Protein Binding and Release by Polymeric Cell-Penetrating Peptide Mimics

There is significant potential in exploiting antibody specificity to develop new therapeutic treatments. However, intracellular protein delivery is a paramount challenge because of the difficulty in transporting large, polar molecules across cell membranes. Cell-penetrating peptide mimics (CPPMs) are synthetic polymers that are versatile materials for intracellular delivery of biological molecules, including nucleic acids and proteins, with superior performance compared to their natural counterparts and commercially available peptide-based reagents. Studies have demonstrated that noncovalent complexation with these synthetic carriers is necessary for the delivery of proteins, but the fundamental interactions dominating CPPM-protein complexation are not well understood. Beyond these interactions, the mechanism of release for many noncovalent carriers is not well established. Herein, interactions expected to be critical in CPPM-protein binding and unbinding were explored, including hydrogen bonding, electrostatics, and hydrophobic interactions. Despite the guanidinium-rich functionality of these polymeric carriers, hydrogen bonding was shown not to be a dominant interaction in CPPM-protein binding. Fluorescence quenching assays were used to decouple the effect of electrostatic and hydrophobic interactions between amphiphilic CPPMs and proteins. Furthermore, by conducting competition assays with other proteins, unbinding of protein cargoes from CPPM-protein complexes was demonstrated and provided insight into mechanisms of protein release. This work offers understanding toward the role of carrier and cargo binding and unbinding in intracellular outcomes. In turn, an improved fundamental understanding of noncovalent polymer-protein complexation will enable more effective methods for intracellular protein delivery.

Utilization of an Intracellular Calcium Mobilization Assay for the Screening of Transduced FK506-Binding Proteins

FK506-binding proteins (FKBPs) belong to the immunophilin family and are linked to various disease states, including the inflammatory response. The inhibition of cytokine and chemokine expression in addition to positive effects of FKBPs on corneal inflammation in animal models suggests that they may be used for ophthalmic delivery in the treatment of dry eye disease. To pass the effective barriers protecting eye tissues, testing the transduction domains of FKBPs is essential. However, monitoring their transduction efficiencies is not a simple task. The quantitative measurement of FKBP interactions was performed using a cell model with a specific G protein-coupled receptor, as FKBPs had been known to act at the inositol 1,4,5-trisphosphate receptor (IP₃R) leading to the inhibition of intracellular calcium mobilization. Because of its luminescence amplitude and stability, human urotensin II receptor was expressed in aequorin parental cells to measure the action of selected FKBPs. This luminescence-based functional assay platform exhibited a high signal-to-background ratio of more than 100 and a Z' factor at 0.6204. As expected, changes in the sequence of the transduction domain affected the function of the FKBPs. The intracellular calcium mobilization assay with selected FKBPs represented a robust and reliable platform to screen initial candidates. Although the precise nature of the control that FKBPs exert on the IP₃R is uncertain, this approach can be used to develop innovative anti-inflammatory treatments for dry eye disease by optimizing protein transduction domain sequences.

Sequence segregation improves non-covalent protein delivery

The impermeability of the plasma membrane towards large, hydrophilic biomolecules is a major obstacle in their use and development against intracellular targets. To overcome such limitations, protein transduction domains (PTDs) have been used as protein carriers, however they often require covalent fusion to the protein for efficient delivery. In an effort to develop more efficient and versatile biological vehicles, a series of PTD-inspired polyoxanorbornene-based synthetic mimics with identical chemical compositions but different hydrophobic/hydrophilic segregation were used to investigate the role of sequence segregation on protein binding and uptake into Jurkat T cells and HEK293Ts. This series was composed of a strongly segregated block copolymer, an intermediately segregated gradient copolymer, and a non-segregated homopolymer. Among the series, the block copolymer maximized both protein binding and translocation efficiencies, closely followed by the gradient copolymer, resulting in two protein transporter molecules more efficacious than currently commercially available agents. These two polymers were also used to deliver the biologically active Cre recombinase into a loxP-reporter T cell line. Since exogenous Cre must reach the nucleus and retain its activity to induce gene recombination, this in vitro experiment better exemplifies the broad applicability of this synthetic system. This study shows that increasing segregation between hydrophobic and cationic moieties in these polymeric mimics improves non-covalent protein delivery, providing crucial design parameters for the creation of more potent biological delivery agents for research and biomedical applications.

In vitro and in vivo delivery of therapeutic proteins using cell penetrating peptides

The failure of proteins to penetrate mammalian cells or target tumor cells restricts their value as therapeutic tools in a variety of diseases such as cancers. Recently, protein transduction domains (PTDs) or cell penetrating peptides (CPPs) have been shown to promote the delivery of therapeutic proteins or peptides into live cells. The successful delivery of proteins mainly depends on their physicochemical properties. Although, linear cell penetrating peptides are one of the most effective delivery vehicles; but currently, cyclic CPPs has been developed to potently transport bioactive full-length proteins into cells. Up to now, several small protein transduction domains from viral proteins including Tat or VP22 could be fused to other peptides or proteins to entry them in various cell types at a dose-dependent approach. A major disadvantage of PTD-fusion proteins is primary uptake into endosomal vesicles leading to inefficient release of the fusion proteins into the cytosol. Recently, non-covalent complex formation (Chariot) between proteins and CPPs has attracted a special interest to overcome some delivery limitations (e.g., toxicity). Many preclinical and clinical trials of CPP-based delivery are currently under evaluation. Generally, development of more efficient protein transduction domains would significantly increase the potency of protein therapeutics. Moreover, the synergistic or combined effects of CPPs with other delivery systems for protein/peptide drug delivery would promote their therapeutic effects in cancer and other diseases. In this review, we will describe the functions and implications of CPPs for delivering the therapeutic proteins or peptides in preclinical and clinical studies.

Studying Protein Function and the Role of Altered Protein Expression by Antibody Interference and Three-dimensional Reconstructions

A strict management of protein expression is not only essential to every organism alive, but also an important strategy to investigate protein functions in cellular models. Therefore, recent research invented different tools to target protein expression in mammalian cell lines or even animal models, including RNA and antibody interference. While the first strategy has gathered much attention during the past two decades, peptides  mediating a translocation of antibody cargos across cellular membranes and into cells, obtained much less interest. In this publication, we provide a detailed protocol how to utilize a peptide carrier named Chariot in human embryonic kidney cells as well as in primary hippocampal neurons to perform antibody interference experiments and further illustrate the application of three-dimensional reconstructions in analyzing protein function. Our findings suggest that Chariot is, probably due to its nuclear localization signal, particularly well-suited to target proteins residing in the soma and the nucleus. Remarkably, when applying Chariot to primary hippocampal cultures, the reagent turned out to be surprisingly well accepted by dissociated neurons.

Fusion with pep-1, a cell-penetrating peptide, enhances the transmembrane ability of human epidermal growth factor

Administration of macromolecule compositions in medicine and cosmetics always exhibited low bioavailability due to the limitation of transmembrane transport. Here, human epidermal growth factor (hEGF) was fused with glutathione S-transferase (GST) and Pep-1, the first commercial cell-penetrating peptide, in Escherichia coli. The fusion protein was firstly purified with the affinity chromatography, and then the GST tag was released by TEV protease. Final purification was achieved by the ion exchange chromatography. The biological activities and the transmembrane ability of the obtained products were determined using scratch wound-healing assay, MTT analysis, and immunofluorescence assay. The results showed that both rhEGF and Pep-1-fused hEGF were soluble expressed in E. coli. The fusion of Pep-1 could markedly increase the transmembrane ability of EGF, whereas it did not interfere with the growth-stimulating and migration-promoting functions of hEGF on fibroblasts. This research provided a novel strategy for the transmembrane transport of protein-derived cosmetics or drugs.

Protein transport across membranes: Comparison between lysine and guanidinium-rich carriers

The mechanism(s) by which certain small peptides and peptide mimics carry large cargoes across membranes through exclusively non-covalent interactions has been difficult to resolve. Here, we use the droplet-interface bilayer as a platform to characterize distinct mechanistic differences between two such carriers: Pep-1 and a guanidinium-rich peptide mimic we call D9. While both Pep-1 and D9 can carry an enzyme, horseradish peroxidase (HRP) across a lipid bilayer, we found that they do so by different mechanisms. Specifically, Pep-1 requires voltage or membrane asymmetry while D9 does not. In addition, D9 can facilitate HRP transport without pre-forming a complex with HRP. By contrast, complex formation is required by Pep-1. Both carriers are capable of forming pores in membranes but our data hints that these pores are not responsible for cargo transport. Overall, D9 appears to be a more potent and versatile transporter when compared with Pep-1 because D9 does not require an applied voltage or other forces to drive transport. Thus, D9 might be used to deliver cargo across membranes under conditions where Pep-1 would be ineffective.

Enhanced non-endocytotic uptake of mesoporous silica nanoparticles by shortening the peptide transporter arginine side chain

Mesoporous silica nanoparticles (MSNs) are multifunctional nanocarriers with potential biomedical applications. However, MSNs are frequently trapped in endosomes upon cellular uptake through endocytosis, requiring endosomal escape. Herein, enhanced nonendocytosis was observed for 300 nm MSNs by conjugating peptides with noncanonical arginine analogs.

Synthesis and evaluation of a photochromic surfactant for organic reactions in aqueous media

A novel photochromic azobenzene-based surfactant was described for organic chemistry in water. The molecule 4-butylazobenzyl-4'-triazologlucuronic acid sodium salt thus synthesized can be isomerized from its trans to its cis form reversibly in solution by simple light irradiation. That property allowed the recyclability of a model acetylation reaction performed in the surfactant media, compared to the well-known, commercially available sodium dodecyl sulfate surfactant media.

Poly(amidoamine) polymers: soluble linear amphiphilic drug-delivery systems for genes, proteins and oligonucleotides

Polymer-drug and polymer-protein conjugates are emerging as a robust and well-characterized class of therapeutic entity. Although there are no low-molecular-weight soluble polymer conjugates in routine clinical use, there are many examples of routinely used high-molecular-weight drugs conjugated to soluble polymers (e.g., Oncospar). Advances in synthetic polymer chemistry have fostered the development of linear poly(amidoamine)s (PAA)s that impart both biodegradability, 'smart' (pH responsive) biological activity and biocompatibility. In their linear form, such as hyper-branched poly(amidoamine) (PAMAM) dendrimers, linear PAAs can be used to deliver large therapeutic entities such as peptides, proteins and genes to either the cytosol or nucleus. Furthermore, these polymers offer great potential in vivo due to their ability to either target the liver or be directed away from the liver and enter tumor mass via the enhanced permeability and retention (EPR) effect. PAAs also exhibit minimal toxicity (dependent upon backbone chemistry), relative to well-characterized polymers used for gene delivery. The propensity of PAAs to modulate intracellular trafficking resulting in their cytosolic translocation has also recently been quantified in vivo and is the primary focus of this article.

Cationic polyfluorenes for intracellular delivery of proteins

Two cationic polyfluorene derivatives, quaternary amine 1 and guanidine 2 sheathed systems, were prepared as potential carriers to mediate import of proteins into cells without requiring covalent attachment to the protein. Neither polymer showed significant cytotoxicities (IC(50) 100 μM) when exposed to Clone 9 rat liver cells. Both polymers were shown to mediate import of a series of four proteins chosen because they have different pI values, sizes, and variable organic fluor attachments. Once inside the cells, the quaternary amine system 1 released more of its cargo into regions outside the lysosomes. In one exploratory experiment, pyrenebutyrate was shown to accelerate import of a protein system by polymer 1.

Potentiation of ribonuclease cytotoxicity by a poly(amidoamine) dendrimer

Variants of bovine pancreatic ribonuclease (RNase A) engineered to evade the endogenous ribonuclease inhibitor protein (RI) are toxic to human cancer cells. Increasing the basicity of these variants facilitates their entry into the cytosol and thus increases their cytotoxicity. The installation of additional positive charge also has the deleterious consequence of decreasing ribonucleolytic activity or conformational stability. Here, we report that the same benefit can be availed by co-treating cells with a cationic dendrimer. We find that adding the generation 2 poly(amidoamine) dendrimer in trans increases the cytotoxicity of RI-evasive RNase A variants without decreasing their activity or stability. The increased cytotoxicity is not due to increased RI-evasion or cellular internalization, but likely results from improved translocation into the cytosol after endocytosis. These data indicate that co-treatment with highly cationic molecules could enhance the efficacy of ribonucleases as chemotherapeutic agents.

Fast membrane association is a crucial factor in the peptide pep-1 translocation mechanism: a kinetic study followed by surface plasmon resonance

The use of peptide carriers, termed "cell-penetrating peptides (CPPs)" has attracted much attention due to their potential for cellular delivery of hydrophilic molecules with pharmacological interest, overcoming the membrane barrier. These peptides are able to deliver attached cargos in a nontoxic manner, with the uptake mechanisms being either endosomally or physically driven. Pep-1 is a CPP of particular interest, not only due to outstanding delivery rates but also because its mechanism of membrane translocation is exclusively physically driven which appears to be dependent on a very high affinity for the phospholipid bilayer in the cell membrane. In this study, pep-1-lipid interactions were further explored by characterization of the pep-1-lipid association/dissociation by surface plasmon resonance. Although a high affinity of pep-1 for lipid bilayers was observed in all conditions tested, negatively charged phospholipids resulted in a larger peptide/lipid ratio. We also show that pep-1-membrane interaction is a fast process described by a multistep model initiated by peptide adsorption, primarily governed by electrostatic attractions, and followed by peptide insertion in the hydrophobic membrane core. In the context of a cell-based process, the translocation of pep-1 is a physical mechanism promoted by peptide primary amphipathicity and asymmetric properties of the membrane. This explains the high efficiency rates of pep-1 when compared with other CPPs.

A ratiometric pH reporter for imaging protein-dye conjugates in living cells

A molecule that transfers energy through bonds from a donor to an acceptor was prepared with a pH sensitive donor function (fluorescein). At pH values above 6.5, minimal energy transfer occurred, and the probe emitted green fluorescence (ca. 520 nm) when excited at the donor (488 nm). Below pH 6.0 however, energy transfer is efficient; hence excitation at the donor causes emission at the acceptor part (600 nm). This probe was used to image a conjugate of the probe with bovine serum albumin that was imported into endosomes or in the cytosol using the noncovalently bound carrier, Pep-1, at 37 and 4 degrees C, respectively. The more acidic environment of the endosomes was conspicuous from the red fluorescence of the probe.