Heparin binding by the HIV-1 tat protein transduction domain

Expression, Purification, Characterization and Cellular Uptake of MeCP2 Variants

The transcriptional regulator Methyl-CpG-binding protein 2 (MeCP2) is an intrinsically disordered protein, mutations in which, are implicated in the onset of Rett Syndrome, a severe and debilitating neurodevelopmental disorder. Delivery of this protein fused to the cell-penetrating peptide TAT could allow for the intracellular replenishment of functional MeCP2 and hence potentially serve as a prospective Rett Syndrome therapy. This work outlines the expression, purification and characterization of various TAT-MeCP2 constructs as well as their full-length and shortened eGFP fusion variants. The latter two constructs were used for intracellular uptake studies with subsequent analysis via western blotting and live-cell imaging. All purified MeCP2 samples exhibited high degree of stability and very little aggregation propensity. Full length and minimal TAT-MeCP2-eGFP were found to efficiently transduce into human dermal and murine fibroblasts and localize to cell nuclei. These findings clearly support the utility of MeCP2-based protein replacement therapy as a potential Rett Syndrome treatment option.

CCAAT/Enhancer-Binding Protein ε27 Antagonism of GATA-1 Transcriptional Activity in the Eosinophil Is Mediated by a Unique N-Terminal Repression Domain, Is Independent of Sumoylation and Does Not Require DNA Binding

CCAAT/enhancer binding protein epsilon (C/EBPε) is required for eosinophil differentiation, lineage-specific gene transcription, and expression of C/EBPε³² and shorter 27kD and 14kD isoforms is developmentally regulated during this process. We previously defined the 27kD isoform (C/EBPε²⁷) as an antagonist of GATA-1 transactivation of the eosinophil's major basic protein-1 (MBP1) P2-promoter, showing C/EBPε²⁷ and GATA-1 physically interact. In the current study, we used a Tat-C/EBPε²⁷ fusion protein for cell/nuclear transduction of an eosinophil myelocyte cell line to demonstrate that C/EBPε²⁷ is a potent repressor of MBP1 transcription. We performed structure-function analyses of C/EBPε²⁷ mapping its repressor domains, comparing it to C/EBPε³² and C/EBPε¹⁴, using GATA-1 co-transactivation of the MBP1-P2 promoter. Results show C/EBPε²⁷ repression of GATA-1 is mediated by its unique 68aa N-terminus combined with previously identified RDI domain. This repressor activity does not require, but is enhanced by, DNA binding via the basic region of C/EBPε²⁷ but independent of sumoylation of the RDI core "VKEEP" sumoylation site. These findings identify the N-terminus of C/EBPε²⁷ as the minimum repressor domain required for antagonism of GATA-1 in the eosinophil. C/EBPε²⁷ repression of GATA-1 occurs via a combination of both C/EBPε²⁷-GATA-1 protein-protein interaction and C/EBPε²⁷ binding to a C/EBP site in the MBP1 promoter. The C/EBPε²⁷ isoform may serve to titrate and/or turn off eosinophil granule protein genes like MBP1 during eosinophil differentiation, as these genes are ultimately silenced in the mature cell. Understanding the functionality of C/EBPε²⁷ in eosinophil development may prove promising in developing therapeutics that reduce eosinophil proliferation in allergic diseases.

Emerging treatments for progressive myoclonus epilepsies

Introduction: Progressive myoclonus epilepsies (PMEs) are a group of neurodegenerative diseases, invariably leading to severe disability or fatal outcome in a few years or decades. Nowadays, PMEs treatment remains challenging with a significant burden of disability for patients. Pharmacotherapy is primarily used to treat seizures, which impact patients' quality of life. However, new approaches have emerged in the last few years, which try to curb the neurological deterioration of PMEs through a better knowledge of the pathogenetic process. This is a review on the newest therapeutic options for the treatment of PMEs.Areas covered: Experimental and clinical results on novel therapeutic approaches for the different forms of PME are reviewed and discussed. Special attention is primarily focused on the efficacy and tolerability outcomes, trying to infer the role novel approaches may have in the future.Expert opinion: The large heterogeneity of disease-causing mechanisms prevents researchers from identifying a single approach to treat PMEs. Understanding of pathophysiologic processes is leading the way to targeted therapies, which, through enzyme replacement or underlying gene defect correction have already proved to potentially strike on neurodegeneration.

Tat basic domain: A "Swiss army knife" of HIV-1 Tat?

Tat (transactivator of transcription) regulates transcription from the HIV provirus. It plays a crucial role in disease progression, supporting efficient replication of the viral genome. Tat also modulates many functions in the host genome via its interaction with chromatin and proteins. Many of the functions of Tat are associated with its basic domain rich in arginine and lysine residues. It is still unknown why the basic domain exhibits so many diverse functions. However, the highly charged basic domain, coupled with the overall structural flexibility of Tat protein itself, makes the basic domain a key player in binding to or associating with cellular and viral components. In addition, the basic domain undergoes diverse posttranslational modifications, which further expand and modulate its functions. Here, we review the current knowledge of Tat basic domain and its versatile role in the interaction between the virus and the host cell.

Tat-MyoD fused proteins, together with C2c12 conditioned medium, are able to induce equine adult mesenchimal stem cells towards the myogenic fate

The Tat protein is able to translocate through the plasma membrane and when it is fused with other peptides may acts as a protein transduction system. This ability appears particularly interesting to induce tissue-specific differentiation when the Tat protein is associated to transcription factors. In the present work, the potential of the complex Tat-MyoD in inducing equine peripheral blood mesenchymal stem cells (PB-MSCs) towards the myogenic fate, was evaluated. Results showed that the internalization process of Tat-MyoD happens only in serum free conditions and that the nuclear localization of the fused complex is observed after 15 hours of incubation. However, the supplement of Tat-MyoD only was not sufficient to induce myogenesis and, therefore, in order to achieve the myogenic differentiation of PB-MSCs, conditioned medium from C2C12 cells was added without direct contact. Real Time PCR and immunofluorescence methods evaluated the establishment of a myogenic program. Our results suggest that TAT- transduction of Tat-MyoD, when supported by conditioned medium, represents a useful methodology to induce myogenic differentiation.

Heparin-Regulated Prodrug-Type Macromolecular Theranostic Systems for Cancer Therapy

Heparin is a kind of naturally occurring polymer with excellent biocompatibility and solubility. It is characterized by dense of negative charge, higher than any endogenous components. Heparin can bind with various cationic peptides and proteins, thereby providing a useful noncovalent linkage for building a drug delivery system. As a case in point, heparin/cell-penetrating peptides (CPP) interaction is strong, and remains stable in vivo. They can be used to modify different proteins, respectively, and subsequently, by simply mixing the modified proteins, a protein-protein conjugate can be form via the stable heparin/CPP linkage. This linkage could not be broken unless addition of protamine that bears higher cationic charge density than CPP, and CPP thus can be substituted and released. Of note, heparin is a potent antagonist of CPP, and their binding naturally inhibits CPP-mediated drug cell penetration. Based on this method, we developed a heparin-regulated macromolecular prodrug-type system, termed ATTEMPTS, for drug targeting delivery. In this review article, we mainly summary the application of ATTEMPTS in delivery of various macromolecular drugs for cancer therapy, and also introduce the heparin-regulated nanoprobes for tumor imaging.

Delivery of Soluble Heme Oxygenase 1 Cell-Penetrating Peptide into Liver Cells in in vitro and ex vivo Models of Cold Ischemia

BACKGROUND/PURPOSE

Liver transplantation is the treatment of choice in patients with end-stage liver disease. During liver transplantation, ischemia-reperfusion injury (IRI) occurs, which is an inevitable consequence of the transplantation process. To reduce the extent of cellular injury, one of the proteins that have been extensively investigated is heme oxygenase 1 (HO-1), which plays an important role in protecting the organs against IRI. The aim of this study was to introduce an active and functional HO-1 protein conjugated to a cell-penetrating peptide (CPP) in vitro and ex vivo into liver cells in hypothermic and anoxic conditions and to assert its cytoprotective effects.

METHODS

We generated an enzymatically active soluble (s)HO-1-CPP recombinant protein. The ability of the sHO-1-CPP protein to penetrate McA-RH7777, Clone 9, and Hep G2 cells, primary hepatocytes, and Kupffer and human umbilical vein endothelial cells in vitro, as well as its ability to penetrate a whole liver ex vivo under hypothermic and anoxic conditions, was assessed. An in vitro hypoxia-reoxygenation (HR) model was used to determine the cytoprotective effect of the sHO-1-CPP protein.

RESULTS

We showed that our recombinant protein sHO-1-CPP can cross cell membranes into rodent and human liver cells in vitro, and the results were further validated ex vivo, where rodent livers were perfused with an organ preservation solution supplemented with sHO-1-CPP under anoxic and hypothermic conditions. Immunohistochemistry revealed an intracellular localization of sHO-1-CPP in zones 1-3 of the perfused livers. The CPP did not exert any significant toxicity on the cells. Treating cells with sHO-1-CPP showed significant cytoprotection in the in vitro HR model.

CONCLUSIONS

Our findings show that the recombinant protein sHO-1-CPP can be successfully delivered to cells of a whole organ in an ex vivo hypothermic and anoxic perfusion model and that it provides cytoprotection to hepatocytes in an in vitro HR model. These results hold great potential for future repair and protection of donor organs. Future experiments are planned to confirm these data in in vivo models of IRI.

Inhibitory effect of genetically engineered mesenchymal stem cells with Apoptin on hepatoma cells in vitro and in vivo

Hepatocellular carcinoma (HCC) is an aggressive tumor and has become one of the most frequent causes of cancer death in the world. The rate of post-operative recurrence and metastasis are still high even though after surgical resection. It is a difficult problem with extraordinary importance for the clinical treatment. So stem cell therapy becomes one of the anti-tumor biotherapy methods which is exploring. Due to the feature of homing to tumor site and immunosuppressive, mesenchymal stem cells (MSCs) have the capacity of gene treatment to tumor as a vehicle. Apoptin derived from chicken anemia virus is one kind of protein with an inherent ability to lyse cancer cells while leaving normal cells unharmed. Adenovirus (Ad) vectors can be modified to deliver therapeutic genes with the advantages of low toxicity and high transfer capacity. Now it has not been reported that combining MSCs and Adenovirus with Apoptin are used in HCC treatment. This study intends to construct recombinant adenovirus which expresses Apoptin and then infects human bone marrow MSCs, and explore the migration of MSCs to the hepatoma cells and inhibitory effect of genetically engineered mesenchymal stem cells with Apoptin on hepatoma cells in vitro and in vivo. Our research successfully established the recombinant Ad which was constructed by Ad system, and obtained MSCs which could secrete Apoptin. We found that both the modified MSCs with Apoptin and their conditional medium significantly inhibited the proliferation of liver cancer cells HepG2, which provided a novel means and experimental basis for stem cell treatment for HCC. This study tries to search for a stem cell therapy for cancers, which will provide a new approach and experimental basis for the clinical treatment of cancer. At the same time, this research will also provide experimental basis for a novel in vivo drug delivery system through stem cells as vehicle, which will resolve immune rejection induced by repeated applications of drug directly delivered by Ad vectors and reduce the high cost of a large-scale production and purification of exogenous drugs.

T-Cell Signaling in HIV-1 Infection

HIV exploits the T-cell signaling network to gain access to downstream cellular components, which serves as effective tools to break the cellular barriers. Multiple host factors and their interaction with viral proteins contribute to the complexity of HIV-1 pathogenesis and disease progression. HIV-1 proteins gp120, Nef, Tat and Vpr alter the T-cell signaling pathways by activating multiple transcription factors including NF-ĸB, Sp1 and AP-1. HIV-1 evades the immune system by developing a multi-pronged strategy. Additionally, HIV-1 encoded proteins influence the apoptosis in the host cell favoring or blocking T-cell apoptosis. Thus, T-cell signaling hijacked by viral proteins accounts for both viral persistence and immune suppression during HIV-1 infection. Here, we summarize past and present studies on HIV-1 T-cell signaling with special focus on the possible role of T cells in facilitating viral infection and pathogenesis

Improving the endosomal escape of cell-penetrating peptides and their cargos: strategies and challenges

Cell penetrating peptides (CPPs) can deliver cell-impermeable therapeutic cargos into cells. In particular, CPP-cargo conjugates tend to accumulate inside cells by endocytosis. However, they often remain trapped inside endocytic organelles and fail to reach the cytosolic space of cells efficiently. In this review, the evidence for CPP-mediated endosomal escape is discussed. In addition, several strategies that have been utilized to enhance the endosomal escape of CPP-cargos are described. The recent development of branched systems that display multiple copies of a CPP is presented. The use of viral or synthetic peptides that can disrupt the endosomal membrane upon activation by the low pH of endosomes is also discussed. Finally, we survey how CPPs labeled with chromophores can be used in combination with light to stimulate endosomal lysis. The mechanisms and challenges associated with these intracellular delivery methodologies are discussed.

Exploring transduction mechanisms of protein transduction domains (PTDs) in living cells utilizing single-quantum dot tracking (SQT) technology

Specific protein domains known as protein transduction domains (PTDs) can permeate cell membranes and deliver proteins or bioactive materials into living cells. Various approaches have been applied for improving their transduction efficacy. It is, therefore, crucial to clarify the entry mechanisms and to identify the rate-limiting steps. Because of technical limitations for imaging PTD behavior on cells with conventional fluorescent-dyes, how PTDs enter the cells has been a topic of much debate. Utilizing quantum dots (QDs), we recently tracked the behavior of PTD that was derived from HIV-1 Tat (TatP) in living cells at the single-molecule level with 7-nm special precision. In this review article, we initially summarize the controversy on TatP entry mechanisms; thereafter, we will focus on our recent findings on single-TatP-QD tracking (SQT), to identify the major sequential steps of intracellular delivery in living cells and to discuss how SQT can easily provide direct information on TatP entry mechanisms. As a primer for SQT study, we also discuss the latest findings on single particle tracking of various molecules on the plasma membrane. Finally, we discuss the problems of QDs and the challenges for the future in utilizing currently available QD probes for SQT. In conclusion, direct identification of the rate-limiting steps of PTD entry with SQT should dramatically improve the methods for enhancing transduction efficiency.

A HA2-Fusion tag limits the endosomal release of its protein cargo despite causing endosomal lysis

BACKGROUND

Protein transduction domains (PTDs) can be fused to a protein to render it cell-permeable. The delivery efficiencies of PTDs are, however, often poor because PTD-protein conjugates cannot escape from endosomes. A potential solution to this problem consists in adding HA2 analogs to the PTD-protein construct as these peptides can cause endosomal lysis upon acidification of the endosomal lumen. To date, however, the utility of HA2-based PTDs has not been clearly established.

METHODS

We investigate the biophysical and cellular properties of the glutamate-rich HA2 analog E5 fused to the model protein TAT-mCherry.

RESULTS

E5-TAT-mCherry causes the release of fluorescent dextrans trapped with the protein inside endosomes. Yet, E5-TAT-mCherry itself is not released in the cytosol of cells, indicating that the protein remained trapped inside endosomes even after endosomal lysis takes place. Cytosolic delivery of the protein could be achieved, however, by insertion of a disulfide bond between E5 and its cargo.

CONCLUSIONS

These results show that E5 causes the retention of its fused protein inside endosomes even after lysis takes place.

GENERAL SIGNIFICANCE

These data establish that HA2 analogs might not be useful PTDs unless cleavable linkers are engineered between PTD and protein cargo.

Systemic delivery of NEMO binding domain/IKKγ inhibitory peptide to young mdx mice improves dystrophic skeletal muscle histopathology

The activation of nuclear factor κB (NF-κB) contributes to muscle degeneration that results from dystrophin deficiency in human Duchenne muscular dystrophy (DMD) and in the mdx mouse. In dystrophic muscle, NF-κB participates in inflammation and failure of muscle regeneration. Peptides containing the NF-κB Essential Modulator (NEMO) binding domain (NBD) disrupt the IκB kinase complex, thus blocking NF-κB activation. The NBD peptide, which is linked to a protein transduction domain to achieve in vivo peptide delivery to muscle tissue, was systemically delivered to mdx mice for 4 or 7 weeks to study NF-κB activation, histological changes in hind limb and diaphragm muscle and ex vivo function of diaphragm muscle. Decreased NF-κB activation, decreased necrosis and increased regeneration were observed in hind limb and diaphragm muscle in mdx mice treated systemically with NBD peptide, as compared to control mdx mice. NBD peptide treatment resulted in improved generation of specific force and greater resistance to lengthening activations in diaphragm muscle ex vivo. Together these data support the potential of NBD peptides for the treatment of DMD by modulating dystrophic pathways in muscle that are downstream of dystrophin deficiency.

A novel human derived cell-penetrating peptide in drug delivery

Cell-penetrating peptides can carry a variety of biologically active molecules into cells. Here we have identified a novel CPP derived from the C-terminus of human extracellular superoxide dismutase (hC-SOD3) which was shown to be located throughout in the cytoplasm and nucleus by fluorescence microscopy investigation. Furthermore, when apoptin fused to hC-SOD3, it was translocated efficiently into HeLa cells resulting in antitumor activities. This study shows that hC-SOD3 has the potential to penetrate and translocate cargo molecules into cells and has no cytotoxicity at effective concentration.

Modeling of the endosomolytic activity of HA2-TAT peptides with red blood cells and ghosts

HA2-TAT is a peptide-based delivery agent that combines the pH-sensitive HA2 fusion peptide from influenza and the cell-penetrating peptide TAT from HIV. This chimeric peptide is engineered to induce the cellular uptake of macromolecules into endosomes via the TAT moiety and to respond to the acidifying lumen of endosomes to cause membrane leakage and release of macromolecules into cells via the HA2 moiety. The question of how HA2 and TAT affect the properties of one another remains, however, unanswered, and the behavior of the peptide inside endosomes is mostly uncharacterized. To address these issues, the binding and membrane leakage activity of a glutamic acid-enriched analogue E5-TAT was assessed with red blood cells and giant unilamellar vesicles as membrane models for endosomes. Hemolysis and microscopy assays reveal that E5-TAT binds to membranes in a pH-dependent manner and causes membrane leakage by inducing the formation of pores through which macromolecules can escape. The TAT moiety contributes to this activity by causing a shift in the pH response of E5 and by binding to negatively charged phospholipids. On the other hand, TAT binding to glycosaminoglycans reduces the lytic activity of E5-TAT. Addition of TAT to the C-terminus of E5 can therefore either increase or inhibit the activity of E5 depending on the cellular components present at the membrane. Taken together, these results suggest a model for the endosomolytic activity of the peptide and provide the basis for the molecular design of future delivery agents.

Internalization and kinetics of nuclear migration of protein-only, arginine-rich nanoparticles

Understanding the intracellular trafficking of nanoparticles internalized by mammalian cells is a critical issue in nanomedicine, intimately linked to therapeutic applications but also to toxicity concerns. While the uptake mechanisms of carbon nanotubes and polymeric particles have been investigated fairly extensively, there are few studies on the migration and fate of protein-only nanoparticles other than natural viruses. Interestingly, protein nanoparticles are emerging as tools in personalized medicines because of their biocompatibility and functional tuneability, and are particularly promising for gene therapy and also conventional drug delivery. Here, we have investigated the uptake and kinetics of intracellular migration of protein nanoparticles built up by a chimerical multifunctional protein, and functionalized by a pleiotropic, membrane-active (R9) terminal peptide. Interestingly, protein nanoparticles are first localized in endosomes, but an early endosomal escape allows them to reach and accumulate in the nucleus (but not in the cytoplasm), with a migration speed of 0.0044 ± 0.0003 μm/s, ten-fold higher than that expected for passive diffusion. Interestingly, the plasmatic, instead of the nuclear membrane is the main cellular barrier in the nuclear way of R9-assisted protein-only nanoparticles.

Revised role of glycosaminoglycans in TAT protein transduction domain-mediated cellular transduction

Cellular uptake of the human immunodeficiency virus TAT protein transduction domain (PTD), or cell-penetrating peptide, has previously been surmised to occur in a manner dependent on the presence of heparan sulfate proteoglycans that are expressed ubiquitously on the cell surface. These acidic polysaccharides form a large pool of negative charge on the cell surface that TAT PTD binds avidly. Additionally, sulfated glycans have been proposed to aid in the interaction of TAT PTD and other arginine-rich PTDs with the cell membrane, perhaps aiding their translocation across the membrane. Surprisingly, however, TAT PTD-mediated induction of macropinocytosis and cellular transduction occurs in the absence of heparan sulfate and sialic acid. Using labeled TAT PTD peptides and fusion proteins, in addition to TAT PTD-Cre recombination-based phenotypic assays, we show that transduction occurs efficiently in mutant Chinese hamster ovary cell lines deficient in glycosaminoglycans and sialic acids. Similar results were obtained in cells where glycans were enzymatically removed. In contrast, enzymatic removal of proteins from the cell surface completely ablated TAT PTD-mediated transduction. Our findings support the hypothesis that acidic glycans form a pool of charge that TAT PTD binds on the cell surface, but this binding is independent of the PTD-mediated transduction mechanism and the induction of macropinocytotic uptake by TAT PTD.

Cellular uptake mechanisms and potential therapeutic utility of peptidic cell delivery vectors: progress 2001-2006

Cell delivery vectors (CDVs) are short amphipathic and cationic peptides and peptide derivatives, usually containing multiple lysine and arginine residues. They possess inherent membrane activity and can be conjugated or complexed with large impermeable macromolecules and even microscopic particles to facilitate cell entry. Various mechanisms have been proposed but it is now becoming clear that the main port of entry into cells of such CDV constructs involves adsorptive-mediated endocytosis rather than direct penetration of the plasma membrane. It is still unclear, however, how and to what extent CDV constructs are capable of exiting endosomal compartments and reaching their intended cellular site of action, usually the cytosol or the nucleus. Furthermore, although many CDVs can mediate cellular uptake of their cargo and appear comparatively non-toxic to cells in tissue culture, the utility of CDVs for in vivo applications remains poorly understood. Whatever the mechanisms of cell entry and disposition, the overriding question as far as potential pharmacological application of CDV conjugates is concerned is whether or not a therapeutic margin can be achieved by their administration. Such a margin will only result if the intracellular concentration in the target tissues necessary to elicit the biological effect of the CDV cargo can be achieved at systemic CDV exposure levels that are non-toxic to both target and bystander cells. It is proposed that the focus of CDV research now be shifted from mechanistic in vitro studies with labeled but otherwise unconjugated CDVs to in vivo pharmacological and toxicological studies using CDV-derivatized and other cationized forms of inherently non-permeable macromolecules of true therapeutic interest.

TAT transduction: the molecular mechanism and therapeutic prospects

Research into the mechanism of protein transduction has undergone a renaissance in the past five years as many groups have sought to understand the behavior of transducing peptides to harness their enormous therapeutic and diagnostic potential. The field has benefited greatly from rigorous cell biological and biophysical studies of the mechanism used by cell penetrating peptides to enter cells and deliver their cargo. The recent identification of fluid phase endocytosis as the mode of cellular entry for TAT and other protein transduction domains has enhanced our understanding of how transduction facilitates intracellular delivery. Many outstanding questions and contradictions still remain to be resolved in the field. Nevertheless, the current body of work regarding the mechanism of uptake gives a much clearer picture of how these macromolecules enter cells and how we might enhance the bioavailability to take advantage of them clinically.

Enhancement of gene transfer using YIGSR analog of Tat-derived peptide

Cell penetrating peptide based gene carriers are notably known for low level of gene transfer. To remedy this, as laminin receptor (LR) has been previously linked to tumor metastasis, the LR-binding domain (YIGSR) as well as a scrambled sequence (SGIYR) were added to Tat-derived peptide sequence (YIGSR-Tat and SGIYR-Tat respectively). Peptides cellular uptake was assessed with high-LR (HT1080) and low-LR (HT29) cell lines by flow cytometry. Their ability to form complexes with DNA was examined using YOPRO-1 fluorescence assay and their transfection efficiencies evaluated using a luciferase reporter gene assay. DNA complexes were formed at (+/-) charge ratios as low as 2:1. While no conclusion could be drawn on the effect of YIGSR sequence on peptides uptake in both cell lines, a significant improvement in gene transfection in HT1080 cells was achieved using YIGSR-Tat compared to Tat and SGIYR-Tat. Additionally this increased efficiency was inhibited by excess free YIGSR. No significant difference in transfection efficiency was observed between Tat, SGIYR-Tat and YIGSR-Tat based complexes in HT29 cells. These studies demonstrate that attachment of receptor-binding ligand (YIGSR) to Tat-derived peptide can improve the efficiency of gene transfer in LR-positive cells (HT1080).

A novel use of TAT-EGFP to validate techniques to alter osteosarcoma cell surface glycosaminoglycan expression

Several methods to alter cell surface glycosaminoglycan (GAG) expression have previously been described, including treatments with chlorate to reduce the addition of charged sulfate groups, xyloside compounds to displace GAGs from their core proteins, and GAG lyases, such as heparinase and chondroitinase, to release GAG fragments from the cell layer. While these methods are useful in identifying cellular mechanisms which are dependent on GAGs, they must be stringently validated to assess results in the appropriate context. To determine the most useful technique for the evaluation of GAG function in osteogenesis, MG-63 osteosarcoma cells were systematically treated with these agents and evaluated for changes in cell surface GAGs using a TAT-EGFP fusion protein. TAT, a protein transduction domain from the HIV-1 virus, requires cell surface GAGs to traverse cell membranes. The EGFP component provides a method to assess protein entry into cells in both qualitative and quantitative tests. Here, TAT-EGFP transduction analysis confirmed radiochemical and physiological data that chlorate effectively disrupts GAG expression. TAT-EGFP entry into cells was also inhibited by the exogenous application of commercial heparin and GAGs extracted from MG-63 cells as well as by the pre-treatment of cells with chondroitinase ABC. However, neither heparinase III treatment nor the addition of exogenous chondroitin-6-sulfate affected TAT-EGFP entry into cells. In addition, xyloside-beta-D-naphthol and xyloside-beta-D-cis/trans-decahydro-2-naphthol treatment could not induce significant phenotypic change in these cells, and the unaffected TAT-EGFP transduction confirmed that this was due to an inability to efficiently prime GAG synthesis. The use of TAT-EGFP is thus a useful technique to specifically evaluate cell surface GAG expression in a simple, quantifiable manner, and avoids the complications involved with conventional radiochemical assays or analytical chromatography.

The taming of the cell penetrating domain of the HIV Tat: myths and realities

Protein transduction with cell penetrating peptides over the past several years has been shown to be an effective way of delivering proteins in vitro and now several reports have also shown valuable in vivo applications in correcting disease states. An impressive bioinspired phenomenon of crossing biological barriers came from HIV transactivator Tat protein. Specifically, the protein transduction domain of HIV Tat has been shown to be a potent pleiotropic peptide in protein delivery. Various approaches such as molecular modeling, arginine guanidinium head group structural strategy, multimerization of PTD sequence and phage display system have been applied for taming of the PTD. This has resulted in identification of PTD variants which are efficient in cell membrane penetration and cytoplasmic delivery. In spite of these state of the art technologies, the dilemma of low protein transduction efficiency and target specific delivery of PTD fusion proteins remains unsolved. Moreover, some misconceptions about PTD of Tat in the literature require considerations. We have assembled critical information on secretory, plasma membrane penetration and transcellular properties of Tat and PTD using molecular analysis and available experimental evidences.

Influence of protein transduction domains on intracellular delivery of macromolecules

As the plasma membrane and blood-brain barrier selectively restrict the entry of most compounds into cells to < 500 Da, delivering macromolecules into cells was, until recently, little more than a goal. However, with significant effort to capitalise on therapeutic targets available in the post-genomic era, novel approaches for delivering therapeutic macromolecules are being rapidly developed. The discovery of small cationic peptides, termed peptide/protein transduction domains or cell-penetrating peptides, which cross biological membranes, has emerged as a venerable Trojan horse to transport large, biologically active molecules, such as peptides, proteins and oligonucleotides, into mammalian cells in vitro, as well as in preclinical models and clinical trials in vivo. This review discusses the implications of peptide/protein transduction domain-mediated delivery of macromolecules and their possible uses as important primary drug delivery agents.

Internalization of cationic peptides: the road less (or more?) traveled

Facilitating the entry of molecules into mammalian cells is of great interest to fields as diverse as cell biology and drug delivery. The discovery of natural protein transduction domains and the development of artificial ones, including polyarginine, provides a means to achieve this goal. Here, we comment on key chemical and biological aspects of cationic peptide internalization, including the physiological relevance of this process.

Transactivation and signaling functions of Tat are not correlated: biological and immunological characterization of HIV-1 subtype-C Tat protein

Background

Of the diverse subtypes of Human Immunodeficiency Virus Type-1 (HIV-1), subtype-C strains cause a large majority of infections worldwide. The reasons for the global dominance of HIV-1 subtype-C infections are not completely understood. Tat, being critical for viral infectivity and pathogenesis, may differentially modulate pathogenic properties of the viral subtypes. Biochemical studies on Tat are hampered by the limitations of the current purification protocols. Tat purified using standard protocols often is competent for transactivation activity but defective for a variety of other biological functions. Keeping this limitation in view, we developed an efficient protein purification strategy for Tat.

Results

Tat proteins obtained using the novel strategy described here were free of contaminants and retained biological functions as evaluated in a range of assays including the induction of cytokines, upregulation of chemokine coreceptor, transactivation of the viral promoter and rescue of a Tat-defective virus. Given the highly unstable nature of Tat, we evaluated the effect of the storage conditions on the biological function of Tat following purification. Tat stored in a lyophilized form retained complete biological activity regardless of the storage temperature. To understand if variations in the primary structure of Tat could influence the secondary structure of the protein and consequently its biological functions, we determined the CD spectra of subtype-C and -B Tat proteins. We demonstrate that subtype-C Tat may have a relatively higher ordered structure and be less flexible than subtype-B Tat. We show that subtype-C Tat as a protein, but not as a DNA expression vector, was consistently inferior to subtype-B Tat in a variety of biological assays. Furthermore, using ELISA, we evaluated the anti-Tat antibody titers in a large number of primary clinical samples (n = 200) collected from all four southern Indian states. Our analysis of the Indian populations demonstrated that Tat is non-immunodominant and that a large variation exists in the antigen-specific antibody titers.

Conclusion

Our report not only describes a simple protein purification strategy for Tat but also demonstrates important structural and functional differences between subtype-B and -C Tat proteins. Furthermore, this is the first report of protein purification and characterization of subtype-C Tat.

Cellular uptake of S413-PV peptide occurs upon conformational changes induced by peptide-membrane interactions

In face of accumulated reports demonstrating that uptake of some cell-penetrating peptides occurs through previously described endocytic pathways, or is a consequence of cell fixation artifacts, we conducted a systematic analysis on the mechanism responsible for the cellular uptake of the S4(13)-PV karyophilic cell-penetrating peptide. The results reviewed here show that the S4(13)-PV peptide is able to very efficiently accumulate inside live cells in a rapid, non-toxic and dose-dependent manner, through a mechanism distinct from endocytosis. Comparative analysis of peptide uptake by mutant cells lacking heparan sulfate proteoglycans demonstrates that, although not mandatory, their presence at cell surface facilitates the cellular uptake of the S4(13)-PV peptide. Furthermore, we demonstrate that upon interaction with lipid vesicles, the S4(13)-PV peptide undergoes significant conformational changes that are consistent with the formation of helical structures. Such conformational changes occur concomitantly with a penetration of the peptide into the lipid bilayer, strongly suggesting that the resulting helical structures are crucial for the non-endocytic cellular uptake of the S4(13)-PV peptide. Overall, our data support that, rather than endocytosis, the cellular uptake of the S4(13)-PV cell-penetrating peptide is a consequence of its direct translocation through cell membranes following conformational changes induced by peptide-membrane interactions.

On the mechanisms of the internalization of S4(13)-PV cell-penetrating peptide

Cell-penetrating peptides have been shown to translocate across eukaryotic cell membranes through a temperature-insensitive and energy-independent mechanism that does not involve membrane receptors or transporters. Although cell-penetrating peptides have been successfully used to mediate the intracellular delivery of a wide variety of molecules of pharmacological interest both in vitro and in vivo, the mechanisms by which cellular uptake occurs remain unclear. In the face of recent reports demonstrating that uptake of cell-penetrating peptides occurs through previously described endocytic pathways, or is a consequence of fixation artifacts, we conducted a critical re-evaluation of the mechanism responsible for the cellular uptake of the S4(13)-PV karyophilic cell-penetrating peptide. We report that the S4(13)-PV peptide is able to accumulate inside live cells very efficiently through a rapid, dose-dependent and non-toxic process, providing clear evidence that the cellular uptake of this peptide cannot be attributed to fixation artifacts. Comparative analysis of peptide uptake into mutant cells lacking heparan sulphate proteoglycans demonstrates that their presence at the cell surface facilitates the cellular uptake of the S4(13)-PV peptide, particularly at low peptide concentrations. Most importantly, our results clearly demonstrate that, in addition to endocytosis, which is only evident at low peptide concentrations, the efficient cellular uptake of the S4(13)-PV cell-penetrating peptide occurs mainly through an alternative, non-endocytic mechanism, most likely involving direct penetration across cell membranes.

TAT-mediated endocytotic delivery of the loop deletion Bcl-2 protein protects neurons against cell death

Protein delivery mediated by protein transduction domains (PTD) such as the HIV-1 TAT-PTD has emerged as a promising approach for neuroprotection. The objective of this study was to generate and evaluate the neuroprotective potential of TAT fusion proteins using constructs based on Bcl-2 anti-death family proteins. A TAT-Bcl-2 construct with the loop domain deleted (TAT-Bcl-2Deltaloop) was tested for its ability to transduce neuronal cells and to promote survival. The potential mechanism of TAT-mediated protein internalization in neural cells was also investigated. The purified TAT-Bcl-2Deltaloop binds to neural cell and rat brain mitochondria, and transduces cultured neural cell lines and primary cortical neurons when used at nm concentrations. Effective internalization of TAT-Bcl-2Deltaloop occurs at 37 degrees C but not at 4 degrees C, consistent with an endocytotic process. Both cell association and internalization require interaction of TAT-Bcl-2Deltaloop with cell surface heparan sulfate proteoglycans. TAT-mediated protein delivery in neuronal cells occurs through a lipid raft-dependent endocytotic process, inhibited by the cholesterol-sequestering agent nystatin. Transducible loop deleted Bcl-2 increases the survival of cortical neurons following trophic factor withdrawal and also rescues neural cell lines from staurosporine-induced death. These results support the concept of using protein transduction of Bcl-2 constructs for neuroprotection.

Inhibition of mitochondrial neural cell death pathways by protein transduction of Bcl-2 family proteins

Bcl-2 and other closely related members of the Bcl-2 family of proteins inhibit the death of neurons and many other cells in response to a wide variety of pathogenic stimuli. Bcl-2 inhibition of apoptosis is mediated by its binding to pro-apoptotic proteins, e.g., Bax and tBid, inhibition of their oligomerization, and thus inhibition of mitochondrial outer membrane pore formation, through which other pro-apoptotic proteins, e.g., cytochrome c, are released to the cytosol. Bcl-2 also exhibits an indirect antioxidant activity caused by a sub-toxic elevation of mitochondrial production of reactive oxygen species and a compensatory increase in expression of antioxidant gene products. While classic approaches to cytoprotection based on Bcl-2 family gene delivery have significant limitations, cellular protein transduction represents a new and exciting approach utilizing peptides and proteins as drugs with intracellular targets. The mechanism by which proteins with transduction domains are taken up by cells and delivered to their targets is controversial but usually involves endocytosis. The effectiveness of transduced proteins may therefore be limited by their release from endosomes into the cytosol.

Delivery of bioactive molecules into the cell: the Trojan horse approach

In recent years, vast amounts of data on the mechanisms of neural de- and regeneration have accumulated. However, only in disproportionally few cases has this led to efficient therapies for human patients. Part of the problem is to deliver cell death-averting genes or gene products across the blood-brain barrier (BBB) and cellular membranes. The discovery of Antennapedia (Antp)-mediated transduction of heterologous proteins into cells in 1992 and other "Trojan horse peptides" raised hopes that often-frustrating attempts to deliver proteins would now be history. The demonstration that proteins fused to the Tat protein transduction domain (PTD) are capable of crossing the BBB may revolutionize molecular research and neurobiological therapy. However, it was only recently that PTD-mediated delivery of proteins with therapeutic potential has been achieved in models of neural degeneration in nerve trauma and ischemia. Several groups have published the first positive results using protein transduction domains for the delivery of therapeutic proteins in relevant animal models of human neurological disorders. Here, we give an extensive review of peptide-mediated protein transduction from its early beginnings to new advances, discuss their application, with particular focus on a critical evaluation of the limitations of the method, as well as alternative approaches. Besides applications in neurobiology, a large number of reports using PTD in other systems are included as well. Because each protein requires an individual purification scheme that yields sufficient quantities of soluble, transducible material, the neurobiologist will benefit from the experiences of other researchers in the growing field of protein transduction.

Pathway for polyarginine entry into mammalian cells

Cationic peptides known as protein transduction domains (PTDs) provide a means to deliver molecules into mammalian cells. Here, nonaarginine (R(9)), the most efficacious of known PTDs, is used to elucidate the pathway for PTD internalization. Although R(9) is found in the cytosol as well as the nucleolus when cells are fixed, this peptide is observed only in the endocytic vesicles of live cells. Colocalization studies with vesicular markers confirm that PTDs are internalized by endocytosis rather than by crossing the plasma membrane. The inability of R(9) to enter living cells deficient in heparan sulfate (HS) suggests that binding to HS is necessary for PTD internalization. This finding is consistent with the high affinity of R(9) for heparin (K(d) = 109 nM). Finally, R(9) is shown to promote the leakage of liposomes but only at high peptide:lipid ratios. These and other data indicate that the PTD-mediated delivery of molecules into live mammalian cells involves (1) binding to cell surface HS, (2) uptake by endocytosis, (3) release upon HS degradation, and (4) leakage from endocytic vesicles.

Targeting plasmid-encoded proteins to the antigen presentation pathways

The antigen presentation pathways constitute a fulcrum on which adaptive immunity is balanced, and their manipulation should allow us to induce designer immune responses. The ease and rapidity with which DNA vaccines can be constructed and altered make them ideal candidates with which to test the various targeting strategies that have been conceived to date. These approaches and the mechanisms that may (or may not) underlie their success are reviewed in this article.

Polyanions and the proteome

The behavior of the proteome reflects spatial and temporal organization both within and without cells. We propose that various macromolecular entities possessing polyanionic character such as proteoglycans, lipid bilayer surfaces, microtubules, microfilaments, and polynucleotides may provide a functional network that mediates a variety of cellular phenomena. The interaction of proteins with this array of polyanions is characterized by a lower degree of specificity than seen with most commonly recognized macromolecular interactions. In this commentary, potential roles for this polyanion network in diverse functions such as protein/protein interactions, protein folding and stabilization, macromolecular transport, and various disease processes are all considered, as well as the use of polyanions as therapeutic agents. The role of small polyanions in the regulation of protein/polyanion interactions is also postulated. We provide preliminary experimental analysis of the extent to which proteins interact with polyanions inside cells using a combination of two-dimensional chromatographic and electrophoretic methods and antibody arrays. We conclude that many hundreds to thousands of such interactions are present in cells and argue that future understanding of the proteome will require that the "polyanion world" be taken into account.

Full-length proteins attached to the HIV tat protein transduction domain are neither transduced between cells, nor exhibit enhanced immunogenicity

Several proteins have been accorded the unusual ability to translocate across cell membranes in a receptor-independent and temperature-independent manner, and this activity has been mapped to a highly basic series of residues currently termed a 'protein transduction domain' (PTD). This translocatory attribute, if authentic, would be valuable for purposes of gene therapy and vaccination. We have evaluated the PTD from the human immunodeficiency virus type 1 (HIV) tat protein and we conclude that, when synthesized de novo, (1) the HIV tat PTD does not enhance the immunogenicity of a full-length protein to which it is tethered; and (2) the HIV tat PTD does not cause intercellular transfer of an attached marker protein, as judged by careful quantitative analyses. From our data, and from a review of published materials, we suggest that contrary to current dogma there is little evidence that these supposedly translocatory proteins can move between live cells. Furthermore, we suggest that PTDs do not act to enhance translocation, but instead merely to increase binding to the cell surface; in which case, the term 'protein transduction domain', and the related acronym, are misnomers which should be abandoned. Our conclusions explain why the most dramatic demonstrations of PTD efficacy have been obtained using fixed cells and/or denatured proteins, and have obvious implications for gene therapy and vaccination.

Efficiency of protein transduction is cell type-dependent and is enhanced by dextran sulfate

Protein transduction domains (PTDs), both naturally occurring and synthetic, have been increasingly utilized to deliver biologically active agents to a variety of cell types in vitro and in vivo. We report that in addition to previously characterized arginine-rich PTDs, including TAT, lysine homopolymers were able to mediate transduction of a wide variety of cell types, as measured by flow cytometric and enzymatic assays. The efficiency of PTD-mediated transduction was influenced by the cell type tested, although polylysine homopolymers demonstrate levels of internalization that consistently exceeded those of TAT and arginine homopolymers. Transduction of arginine/lysine-rich PTDs occurred at 4 degrees C and following depletion of cellular ATP pools, albeit generally at reduced levels. Although transduction was reduced in Chinese hamster ovary mutant lines deficient in either heparan sulfate or glycosaminoglycan synthesis, uptake was restored to wild-type levels by incubating target cells with dextran sulfate. The enhancement of transduction by dextran sulfate suggests that electrostatic interactions play an important first step in the process by which PTDs and their cargo traverse the plasma membrane.

Protein transduction technology

Intracellular delivery of macromolecules remains problematic because of the bioavailability restriction imposed by the cell membrane. Recent studies on protein transduction domains have circumvented this barrier, however, and have resulted in the delivery of peptides, full-length proteins, iron beads, liposomes, and radioactive isotopes into cells in culture and animal models in vivo.