Protein and Antibody Delivery into Difficult-to-Transfect Cells by Polymeric Peptide Mimics

Combating bone marrow failure with polymer materials

Bone marrow failure (BMF) has become one of the most studied autoimmune disorders, particularly due to its prevalence both as an inherited disease, but also as a result of chemotherapies. BMF is associated with severe symptoms such as bleeding episodes and susceptibility to infections, and often has underlying characteristics, such as anemia, thrombocytopenia, and neutropenia. The current treatment landscape for BMF requires stem cell transplantation or chemotherapies to induce immune suppression. However, there is limited donor cell availability or dose related toxicity associated with these treatments. Optimizing these treatments has become a necessity. Polymer-based materials have become increasingly popular, as current research efforts are focused on synthesizing novel cell matrices for stem cell expansion to solve limited donor cell availability, as well as applying polymer delivery vehicles to intracellularly deliver cargo that can aid in immunosuppression. Here, we discuss the importance and impact of polymer materials to enhance therapeutics in the context of BMF.

E3MPH16: An efficient endosomolytic peptide for intracellular protein delivery

To facilitate the introduction of proteins, such as antibodies, into cells, a variety of delivery peptides have been engineered. These peptides are typically highly cationic and somewhat hydrophobic, enabling cytosolic protein delivery at the cost of causing cell damage by rupturing membranes. This balance between delivery effectiveness and cytotoxicity presents obstacles for their real-world use. To tackle this problem, we designed a new endosome-disruptive cytosolic delivery peptide, E3MPH16, inspired by mastoparan X (MP). E3MPH16 was engineered to incorporate three Glu (E3) and 16 His (H16) residues at the N- and C-termini of MP, respectively. The negative charges of E3 substantially mitigate the cell-surface damage induced by MP. The H16 segment is known to enhance cell-surface adsorption and endocytic uptake of the associated molecules. With these modifications, E3MPH16 was successfully trapped within endosomes. The acidification of endosomes is expected to protonate the side chains of E3 and H16, enabling E3MPH16 to rupture endosomal membranes. As a result, nearly 100% of cells achieved cytosolic delivery of a model biomacromolecule, Alexa Fluor 488-labeled dextran (10 kDa), via endosomal escape by co-incubation with E3MPH16. The delivery process also suggested the involvement of macropinocytosis and caveolae-mediated endocytosis. With the assistance of E3MPH16, Cre recombinase and anti-Ras-IgG delivered into HEK293 cells and HT1080 cells enabled gene recombination and inhibited cell proliferation, respectively. The potential for in vivo application of this intracellular delivery method was further validated by topically injecting the green fluorescent protein fused with a nuclear localization signal (NLS-GFP) along with E3MPH16 into Colon-26 tumor xenografts in mice.

A live mammalian cells electroporation array for on-chip immunofluorescence

The detection of intracellular proteins in vitro is commonly realized with immunofluorescence techniques, through which antibodies or markers are delivered into fixed cells and recognize specific proteins. Many innovative techniques, however, avoid cells fixation by chemical compounds and, among the others, electroporation is widely used. Here we demonstrate that in situ electroporation on thin film SiO2 capacitive microelectrodes can be realized with high efficiency to deliver fluorescent markers and antibodies into mammalian cell lines and primary neuronal cells to detect intracellular proteins, like actin. The results presented in this work open the way to the use of this technique for the detection of potentially any target protein, even through subsequent electroporations.

Alcohol-containing protein transduction domain mimics

The application and design of protein transduction domains (PTDs) and protein transduction domain mimics (PTDMs) have revolutionized the field of biomacromolecule delivery. Our group has previously synthesized block copolymer PTDMs with well-defined hydrophobic and cationic blocks via ring-opening metathesis polymerization (ROMP). We have optimized the balance of hydrophobicity and cationic density to intracellularly deliver model proteins, active proteins, and antibodies. Despite the presence of serine, threonine, and tyrosine in naturally occurring PTDs, synthetic analogs have yet to be studied in PTDMs. In our present work, we introduce different alcohol groups to our PTDM structures as a new design parameter. A library of nine novel PTDMs were synthesized to incorporate alcohol groups of varying structures and evaluated based on their ability to intracellularly deliver fluorescently labeled antibodies. One PTDM in this novel library, named PTDM4, incorporates alcohol groups in both the hydrophobic and cationic blocks and was found to be the best performing PTDM with almost twice the median fluorescence intensity of the delivered antibody and half the cationic density compared to our positive control, a PTDM thoroughly studied by our group. PTDM4 was further studied by intracellularly delivering the active enzyme, TAT-Cre Recombinase. The activity of TAT-Cre Recombinase delivered by PTDM4 was comparable to that of the positive control, again with half the cationic density. This study is one of the first to examine the effects of alcohol groups on intracellular antibody and active enzyme delivery.

A hybridized mechano-electroporation technique for efficient immune cell engineering

Immune cell engineering, which involves genetic modification of T cells, natural killer cells, and macrophages, is shifting the paradigm in immunotherapy for treating hematologic malignancies. These modified cells can be viewed as living drugs and offer advantages, including dynamic functionality, active local trafficking, and boosting the immune system while recognizing and eliminating malignant cells. Among the current technologies employed for the modification of immune cell functions, electroporation stands as a predominant approach, but it suffers from heterogeneity arising from the treatment of a bulk population of immune cells during the manufacturing procedures. To address this challenge of the field, here we present a hybrid approach to induce consecutive gentle mechanical and electric shocks. This approach enhances the treatment homogeneity and improves outcomes in difficult-to-load immune cells. The hybrid approach aims to enhance the treatment homogeneity by passing individual immune cells through a microengineered filter membrane with micropores smaller than the cell diameter. This facilitates the creation of transient pores in the cell membrane, followed by efficient delivery of biomolecules through the complementary use of a gentle electric shock. Using this hybrid mechano-electroporation (HMEP) system, we could successfully deliver fluorescein isothiocyanate (FITC) dextran molecules from the smallest (4 kDa) to the largest (2000 kDa) size and GFP expressing plasmid DNA into different immune cell types. We also provide insight into the delivery performance of the HMEP system in comparison with the benchtop electroporation since both methods hinge on membrane disruption as their permeabilization mechanism. Immune cells treated with the HMEP protocol demonstrated higher delivery efficiencies while maintaining cell viability compared to those experiencing conventional electroporation. Therefore, membrane-based mechanoporation can be a cost-effective and efficient approach to pre-treat the hard-to-deliver immune cells before electroporation, elevating the treatment homogeneity and delivery of exogenous cargoes to a higher level.

Delivery of Transcriptional Factors for Activating Antioxidant Defenses against Inflammatory Bowel Disease

Oxidative stress caused by the overproduction of reactive oxygen species (ROS) plays an important role in inflammatory bowel disease (IBD). It is well-known that the Nrf2-ARE (antioxidative response element) pathway is important in the regulation mechanism of antioxidant defense. Therefore, Nrf2 activation may be an effective therapeutic strategy for IBD. Here, we reported the development of a nucleus-targeted Nrf2 delivery nanoplatform, termed N/LC, that could accumulate in inflamed colonic epithelium, reduce inflammatory responses, and restore epithelium barriers in a murine model of acute colitis. N/LC nanocomposites could quickly escape from lysosomes, so Nrf2 largely accumulated in the nucleus of colonic cells, activated the Nrf2-ARE signaling pathway, further elevated the expression levels of downstream detoxification and antioxidant genes, and protected cells from oxidative damage. These results suggested that N/LC might be a potential nanoplatform for IBD therapy. The study provided the basis for the biomedical applications of Nrf2-based therapeutics in various diseases.

Polymer dots and glassy organic dots using dibenzodipyridophenazine dyes as water-dispersible TADF probes for cellular imaging

Fluorescence imaging of living cells is key to better understanding cellular morphology and biological processes. Water-dispersible nanoparticles exhibiting thermally activated delayed fluorescence (TADF) have recently emerged as useful probes for time-resolved fluorescence imaging (TRFI), circumventing interference from biological autofluorescence. Many existing approaches, however, require TADF dyes with specific structural features, precluding many high-performance TADF materials from being used in this application. Here, we describe the synthesis of two TADF emitters based on the rigid and strongly electron-withdrawing dibenzo[a,c]dipyrido[3,2-h:2'-3'-j]phenazine-12-yl (BPPZ) motif, and demonstrate two parallel approaches for the encapsulation of these fluorophores to yield water-dispersible nanoparticles suitable for TRFI. First, fluorescent polymer dots (Pdots) were formed by dye encapsulation within cell-penetrating amphiphilic copolymers. Glassy organic nanoparticles (g-Odots) were also prepared, giving nanoparticles with higher photoluminescence quantum yields and improved colour purity. Both approaches yielded nanoparticles suitable for imaging, with reasonable uptake and cytotoxicity on the timescale of standard imaging experiments using human cervical (HeLa) and liver (HepG2) cancer cell lines. This work demonstrates two flexible strategies for preparing water-dispersible TADF nanoparticles for TRFI, both of which should be readily adaptable to nearly any existing hydrophobic TADF dye.

Effects of sidechain isomerism on polymer-based non-covalent protein delivery

We present the importance of functional group isomerism on intracellular protein delivery using polymers containing different isomeric side chains. While the physical properties of polymer/protein complexes are relatively similar, different planarity of the isomers greatly influences the cellular entry efficiency.

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.

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.

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.

Polymers for Biomedical Applications: The Importance of Hydrophobicity in Directing Biological Interactions and Application Efficacy

The past decades have seen significant research effort in the field of polymers for a range of biomedical applications, driven by the promising prospect of these materials for realizing next generation therapeutics in the clinic. In this regard, it is widely accepted that polymer properties such as chemistry, charge, and block composition, as well as properties of their self-assemblies including size, shape, surface chemistry, and biodegradation, all influence and direct their interactions with cells and biological membranes. In particular, polymer hydrophobicity is a property of interest, with growing evidence demonstrating the significant impact that hydrophobic interactions with lipid membranes and proteins can have on biomaterial application efficacy within the body. However, to date, this phenomenon has been relatively underexplored, and therefore there exists no clear universal understanding to direct polymer design. In this Perspective, we highlight important contributions to this field, focusing on seminal studies which investigate experimentally and theoretically how incorporation of hydrophobic moieties within polymer systems can influence their ultimate properties when used in biomedical applications. In this way, we aim to signify future directions in the design of highly performing polymers for biomedicine, making a case for the importance of standardized computational modeling to achieve widely applicable conclusions and facilitate future translational efforts.

A pH-Responsive Phase-Transition Polymer with High Serum Stability in Cytosolic Protein Delivery

Polymer-mediated intracellular protein delivery systems are important for the development of protein-based biotechnologies and therapeutics. However, intracellular delivery of cargo proteins in the presence of serum remains challenging due to competitive binding of serum proteins with the polymers. Here, we reported a dendrimer engineered with a high density of 4-diethylaminophenyl groups on the surface to address this issue. The dendrimer showed a pH-responsive phase-transition behavior and could assemble with cargo proteins into stable nanoparticles in serum solutions. It efficiently delivered cargo proteins into living cells, and exhibited a pH-responsive disassembly behavior after cell internalization. As a result, various cargo proteins were delivered into the cytosol of living cells with maintained bioactivity. This study provided a convenient and efficient strategy to design polymers with high serum-tolerance in cytosolic protein delivery.

Tuning Net Charge in Aliphatic Polycarbonates Alters Solubility and Protein Complexation Behavior

A synthetic strategy yielded polyelectrolytes and polyampholytes with tunable net charge for complexation and protein binding. Organocatalytic ring-opening polymerizations yielded aliphatic polycarbonates that were functionalized with both carboxylate and ammonium side chains in a post-polymerization, radical-mediated thiol-ene reaction. Incorporating net charge into the polymer architecture altered the chain dimensions in phosphate buffered solution in a manner consistent with self-complexation and complexation behavior with model proteins. A net cationic polyampholyte with 5% of carboxylate side chains formed large clusters rather than small complexes with bovine serum albumin, while 50% carboxylate polyampholyte was insoluble. Overall, the aliphatic polycarbonates with varying net charge exhibited different macrophase solution behaviors when mixed with protein, where self-complexation appears to compete with protein binding and larger-scale complexation.

Red-Emissive Cell-Penetrating Polymer Dots Exhibiting Thermally Activated Delayed Fluorescence for Cellular Imaging

Fluorescence imaging in living cells is key to understanding many biological processes, yet autofluorescence from the sample can lower sensitivity and hinder high-resolution imaging. Time-gated measurements using phosphorescent metal complexes can improve imaging, at the cost of potential toxicity from the use of heavy metals. Here, we describe orange/red-emitting polymer dots (Pdots) exhibiting thermally activated delayed fluorescence (TADF) for time-gated imaging. Inspired by the cell invasion mechanism of the HIV TAT protein, the Pdots were formed from block copolymers composed of a hydrophilic guanidine-rich block as a cell-penetrating peptide mimic, and a rigid organic semiconductor block to provide efficient delayed fluorescence. These all-organic polymer nanoparticles were shown to efficiently enter HeLa, CHO, and HepG2 cells within 30 min, with cell viabilities remaining high for Pdot concentrations up to 25 mg mL^-1. Pdot quantum yields were as high as 0.17 in aerated water, with the Pdot structure effectively shielding the TADF emitters from quenching by oxygen. Colocalization experiments revealed that the Pdots primarily accumulate outside of lysosomes, minimizing lysosomal degradation. When used for fixed cellular imaging, Pdot-incubated cells showed high signal-to-background ratios compared to control samples with no Pdot exposure. Using time-resolved spectroscopy, the delayed emission of the TADF materials was effectively separated from that of both a biological serum and a secondary fluorescent dye.

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.

Intracellular Delivery of Active Proteins by Polyphosphazene Polymers

Achieving intracellular delivery of protein therapeutics within cells remains a significant challenge. Although custom formulations are available for some protein therapeutics, the development of non-toxic delivery systems that can incorporate a variety of active protein cargo and maintain their stability, is a topic of great relevance. This study utilized ionic polyphosphazenes (PZ) that can assemble into supramolecular complexes through non-covalent interactions with different types of protein cargo. We tested a PEGylated graft copolymer (PZ-PEG) and a pyrrolidone containing linear derivative (PZ-PYR) for their ability to intracellularly deliver FITC-avidin, a model protein. In endothelial cells, PZ-PYR/protein exhibited both faster internalization and higher uptake levels than PZ-PEG/protein, while in cancer cells both polymers achieved similar uptake levels over time, although the internalization rate was slower for PZ-PYR/protein. Uptake was mediated by endocytosis through multiple mechanisms, PZ-PEG/avidin colocalized more profusely with endo-lysosomes, and PZ-PYR/avidin achieved greater cytosolic delivery. Consequently, a PZ-PYR-delivered anti-F-actin antibody was able to bind to cytosolic actin filaments without needing cell permeabilization. Similarly, a cell-impermeable Bax-BH3 peptide known to induce apoptosis, decreased cell viability when complexed with PZ-PYR, demonstrating endo-lysosomal escape. These biodegradable PZs were non-toxic to cells and represent a promising platform for drug delivery of protein therapeutics.