Synthetic Protein Mimics for Functional Protein Delivery

Intracellular delivery of therapeutic proteins. New advancements and future directions

Achieving the full potential of therapeutic proteins to access and target intracellular receptors will have enormous benefits in advancing human health and fighting disease. Existing strategies for intracellular protein delivery, such as chemical modification and nanocarrier-based protein delivery approaches, have shown promise but with limited efficiency and safety concerns. The development of more effective and versatile delivery tools is crucial for the safe and effective use of protein drugs. Nanosystems that can trigger endocytosis and endosomal disruption, or directly deliver proteins into the cytosol, are essential for successful therapeutic effects. This article aims to provide a brief overview of the current methods for intracellular protein delivery to mammalian cells, highlighting current challenges, new developments, and future research opportunities.

Covalent Labeling-Mass Spectrometry Provides a Molecular Understanding of Noncovalent Polymer-Protein Complexation

The delivery of functional proteins to the intracellular space offers tremendous advantages for the development of new therapeutics but is limited by the passage of these large polar biomacromolecules through the cell membrane. Noncovalent polymer-protein binding that is driven by strong carrier-cargo interactions, including electrostatics and hydrophobicity, has previously been explored in the context of delivery of functional proteins. Appropriately designed polymer-based carriers can take advantage of the heterogeneous surface of protein cargoes, where multiple types of physical binding interactions with polymers can occur. Traditional methods of assessing polymer-protein binding, including dynamic light scattering, circular dichroism spectroscopy, and fluorescence-based assays, are useful in the study of new polymer-based carriers but face a number of limitations. We implement for the first time the method of covalent labeling-mass spectrometry (CL-MS) to probe intermolecular surface interactions within noncovalent polymer-protein complexes. We demonstrate the utility of CL-MS for establishing binding of an amphiphilic block copolymer to negatively charged and hydrophobic surface patches of a model protein, superfolder green fluorescent protein (sfGFP), using diethylpyrocarbonate as a pseudo-specific labeling reagent. In addition, we utilize this method to explore differences at the intermolecular surface as the ratio of polymer to protein increases, particularly in the context of defining effective protein delivery regimes. By promoting an understanding of the intermolecular interactions in polymer-protein binding and identifying sites where polymers bind to protein surfaces, noncovalent polymer carriers can be more effectively designed for protein delivery applications.

Arginine-Rich Polymers with Pore-Forming Capability Enable Efficient Intracellular Delivery via Direct Translocation Across Cell Membrane

Efficient delivery of biomacromolecules or drugs across the cell membrane via endocytosis usually encounters inevitable entrapment in endosomes and subsequent degradation in lyso-endosomes. To address this issue, a series of arginine-rich cell penetrating polymers is designed and synthesized, which internalize into cells by inducing the formation of pores on the cell membrane, thereby crossing the cell membrane via direct translocation that fundamentally avoids endo/lysosomal entrapment. The structure-activity relationship studies show that PTn-R2-C6, which is a type of polymer that has two arginine residues and a flexible hexanoic acid linker in each side chain, exhibits excellent pore-formation ability on the cell membrane. Further investigations indicate that PTn-R2-C6 rapidly transports plasmid DNAs into cytosol through a similar endocytosis-independent pathway, thereby achieving significantly higher transfection efficiency and lower cytotoxicity than the gold-standard transfection reagent PEI 25K. These results suggest the great potential of PTn-R2-C6 as a safe and efficient gene transfection reagent for wide applications including disease treatments, vaccine development, and biomedical research purposes.

Amphipathic poly-β-peptides for intracellular protein delivery

A series of amphipathic poly-β-peptides are designed for intracellular protein delivery. The poly-β-peptides with higher molecular weight and hydrophobic contents exhibit higher protein loading and superior delivery efficiency. The lead material efficiently delivers proteins into cells with reserved bioactivity.

Carbamoylated Guanidine-Containing Polymers for Non-Covalent Functional Protein Delivery in Serum-Containing Media

Despite the high potential of controlling cellular processes and treating various diseases by intracellularly delivered proteins, current delivery systems exhibit poor efficiency due to poor serum stability, cellular entry, and cytosolic availability of proteins. Here, we report a novel functional group, phenyl carbamoylated guanidine (Ph-CG), that greatly enhances the delivery efficiency to various types of cells. Owing to the substantially lowered pK_a , the hydrophobic Ph-CG offers optimized inter-macromolecular interactions via enhanced hydrogen-bonding and hydrophobic interactions. The coplanarity of Ph-CG also leads to the better intracellular entry of protein complexes. Intracellularly delivered apoptosis-inducing enzymes and antibodies significantly induce cell viability inhibitions in a serum-containing medium. The newly developed Ph-CG can be introduced to various existing carriers, leading to the realization of future therapeutic protein delivery.

Increased block copolymer length improves intracellular availability of protein cargo

Amphiphilic protein transduction domain mimics (PTDMs) of various lengths were used for protein delivery in Jurkat T cells. Although longer PTDMs facilitated greater cargo internalization, shorter PTDMs yielded greater cargo activity.

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.

Dynamic Polymer Amphiphiles for Efficient Intracellular and In Vivo Protein Delivery

Intracellular delivery of proteins is receiving considerable attention in biotherapeutics for various diseases by replacing dysfunctional proteins. Successful intracellular protein delivery highly relies on the development of efficient and safe polymeric carriers, which remains a grand challenge due to the lack of strong binding sites on proteins and their distinct molecular sizes and polarities. In this work, a strategy is proposed for efficient intracellular protein delivery by using dynamic polymer supra-amphiphiles, which are prepared by grafting boronated polylysine with a series of lipidated catechols via dynamic covalent catechol-boronate ester bonds. The prepared supra-amphiphiles can coassemble with proteins to form stable nanoparticles in water and also enable the release of bound proteins in cells due to their dynamic features, thereby strongly promoting the intracellular delivery process. The lead supra-amphiphiles screened in the library demonstrate high efficiency in the delivery of various proteins including bovine serum albumin, β-galactosidase, α-chymotrypsin, saporin, R-phycoerythrin, ovalbumin, catalase, and superoxide dismutase, and show great potency in delivering superoxide dismutase to treat ulcerative colitis in vivo. This work provides new opportunities for rational design and facile construction of robust intracellular protein delivery materials by the integration of polymer chemistry and supramolecular engineering strategies.

Non-Covalent Carrier Hydrophobicity as a Universal Predictor of Intracellular Protein Activity

Over the past decade, extensive optimization of polymeric cell-penetrating peptide (CPP) mimics (CPPMs) by our group has generated a substantial library of broadly effective carriers which circumvent the need for covalent conjugation often required by CPPs. In this study, design rules learned from CPPM development were applied to reverse-engineer the first library of simple amphiphilic block copolypeptides for non-covalent protein delivery, namely, poly(alanine-block-arginine), poly(phenylalanine-block-arginine), and poly(tryptophan-block-arginine). This new CPP library was screened for enhanced green fluorescent protein and Cre recombinase delivery alongside a library of CPPMs featuring equivalent side-chain configurations. Due to the added hydrophobicity imparted by the polymer backbone as compared to the polypeptide backbone, side-chain functionality was not a universal predictor of carrier performance. Rather, overall carrier hydrophobicity predicted the top performers for both internalization and activity of protein cargoes, regardless of backbone identity. Furthermore, comparison of protein uptake and function revealed carriers which facilitated high gene recombination despite remarkably low Cre internalization, leading us to formalize the concept of intracellular availability (IA) of the delivered cargo. IA, a measure of cargo activity per quantity of cargo internalized, provides valuable insight into the physical relationship between cellular internalization and bioavailability, which can be affected by bottlenecks such as endosomal escape and cargo release. Importantly, carriers with maximal IA existed within a narrow hydrophobicity window, more hydrophilic than those exhibiting maximal cargo uptake. Hydrophobicity may be used as a scaffold-independent predictor of protein uptake, function, and IA, enabling identification of new, effective carriers which would be overlooked by uptake-based screening methods.

Bifunctional and Bioreducible Dendrimer Bearing a Fluoroalkyl Tail for Efficient Protein Delivery Both In Vitro and In Vivo

Rational design of stimuli-responsive polymers for cytosolic protein delivery with robust efficiency is of great importance but remains a challenging task. Here, we reported a bioreducible and amphiphilic dendrimer bearing a fluoroalkyl tail for this purpose. The fluorolipid was conjugated to the focal point of a cysteamine-cored polyamidoamine dendrimer via disulfide bond, while phenylboronic acid moieties were decorated on dendrimer surface for efficient protein binding. The yielding polymer showed high protein binding capability and complex stability and could efficiently release the cargo proteins in a glutathione-responsive manner. The designed polymer was effective in the delivery of various membrane-impermeable proteins into living cells with reserved bioactivities. In addition, the polymer efficiently delivered a toxin protein saporin into 4T1 breast cancer cells and inhibited the tumor growth in vivo after intravenous injection. The developed polymer in this study is a promising vector for the delivery of membrane-impermeable proteins to treat various diseases.

Accessing Intracellular Targets through Nanocarrier-Mediated Cytosolic Protein Delivery

Protein-based therapeutics have unique therapeutic potential due to their specificity, potency, and low toxicity. The vast majority of intracellular applications of proteins require access to the cytosol. Direct entry to the cytosol is challenging due to the impermeability of the cell membrane to proteins. As a result, multiple strategies have focused on endocytic uptake of proteins. Endosomally entrapped cargo, however, can have very low escape efficiency, with protein degradation occurring in acidic endolysosomal compartments. In this review, we briefly discuss endosomal escape strategies and review the strategy of cell membrane fusion, a recent strategy for direct delivery of proteins into the cell cytoplasm.

A boronic acid-rich dendrimer with robust and unprecedented efficiency for cytosolic protein delivery and CRISPR-Cas9 gene editing

Cytosolic protein delivery is of central importance for the development of protein-based biotechnologies and therapeutics; however, efficient intracellular delivery of native proteins remains a challenge. Here, we reported a boronic acid-rich dendrimer with unprecedented efficiency for cytosolic delivery of native proteins. The dendrimer could bind with both negatively and positively charged proteins and efficiently delivered 13 cargo proteins into the cytosol of living cells. All the delivered proteins kept their bioactivities after cytosolic delivery. The dendrimer ensures efficient intracellular delivery of Cas9 protein into various cell lines and showed high efficiency in CRISPR-Cas9 genome editing. The rationally designed boronic acid-rich dendrimer permits the development of an efficient platform with high generality for the delivery of native proteins.

Critical role of Tim-3 mediated autophagy in chronic stress induced immunosuppression

Background

Psychological and physical stress can either enhance or suppress immune functions depending on a variety of factors such as duration and severity of stressful situation. Chronic stress exerts a significantly suppressive effect on immune functions. However, the mechanisms responsible for this phenomenon remain to be elucidated. Autophagy plays an essential role in modulating cellular homeostasis and immune responses. However, it is not known yet whether autophagy contributes to chronic stress-induced immunosuppression. T cell immunoglobulin and mucin domain 3 (Tim-3) has shown immune-suppressive effects and obviously positive regulation on cell apoptosis. Tim-3 combines with Tim-3 ligand galectin-9 to modulate apoptosis. However, its impact on autophagy and chronic stress-induced immunosuppression is not yet identified.

Results

We found remarkably higher autophagy level in the spleens of mice that were subjected to chronic restraint stress compared with the control group. We also found that inhibition of autophagy by the autophagy inhibitor 3-methyladenine (3-MA) significantly attenuated chronic stress-induced alterations of pro-inflammatory and anti-inflammatory cytokine levels. We further elucidated that 3-MA dramatically inhibited the reduction of lymphocyte numbers. Moreover, chronic stress dramatically enhanced the expression of Tim-3 and galectin-9. Inhibition of Tim-3 by small interfering RNA against Tim-3 significantly decreased the level of autophagy and immune suppression in isolated primary splenocytes from stressed mice. In addition, α-lactose, a blocker for the interaction of Tim-3 and galectin-9, also decreased the autophagy level and immune suppression.

Conclusion

Chronic stress induces autophagy, resulting with suppression of immune system. Tim-3 and galectin-9 play a crucial regulatory role in chronic stress-induced autophagy. These studies suggest that Tim-3 mediated autophagy may offer a novel therapeutic strategy against the deleterious effects of chronic stress on the immune system.

Protein Transduction Domain Mimic (PTDM) Self-Assembly?

Intracellular protein delivery is an invaluable tool for biomedical research, as it enables fundamental studies of cellular processes and creates opportunities for novel therapeutic development. Protein delivery reagents such as cell penetration peptides (CPPs) and protein transduction domains (PTDs) are frequently used to facilitate protein delivery. Herein, synthetic polymer mimics of PTDs, called PTDMs, were studied for their ability to self-assemble in aqueous media as it was not known whether self-assembly plays a role in the protein binding and delivery process. The results obtained from interfacial tensiometry (IFT), transmission electron microscopy (TEM), transmittance assays (%T), and dynamic light scattering (DLS) indicated that PTDMs do not readily aggregate or self-assemble at application-relevant time scales and concentrations. However, additional DLS experiments were used to confirm that the presence of protein is required to induce the formation of PTDM-protein complexes and that PTDMs likely bind as single chains.

Enhanced TAT-Cre Protein Transduction for Efficient Gene Recombination in T cells

Genetic manipulation has increased our understanding of gene function and led to the discovery of new therapeutic targets. Cre/LoxP DNA recombination is widely used for genetic studies in mammalian cells. The direct delivery of Cre recombinase fused to protein transduction domains (PTDs), such as TAT, has been described as a valid alternative to the conditional, site-specific Cre expression in transgenic mice. However, efficiently conveying proteins into live cells, especially primary T cells, remains a major challenge. In this study, we show that one of our recently developed PTDs synthetic mimic greatly enhances the cellular uptake of the TAT-Cre fusion protein, enabling significantly smaller amounts of the protein to be used. We used this technique in primary mouse T cells to successfully delete, ex vivo, two essential genes involved in regulating T cell activation, Notch1 and Rbpjκ. Ex vivo gene deletion resulted in substantial protein reduction, comparable to that obtained in vivo when Cre-expressing Notch1-floxed (MxCre^±Notch1^fl/fl) mice were treated with polyinosinic-polycytidylic acid (polyl/C), but in considerably less time, and without altering normal cell physiology. These results highlight several key advantages that include the ability to use less expensive protein (TAT-Cre), a major reduction in total experimental time and labor, and fewer side effects on the treated cells. This method should offer new opportunities for immunological studies, especially in the context of identifying novel therapeutic targets.

Transduction Methods for Cytosolic Delivery of Proteins and Bioconjugates into Living Cells

The human organism and its constituting cells rely on interplay between multiple proteins exerting specific functions. Progress in molecular biotechnologies has facilitated the production of recombinant proteins. When administrated to patients, recombinant proteins can provide important healthcare benefits. To date, most therapeutic proteins must act from the extracellular environment, with their targets being secreted modulators or extracellular receptors. This is because proteins cannot passively diffuse across the plasma membrane into the cytosol. To expand the scope of action of proteins for cytosolic targets (representing more than 40% of the genome) effective methods assisting protein cytosolic entry are being developed. To date, direct protein delivery is extremely tedious and inefficient in cultured cells, even more so in animal models of pathology. Novel techniques are changing this limitation, as recently developed in vitro methods can robustly convey large amount of proteins into cell cultures. Moreover, advances in protein formulation or protein conjugates are slowly, but surely demonstrating efficiency for targeted cytosolic entry of functional protein in vivo in tumor xenograft models. In this review, various methods and recently developed techniques for protein transport into cells are summarized. They are put into perspective to address the challenges encountered during delivery.

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.

Multi-targeting peptides for gene carriers with high transfection efficiency

Non-viral gene carriers for gene therapy have been developed for many years.