Organelle-Targeted Delivery of Biological Macromolecules Using the Protein Transduction Domain: Potential Applications for Peptide Aptamer Delivery into the Nucleus

Journey to the Center of the Cell: Current Nanocarrier Design Strategies Targeting Biopharmaceuticals to the Cytoplasm and Nucleus

New biopharmaceutical molecules, potentially able to provide more personalized and effective treatments, are being identified through the advent of advanced synthetic biology strategies, sophisticated chemical synthesis approaches, and new analytical methods to assess biological potency. However, translation of many of these structures has been significantly limited due to the need for more efficient strategies to deliver macromolecular therapeutics to desirable intracellular sites of action. Engineered nanocarriers that encapsulate peptides, proteins, or nucleic acids are generally internalized into target cells via one of several endocytic pathways. These nanostructures, entrapped within endosomes, must navigate the intracellular milieu to orchestrate delivery to the intended destination, typically the cytoplasm or nucleus. For therapeutics active in the cytoplasm, endosomal escape continues to represent a limiting step to effective treatment, since a majority of nanocarriers trapped within endosomes are ultimately marked for enzymatic degradation in lysosomes. Therapeutics active in the nucleus have the added challenges of reaching and penetrating the nuclear envelope, and nuclear delivery remains a preeminent challenge preventing clinical translation of gene therapy applications. Herein, we review cutting-edge peptide- and polymer-based design strategies with the potential to enable significant improvements in biopharmaceutical efficacy through improved intracellular targeting. These strategies often mimic the activities of pathogens, which have developed innate and highly effective mechanisms to penetrate plasma membranes and enter the nucleus of host cells. Understanding these mechanisms has enabled advances in synthetic peptide and polymer design that may ultimately improve intracellular trafficking and bioavailability, leading to increased access to new classes of biotherapeutics.

The application of aptamer in apoptosis

As an indispensable process of cell life, apoptosis is essential for keeping homeostasis at cell level. Dysregulation of apoptosis is usually involved in the pathological processes of many complex diseases including cancer. With the properties such as high affinity and specificity to their targets, easy of synthesis and modification and good biocompatibility, aptamers have been attractive molecules applied in basic research, diagnostics and therapeutics. This review mainly focuses on the recent researches on application of aptamers in interference of cell apoptosis. Key targets along the intrinsic and extrinsic apoptosis pathways were respectively dissected using aptamers as a tool, providing an insight into the pathological processes, especially for cancer.

Delivery of nucleic acids and nanomaterials by cell-penetrating peptides: opportunities and challenges

Many viral and nonviral systems have been developed to aid delivery of biologically active molecules into cells. Among these, cell-penetrating peptides (CPPs) have received increasing attention in the past two decades for biomedical applications. In this review, we focus on opportunities and challenges associated with CPP delivery of nucleic acids and nanomaterials. We first describe the nature of versatile CPPs and their interactions with various types of cargoes. We then discuss in vivo and in vitro delivery of nucleic acids and nanomaterials by CPPs. Studies on the mechanisms of cellular entry and limitations in the methods used are detailed.

Polymeric microspheres as protein transduction reagents

Discovering the function of an unknown protein, particularly one with neither structural nor functional correlates, is a daunting task. Interaction analyses determine binding partners, whereas DNA transfection, either transient or stable, leads to intracellular expression, though not necessarily at physiologically relevant levels. In theory, direct intracellular protein delivery (protein transduction) provides a conceptually simpler alternative, but in practice the approach is problematic. Domains such as HIV TAT protein are valuable, but their effectiveness is protein specific. Similarly, the delivery of intact proteins via endocytic pathways (e.g. using liposomes) is problematic for functional analysis because of the potential for protein degradation in the endosomes/lysosomes. Consequently, recent reports that microspheres can deliver bio-cargoes into cells via a non-endocytic, energy-independent pathway offer an exciting and promising alternative for in vitro delivery of functional protein. In order for such promise to be fully exploited, microspheres are required that (i) are stably linked to proteins, (ii) can deliver those proteins with good efficiency, (iii) release functional protein once inside the cells, and (iv) permit concomitant tracking. Herein, we report the application of microspheres to successfully address all of these criteria simultaneously, for the first time. After cellular uptake, protein release was autocatalyzed by the reducing cytoplasmic environment. Outside of cells, the covalent microsphere–protein linkage was stable for ≥90 h at 37 °C. Using conservative methods of estimation, 74.3% ± 5.6% of cells were shown to take up these microspheres after 24 h of incubation, with the whole process of delivery and intracellular protein release occurring within 36 h. Intended for in vitro functional protein research, this approach will enable study of the consequences of protein delivery at physiologically relevant levels, without recourse to nucleic acids, and offers a useful alternative to commercial protein transfection reagents such as Chariot™. We also provide clear immunostaining evidence to resolve residual controversy surrounding FACS-based assessment of microsphere uptake.

Protein transduction in human cells is enhanced by cell-penetrating peptides fused with an endosomolytic HA2 sequence

Endocytosis has been proposed as one of the primary mechanisms for cellular entry of cell-penetrating peptides (CPPs) and their cargoes. However, a major limitation of endocytic pathway is entrapment of the CPP-cargo in intracellular vesicles from which the cargo must escape into the cytoplasm to exert its biological activity. Here we demonstrate that a CPP tagged with an endosomolytic fusion peptide derived from the influenza virus hemagglutinin-2 (HA2) remarkably enhances the cytosolic delivery of proteins in human A549 cells. To determine the endosome-disruptive effects, recombinant DNA plasmids containing coding sequences of HA2, CPPs and red fluorescent proteins (RFPs) were constructed. The fusion proteins were purified from plasmid-transformed Escherichia coli, and their effects on protein transduction were examined using live cell imaging and flow cytometry. Our data indicate that endocytosis is the major route for cellular internalization of CPP-HA2-tagged RFP. Mechanistic studies revealed that the fusogenic HA2 peptide dramatically facilitates CPP-mediated protein entry through the release of endocytosed RFPs from endosomes into the cytoplasm. Furthermore, incorporating the HA2 fusion peptide of the CPP-HA2 fusion protein improved cytosolic uptake without causing cytotoxicity. These findings strongly suggest that the CPP-HA2 tag could be an efficient and safe carrier that overcomes endosomal entrapment of delivered therapeutic drugs.

Transduction of human recombinant proteins into mitochondria as a protein therapeutic approach for mitochondrial disorders

Protein therapy is considered an alternative approach to gene therapy for treatment of genetic-metabolic disorders. Human protein therapeutics (PTs), developed via recombinant DNA technology and used for the treatment of these illnesses, act upon membrane-bound receptors to achieve their pharmacological response. On the contrary, proteins that normally act inside the cells cannot be developed as PTs in the conventional way, since they are not able to "cross" the plasma membrane. Furthermore, in mitochondrial disorders, attributed either to depleted or malfunctioned mitochondrial proteins, PTs should also have to reach the subcellular mitochondria to exert their therapeutic potential. Nowadays, there is no effective therapy for mitochondrial disorders. The development of PTs, however, via the Protein Transduction Domain (PTD) technology offered new opportunities for the deliberate delivery of human recombinant proteins inside eukaryotic subcellular organelles. To this end, mitochondrial disorders could be clinically encountered with the delivery of human mitochondrial proteins (engineered via recombinant DNA and PTD technologies) at specific intramitochondrial sites to exert their function. Overall, PTD-mediated Protein Replacement Therapy emerges as a suitable model system for the therapeutic approach for mitochondrial disorders.

Successful TAT-mediated enzyme replacement therapy in a mouse model of mitochondrial E3 deficiency

Medicine today offers no cure for patients suffering from mitochondrial disorders, such as lipoamide dehydrogenase (LAD; also known as E3) deficiency, and treatment is limited to symptomatic care. LAD is one of the components of the α-ketoacid dehydrogenase complexes, which are mitochondrial multienzyme complexes crucial for the metabolism of carbohydrates and amino acids. Recently, we tested the therapeutic approach for treating mitochondrial disorders whereby the activity of multicomponent complexes in the mitochondria is restored by TAT-mediated enzyme replacement therapy (ERT). The LAD deficiency disease was used before as a proof-of-principle in vitro, in patients' cells, utilizing the TAT-LAD fusion protein. In this report, we present successful TAT-mediated ERT in an in vivo mouse model using E3-deficient mice. We demonstrate the delivery of TAT-LAD into E3-deficient mice tissues and that a single administration of TAT-LAD results in a significant increase in the enzymatic activity of the mitochondrial multienzyme complex pyruvate dehydrogenase complex within the liver, heart and, most importantly, the brain of TAT-LAD-treated E3-deficient mice. We believe that this TAT-mediated ERT approach could change the management of mitochondrial disorders and of other metabolic diseases in modern medicine.

CPP-ZFN: a potential DNA-targeting anti-malarial drug

BACKGROUND

Multidrug-resistant Plasmodium is of major concern today. Effective vaccines or successful applications of RNAi-based strategies for the treatment of malaria are currently unavailable. An unexplored area in the field of malaria research is the development of DNA-targeting drugs that can specifically interact with parasitic DNA and introduce deleterious changes, leading to loss of vital genome function and parasite death.

PRESENTATION OF THE HYPOTHESIS

Advances in the development of zinc finger nuclease (ZFN) with engineered DNA recognition domains allow us to design and develop nuclease of high target sequence specificity with a mega recognition site that typically occurs only once in the genome. Moreover, cell-penetrating peptides (CPP) can cross the cell plasma membrane and deliver conjugated protein, nucleic acid, or any other cargo to the cytoplasm, nucleus, or mitochondria. This article proposes that a drug from the combination of the CPP and ZFN systems can effectively enter the intracellular parasite, introduce deleterious changes in its genome, and eliminate the parasite from the infected cells.

TESTING THE HYPOTHESIS

Availability of a DNA-binding motif for more than 45 triplets and its modular nature, with freedom to change number of fingers in a ZFN, makes development of customized ZFN against diverse target DNA sequence of any gene feasible. Since the Plasmodium genome is highly AT rich, there is considerable sequence site diversity even for the structurally and functionally conserved enzymes between Plasmodium and humans. CPP can be used to deliver ZFN to the intracellular nucleus of the parasite. Signal-peptide-based heterologous protein translocation to Plasmodium-infected RBCs (iRBCs) and different Plasmodium organelles have been achieved. With successful fusion of CPP with mitochondrial- and nuclear-targeting peptides, fusion of CPP with 1 more Plasmodium cell membrane translocation peptide seems achievable.

IMPLICATIONS OF THE HYPOTHESIS

Targeting of the Plasmodium genome using ZFN has great potential for the development of anti-malarial drugs. It allows the development of a single drug against all malarial infections, including multidrug-resistant strains. Availability of multiple ZFN target sites in a single gene will provide alternative drug target sites to combat the development of resistance in the future.

HER2- and EGFR-specific affiprobes: novel recombinant optical probes for cell imaging

The human epidermal growth factor receptors, EGFR and HER2, are members of the EGFR family of cell-surface receptors/tyrosine kinases. EGFR- and HER2-positive cancers represent a more aggressive disease with greater likelihood of recurrence, poorer prognosis, and decreased survival rate, compared to EGFR- or HER2-negative cancers. The details of HER2 proto-oncogenic functions are not deeply understood, partially because of a restricted availability of tools for EGFR and HER2 detection (A. Sorkin and L. K. Goh, Exp. Cell Res. 2009, 315, 683-696). We have created photostable and relatively simple-to-produce imaging probes for in vitro staining of EGFR and HER2. These new reagents, called affiprobes, consist of a targeting moiety, a HER2- or EGFR-specific Affibody molecule, and a fluorescent moiety, mCherry (red) or EGFP (green). Our flow cytometry and confocal microscopy experiments demonstrated high specificity and signal/background ratio of affiprobes. Affiprobes are able to stain both live cells and frozen tumor xenograph sections. This type of optical probe can easily be extended for targeting other cell-surface antigens/ receptors.

Reprogramming human fibroblasts using HIV-1 TAT recombinant proteins OCT4, SOX2, KLF4 and c-MYC

It has been shown that human and murine fibroblasts can be reprogrammed by ectopic expression of transcription factors using viral vectors. For the purpose of human therapeutic applications, the integration of viral transgenes into the genome is unlikely to be accepted. We therefore produced recombinant transcription factor proteins in E. coli (OCT4, SOX2, c-MYC and KLF4) carrying the cell penetrating TAT domain from HIV1. The purified proteins were able to enter into mammalian cells when added to tissue culture medium but appeared not to translocate to the nucleus. Further investigation indicated that most of the protein was tied up in the endosomes and was unavailable for reprogramming. Once this problem has been solved it seems likely that protein reprogramming will be the method of choice for clinical applications.

TAT-based drug delivery system--new directions in protein delivery for new hopes?

There has been great progress in the use of TAT-based drug delivery systems for the delivery of different macromolecules into cells in vitro and in vivo, thus circumventing the bioavailability barrier that is a problem for so many drugs. There are many advantages to using this system, such as the ability to deliver these cargoes into all types of cells in culture and into all organs in vivo. This system can even deliver cargoes into the brain across the blood-brain barrier. In addition, the ability to target specific intracellular sub-localizations such as the nuclei, the mitochondria and lysosomes further expands the possibilities of this drug delivery system to the development of sub-cellular organelle-targeted therapy. The therapeutic applications seem almost unlimited, and the use of the TAT-based delivery system has extended from proteins to a large variety of cargoes such as oligonucleotides, imaging agents, low molecular mass drugs, nanoparticles, micelles and liposomes. In this review the most recent advances in the use of the TAT-based drug delivery system will be described, mainly discussing TAT-mediated protein delivery and the use of the TAT system for enzyme replacement therapy.

Mechanisms of peptide amphiphile internalization by SJSA-1 cells in vitro

Self-assembly of peptide amphiphiles into nanostructures makes them attractive for a variety of applications in drug and peptide delivery. We here report on the interactions of micelles composed of a palmitoylated, pro-apoptotic peptide derived from p53 tumor suppressor protein with a human cancer cell line. Characterization of self-assembly in aqueous buffered solutions revealed formation of elongated rod-like micelles above a critical micelle concentration. Our results however demonstrate that monomers instead of micelles are internalized, a finding that correlates with the dynamic nature of the assemblies and the noncovalent interactions that hold them together. Internalization is shown to occur via adsorption-mediated, energy-dependent pathways, resulting in accumulation of the material in endocytic vesicles. We conclude that palmitoylation of peptides is an efficient way to increase peptide permeability inside SJSA-1 cells and that increased micelle stability would be required for intact micelle internalization.

The augmentation of intracellular delivery of peptide therapeutics by artificial protein transduction domains

Protein transduction domains (PTDs), such as HIV-derived Tat, have been successfully used as functional biomaterials for intracellular delivery of anti-cancer macromolecular drugs (protein, peptides, and oligonucleotides). Although there were therefore great expectations regarding the therapeutic potential of PTDs for the development of anti-cancer therapeutics, their clinical application so far has been extremely limited because of the relatively high concentrations required to mediate any effects on cancer cells in vitro or in vivo. In this context, improving the transduction efficiency of PTDs using phage display-based molecular evolution techniques may be useful for creating artificial PTDs with high efficiency and safety. Here, we report an evaluation of transduction efficiency and toxicity of such artificial PTDs (designated mT02 and mT03) compared with Tat. The internalization of mT02 was the most rapid and efficient by a mechanism different from the usual macropinocytosis. Furthermore, we found that artificial PTDs fused with survivin antagonistic peptide potentiate tumor cell-cytostatic activity. Thus, the results of this work provide new insights for designing new-generation peptide therapeutics for a wide variety of cancers as well as those expressing survivin.

Delivery of macromolecules using arginine-rich cell-penetrating peptides: ways to overcome endosomal entrapment

Arginine-rich cell-penetrating peptides (AR-CPPs) are very promising tools for the delivery of therapeutic macromolecules such as peptides, proteins, and nucleic acids. These peptides allow efficient internalization of the linked cargos intracellularly through the endocytic pathway. However, when linked to bulky cargos, entrapment in the endocytic vesicles is a major limitation to the application of these peptides in cytosolic delivery. Attachment of a compatible endosomal escape device is, therefore, necessary to allow cytosolic delivery of the peptide-attached cargo. This review presents different endosomal escape devices currently in application in combination with AR-CPPs. Applications of fusogenic lipids, membrane-disruptive peptides, membrane-disruptive polymers, lysosomotropic agents, and photochemical internalization to enhance the cytosolic delivery of AR-CPPs-attached cargos are presented. The properties of each system and its mechanism of action for the enhancement of endosomal escape are discussed, together with its applications for the delivery of different macromolecules in vitro and, if applicable, in vivo.

Tat-Mediated Peptide Intervention in Analgesia and Anesthesia

Membrane-permeable peptide carriers are attractive drug delivery tools. Among such carriers, the protein transduction domain (PTD) of the human immunodeficiency virus-type 1 Tat protein is most frequently used and has been successfully shown to deliver a large variety of cargoes. The Tat PTD can facilitate the uptake of large, biologically active molecules into mammalian cells, and recent studies have shown that it can mediate the delivery of different cargoes into tissues throughout a living organism. Given that the Tat PTD-mediated delivery is size-independent, this technology could make previously non-applicable large molecules usable to modulate biological function in vivo and treat human diseases. It is likely that the peptide carrier-mediated intracellular delivery process encompasses multiple mechanisms, but endocytic pathways are the predominant internalization routes. Tat PTD has been successfully used in preclinical models for the study of cancer, ischemia, inflammation, analgesia, and anesthesia. Our recent studies have shown that intraperitoneally injected fusion Tat peptide Tat-PSD-95 PDZ2 can be delivered into the spinal cord to dose-dependently disrupt protein-protein interactions between PSD-95 and NMDA receptors. This peptide significantly inhibits chronic inflammatory pain and reduces the threshold for halothane anesthesia. The ability of the Tat PTD to target any cell is advantageous in some respects. However, the drug delivery system will be more attractive if we can modify the Tat PTD to deliver cargo only into desired organs to avoid possible side effects.