed by three types of filamentous proteins - microtubules, actin filaments, and intermediate filaments (IFs) - and enables cells to withstand external and internal forces. Vimentin is the most abundant IF protein in humans and assembles into 10 nm diameter filaments with remarkable mechanical properties, such as high extensibility and stability. It is, however, unclear to which extent these properties are influenced by the electrostatic environment. Here, we study the mechanical properties of single vimentin filaments by employing optical trapping combined with microfluidics. Force-strain curves, recorded at varying ion concentrations and pH values, reveal that the mechanical properties of single vimentin IFs are influenced by pH and ion concentration. By combination with Monte Carlo simulations, we relate these altered mechanics to electrostatic interactions of subunits within the filaments. We thus suggest possible mechanisms that allow cells to locally tune their stiffness without remodeling the entire cytoskeleton.

Rapid release from near-infrared polymer loaded liposomes for photothermal and chemo-combined therapy

A novel near-infrared conjugated polymer, PDPPT, was designed and encapsulated in PEG-protected liposomes to photothermally accelerate the release of DOX.

Self-assembly of Functional Nanostructures by Short Helical Peptide Building Blocks

The self-assembly of short peptide building blocks into well-ordered nanostructures is a key direction in bionanotechnology. The formation of β -sheet organizations by short peptides is well explored, leading to the development of a wide range of functional assemblies. Likewise, many natural proteinaceous materials, such as silk and amyloid fibrils, are based on β-sheet structures. In contrast, collagen, the most abundant protein in mammals, is based on helical arrangement. Similar to β-sheet structures, short helical peptides have been recently discovered to possess a diverse set of functionalities with the potential to fabricate artificial self-assembling materials. Here, we outline the functional roles of self-assembled nanostructures formed by short helical peptides and their potential as artificial materials. We focus on the association between self-assembled mesoscale structures and their material function and demonstrate the way by which this class of building blocks bears the potential for diverse applications, such as the future fabrication of smart devices.

pH-Sensitive Coiled-Coil Peptide-Cross-Linked Hyaluronic Acid Nanogels: Synthesis and Targeted Intracellular Protein Delivery to CD44 Positive Cancer Cells

The clinical translation of protein drugs that act intracellularly is limited by the absence of safe and efficient intracellular protein delivery vehicles. Here, pH-sensitive coiled-coil peptide-cross-linked hyaluronic acid nanogels (HA-cNGs) were designed and investigated for targeted intracellular protein delivery to CD44 overexpressing MCF-7 breast cancer cells. HA-cNGs were obtained with a small size of 176 nm from an equivalent mixture of hyaluronic acid conjugates with GY(EIAALEK)₃GC (E3) and GY(KIAALKE)₃GC (K3) peptides, respectively, at pH 7.4 by nanoprecipitation. Circular dichroism (CD) proved the formation of coiled-coil structures between E3 and K3 peptides at pH 7.4 while fast uncoiling at pH 5.0. HA-cNGs showed facile loading of cytochrome C (CC) and greatly accelerated CC release under mild acidic conditions (18.4%, 76.8%, and 91.4% protein release in 24 h at pH 7.4, 6.0, and 5.0, respectively). Confocal microscopy and flow cytometry displayed efficient internalization of CC-loaded HA-cNGs and effective endosomal escape of CC in MCF-7 cancer cells. Remarkably, HA-cNGs loaded with saporin, a ribosome inactivating protein, exhibited significantly enhanced apoptotic activity to MCF-7 cells with a low IC₅₀ of 12.2 nM. These coiled-coil peptide-cross-linked hyaluronic acid nanogels have appeared as a simple and multifunctional platform for efficient intracellular protein delivery.

Coiled coil protein origami: from modular design principles towards biotechnological applications

This review illustrates the current state in designing coiled-coil-based proteins with an emphasis on coiled coil protein origami structures and their potential.

Stimuli-responsive liposomes for drug delivery

The ultimate goal of drug delivery is to increase the bioavailability and reduce the toxic side effects of the active pharmaceutical ingredient (API) by releasing them at a specific site of action. In the case of antitumor therapy, association of the therapeutic agent with a carrier system can minimize damage to healthy, nontarget tissues, while limit systemic release and promoting long circulation to enhance uptake at the cancerous site due to the enhanced permeation and retention effect (EPR). Stimuli-responsive systems have become a promising way to deliver and release payloads in a site-selective manner. Potential carrier systems have been derived from a wide variety of materials, including inorganic nanoparticles, lipids, and polymers that have been imbued with stimuli-sensitive properties to accomplish triggered release based on an environmental cue. The unique features in the tumor microenvironment can serve as an endogenous stimulus (pH, redox potential, or unique enzymatic activity) or the locus of an applied external stimulus (heat or light) to trigger the controlled release of API. In liposomal carrier systems triggered release is generally based on the principle of membrane destabilization from local defects within bilayer membranes to effect release of liposome-entrapped drugs. This review focuses on the literature appearing between November 2008-February 2016 that reports new developments in stimuli-sensitive liposomal drug delivery strategies using pH change, enzyme transformation, redox reactions, and photochemical mechanisms of activation. WIREs Nanomed Nanobiotechnol 2017, 9:e1450. doi: 10.1002/wnan.1450 For further resources related to this article, please visit the WIREs website.

A pH Switch for β-Sheet Protein Folding

Protein design advancements have led to biotechnological strategies based on more stable and more specific structures. Herein we present a 6-residue sequence (HPATGK) that acts as a stable structure-nucleating turn at physiological and higher pH but is notably unfavorable for chain direction reversal at low pH. When placed into the turn of a β-sheet, this leads to a pH switch of folding. Using a standard 3-stranded β-sheet model, the WW domain, it was found that the pH switch sequence insertion caused minimal change at pH 8 but a ca. 50 °C drop in the melting temperature (Tm ) was observed at pH 2.5: ΔΔGF ≥11.3 kJ mol-1 . Using the strategies demonstrated in this article, the redesign of β-sheets to contain a global, or local, pH-dependent conformational switch should be possible.

Gold nanoshell coated thermo-pH dual responsive liposomes for resveratrol delivery and chemo-photothermal synergistic cancer therapy

Stimuli-responsive drug delivery and release have a great significance in cancer therapy. Herein, a multifunctional responsive drug carrier was designed and developed by loading resveratrol (Res) in chitosan (CTS) modified liposomes, and coated by gold nanoshells (GNS@CTS@Res-lips). The resultant GNS@CTS@Res-lips possess broad near-infrared (NIR) absorbance, high capability, stability, and also high photothermal conversion ability for efficient photothermal therapy (PTT) applications. In addition, the GNS@CTS@Res-lips exhibit the on-demand pH/photothermal-sensitive drug release, and a high loading capacity of Res. Under NIR laser irradiation, the drug delivery system could significantly enhance the cellular uptake of drugs. More importantly, compared to the single chemotherapy or PTT, the carriers with NIR irradiation displayed a higher therapeutic effect for HeLa cells. Therefore, the GNS@CTS@Res-lips with a combination of chemotherapy and PTT will show great potential for application in cancer therapy.

A dual functional liposome specifically targets melanoma cells through integrin and ephrin receptors

A novel bi-functional liposome delivers docetaxel specifically to melanoma cancer cells targeting integrin (α₄β₁) and ephrin (EphA2) receptors and enhances the efficacy of docetaxel.