A 16-mer peptide (RQIKIWFQNRRMKWKK) from antennapedia preferentially targets the Class I pathway

Immunogenicity of a Tripartite Cell Penetrating Peptide Containing a MUC1 Variable Number of Tandem Repeat (VNTR) and A T Helper Epitope

Peptide-based vaccines for cancer have many advantages however, for optimization these immunogens should incorporate peptide epitopes that induce CD8, as well as CD4 responses, antibody and long term immunity. Cell penetrating peptides (CPP) with a capacity of cytosolic delivery have been used to deliver antigenic peptides and proteins to antigen presenting cells to induce cytotoxic T cell, helper T cell and humoral responses in mice. For this study, a tripartite CPP including a mucin 1 (MUC1) variable number of tandem repeat (VNTR) containing multiple T cell epitopes and tetanus toxoid universal T helper epitope peptide (tetCD4) was synthesised (AntpMAPMUC1tet) and immune responses investigated in mice. Mice vaccinated with AntpMAPMUC1tet + CpG show enhanced antigen-specific interferon-gamma (IFN-γ) and IL-4 T cell responses compared with AntpMAPMUC1tet vaccination alone and induced a Th1 response, characterised by a higher ratio of IgG2a antibody/IgG1 antibodies. Furthermore, vaccination generated long term MUC1-specific antibody and T cell responses and delayed growth of MUC1^+ve tumours in mice. This data demonstrates the efficient delivery of branched multiple antigen peptides incorporating CPP and that the addition of CpG augments immune responses.

Library screening of cell-penetrating peptide for BY-2 cells, leaves of Arabidopsis, tobacco, tomato, poplar, and rice callus

Cell-penetrating peptides (CPPs) are used for various applications, especially in the biomedical field. Recently, CPPs have been used as a part of carrier to deliver proteins and/or genes into plant cells and tissues; hence, these peptides are attractive tools for plant biotechnological and agricultural applications, but require more efficient delivery rates and optimization by species before wide-scale use can be achieved. Here, we developed a library containing 55 CPPs to determine the optimal CPP characteristics for penetration of BY-2 cells and leaves of Nicotiana benthamiana, Arabidopsis thaliana, tomato (Solanum lycopersicum), poplar (hybrid aspen Populus tremula × tremuloides line T89), and rice (Oryza sativa). By investigating the cell penetration efficiency of CPPs in the library, we identified several efficient CPPs for all the plants studied except rice leaf. In the case of rice, several CPPs showed efficient penetration into rice callus. Furthermore, we examined the relationship between cell penetration efficiency and CPP secondary structural characteristics. The cell penetration efficiency of Lys-containing CPPs was relatively greater in plant than in animal cells, which could be due to differences in lipid composition and surface charge of the cell membranes. The variation in optimal CPPs across the plants studied here suggests that CPPs must be optimized for each plant species and target tissues of interest.

Comparative Immunogenicity of a Cytotoxic T Cell Epitope Delivered by Penetratin and TAT Cell Penetrating Peptides

Cell penetrating peptides (CPP), including the TAT peptide from the human immunodeficiency virus transactivator of transcription (HIV-TAT) protein and penetratin from Drosophila Antennapedia homeodomain protein, translocate various cargos including peptides and proteins across cellular barriers. This mode of delivery has been harnessed by our group and others to deliver antigenic proteins or peptides into the cytoplasm of antigen processing cells (APC) such as monocyte-derived dendritic cells (MoDC). Antigens or T cell epitopes delivered by CPP into APC in vivo generate antigen-specific cytotoxic T cell and helper T cell responses in mice. Furthermore, mice immunised with these peptides or proteins are protected from a tumour challenge. The functional properties of CPP are dependent on the various cargos being delivered and the target cell type. Despite several studies demonstrating superior immunogenicity of TAT and Antp-based immunogens, none has compared the immunogenicity of antigens delivered by TAT and Antp CPP. In the current study we demonstrate that a cytotoxic T cell epitope from the mucin 1 (MUC1) tumour associated antigen, when delivered by TAT or Antp, generates identical immune responses in mice resulting in specific MUC1 T cell responses as measured by in vivo CTL assays, IFNγ ELISpot assays and prophylactic tumour protection.

Vectorology and factor delivery in induced pluripotent stem cell reprogramming

Induced pluripotent stem cell (iPSC) reprogramming requires sustained expression of multiple reprogramming factors for a limited period of time (10-30 days). Conventional iPSC reprogramming was achieved using lentiviral or simple retroviral vectors. Retroviral reprogramming has flaws of insertional mutagenesis, uncontrolled silencing, residual expression and re-activation of transgenes, and immunogenicity. To overcome these issues, various technologies were explored, including adenoviral vectors, protein transduction, RNA transfection, minicircle DNA, excisable PiggyBac (PB) transposon, Cre-lox excision system, negative-sense RNA replicon, positive-sense RNA replicon, Epstein-Barr virus-based episomal plasmids, and repeated transfections of plasmids. This review provides summaries of the main vectorologies and factor delivery systems used in current reprogramming protocols.

Hepatitis C virus-induced cytoplasmic organelles use the nuclear transport machinery to establish an environment conducive to virus replication

Hepatitis C virus (HCV) infection induces formation of a membranous web structure in the host cell cytoplasm where the viral genome replicates and virions assemble. The membranous web is thought to concentrate viral components and hide viral RNA from pattern recognition receptors. We have uncovered a role for nuclear pore complex proteins (Nups) and nuclear transport factors (NTFs) in the membranous web. We show that HCV infection leads to increased levels of cytoplasmic Nups that accumulate at sites enriched for HCV proteins. Moreover, we detected interactions between specific HCV proteins and both Nups and NTFs. We hypothesize that cytoplasmically positioned Nups facilitate formation of the membranous web and contribute to the compartmentalization of viral replication. Accordingly, we show that transport cargo proteins normally targeted to the nucleus are capable of entering regions of the membranous web, and that depletion of specific Nups or Kaps inhibits HCV replication and assembly.

Transportin 3 and importin α are required for effective nuclear import of HIV-1 integrase in virus-infected cells

Unlike other retroviruses, human immunodeficiency virus type-1 (HIV-1) can infect terminally differentiated cells, due to the ability of its pre-integration complex (PIC) to translocate via the host nuclear pore complex (NPC). The PIC Nuclear import has been suggested to be mediated by the viral integrase protein (IN), via either the importin α or transportin 3 (TNPO3/transportin-SR2) pathways.We show that in virus-infected cells, IN interacts with both importin α and TNPO3, simultaneously or separately, suggesting a multiple use of nuclear import pathways. Disruption of either the IN-importin α or IN-TNPO3 complexes in virus-infected cells by specific cell-permeable-peptides resulted in inhibition of IN and viral cDNA nuclear import. Here we show that peptides which disrupt either one of these complexes block virus infection, indicating involvement of both pathways in efficient viral replication. Formation of IN-importin α and IN-TNPO3 complexes has also been observed in IN-transfected cultured cells. Using specific peptides, we demonstrate that in transfected cells but not in virus infected cells the importin α pathway overrides that of TNPO3. The IN-importin α and IN-TNPO3 complexes were not observed in virus-infected Rev-expressing cells, indicating the Rev protein's ability to disrupt both complexes.Our work suggests that IN nuclear import requires the involvement of both importin α and TNPO3. The ability to inhibit nuclear import of the IN-DNA complex and consequently, virus infection by peptides that interrupt IN's interaction with either importin α or TNPO3 indicates that for efficient infection, nuclear import of IN should be mediated by both nuclear-import receptors.

Whole protein and defined CD8(+) and CD4(+) peptides linked to penetratin targets both MHC class I and II antigen presentation pathways

Cytoplasmic delivery and cross-presentation of proteins and peptides is necessary for processing and presentation of antigens for the generation of cytotoxic T cells. We previously described the use of the 16 amino acid peptide penetratin from the Drosophila Antennapedia homeodomain (penetratin, Antp) to transport cytotoxic T lymphocyte epitopes derived from ovalbumin (OVA) or the Mucin-1 tumor-associated antigen into cells. We have now shown that penetratin covalently conjugated to OVA protein and linked in tandem to CD4(+) and/or CD8(+) T-cell epitopes from OVA-stimulated T cells in vitro (B3Z T-cell hybridoma and OT-I and OT-II T cells). The induction of these responses was directly mediated by the penetratin peptide as linking a nonspecific 16-mer peptide to OVA or mixing did not induce CD8(+) or CD4(+) T-cell responses in vitro. Furthermore, interferon (IFN)-γ-secreting CD4(+) and CD8(+) T cells were induced which suppressed B16.OVA tumor growth in C57BL/6 mice. Tumor protection was mediated by a CD8(+) T-cell-dependent mechanism and did not require CD4(+) help to protect mice 7 days after a boost immunization. Alternatively, 40 days after a boost immunization, the presence of CD4(+) help enhanced antigen-specific IFN-γ-secreting CD8(+) T cells and tumor protection in mice challenged with B16.OVA. Long-term CD8 responses were equally enhanced by antigen-specific and universal CD4 help. In addition, immunization with AntpOVA significantly delayed growth of B16.OVA tumors in mice in a tumor therapy model.

PNA-based antisense oligonucleotides for micrornas inhibition in the absence of a transfection reagent

MicroRNAs (miRNAs) are noncoding RNAs approximately 22 nucleotides in length that play a major role in the regulation of important biological processes, including cellular development, differentiation, and apoptosis. Antisense oligonucleotides against miRNAs are useful tools for studying the biological mechanisms and therapeutic targets of miRNAs. Various antisense oligonucleotides chemistries, including peptide nucleic acids (PNAs), have been developed to enhance nuclease-resistance and affinity and specificity for miRNA targets. PNAs have a greater specificity and affinity for DNA and RNA than do natural nucleic acids, and they are resistant to nucleases-an essential property of an miRNA inhibitor that will be exposed to cellular nucleases. However, the main limiting factor in the use of PNAs is their reduced penetration into cells. Recently, several cell-penetrating peptides (CPPs) have been investigated as a means to overcome the limited penetration of PNAs. Here, we evaluated the ability of 11 CPPs to transport PNAs inside cells in the absence of transfection reagents and then investigated the ability of these CPPs to inhibit miRNAs. Of the 11 CPPs tested, Tat-modified-conjugated PNA showed the most effective penetration into cells in the absence of transfection reagents and most effectively inhibited miRNAs. Our data demonstrate that Tat-modified-conjugated CPP is the most suitable for supporting PNA-mediated miRNA inhibition.

The impact of human leukocyte antigen (HLA) micropolymorphism on ligand specificity within the HLA-B*41 allotypic family

BACKGROUND

Polymorphic differences between human leukocyte antigen (HLA) molecules affect the specificity and conformation of their bound peptides and lead to differential selection of the T-cell repertoire. Mismatching during allogeneic transplantation can, therefore, lead to immunological reactions.

DESIGN AND METHODS

We investigated the structure-function relationships of six members of the HLA-B*41 allelic group that differ by six polymorphic amino acids, including positions 80, 95, 97 and 114 within the antigen-binding cleft. Peptide-binding motifs for B*41:01, *41:02, *41:03, *41:04, *41:05 and *41:06 were determined by sequencing self-peptides from recombinant B*41 molecules by electrospray ionization tandem mass spectrometry. The crystal structures of HLA-B*41:03 bound to a natural 16-mer self-ligand (AEMYGSVTEHPSPSPL) and HLA-B*41:04 bound to a natural 11-mer self-ligand (HEEAVSVDRVL) were solved.

RESULTS

Peptide analysis revealed that all B*41 alleles have an identical anchor motif at peptide position 2 (glutamic acid), but differ in their choice of C-terminal pΩ anchor (proline, valine, leucine). Additionally, B*41:04 displayed a greater preference for long peptides (>10 residues) when compared to the other B*41 allomorphs, while the longest peptide to be eluted from the allelic group (a 16mer) was obtained from B*41:03. The crystal structures of HLA-B*41:03 and HLA-B*41:04 revealed that both alleles interact in a highly conserved manner with the terminal regions of their respective ligands, while micropolymorphism-induced changes in the steric and electrostatic properties of the antigen-binding cleft account for differences in peptide repertoire and auxiliary anchoring.

CONCLUSIONS

Differences in peptide repertoire, and peptide length specificity reflect the significant functional evolution of these closely related allotypes and signal their importance in allogeneic transplantation, especially B*41:03 and B*41:04, which accommodate longer peptides, creating structurally distinct peptide-HLA complexes.

Over-expression of the HIV-1 Rev promotes death of nondividing eukaryotic cells

Expression of the human immunodeficiency virus type 1 (HIV-1) Rev protein is essential for completion of the viral life cycle. Rev mediates nuclear export of partially spliced and unspliced viral transcripts and therefore bears a nuclear localization signal (NLS) as well as a nuclear export signal (NES), which allow its nucleocytoplasmic shuttling. Attempts to express the wild-type Rev protein in eukaryotic human cultured cells have encountered difficulties and so far have failed. Here we show that accumulation of Rev, which occurs in nondividing Rev-expressing cells or when such cells reach confluency, results in death of these cells. Cell death was also promoted by addition of a cell permeable peptide bearing the Rev-NES sequence, but not by the Rev-NLS peptide. Our results probably indicate that binding of excess amounts of the Rev protein or the NES peptide to the exportin receptor CRM1 results in cells' death.

A membrane penetrating multiple antigen peptide (MAP) incorporating ovalbumin CD8 epitope induces potent immune responses in mice

Cell penetrating peptides (CPP) represent a novel approach to facilitate cytoplasmic delivery of macromolecules. The DNA binding domain of Drosophila Antennapedia contains 60 amino acids and consists of 3 α-helices, with internalizing activity mapped to a 16-amino acid peptide penetratin (Antp) within the third α-helix. Here, we report on the use of penetratin to deliver a multiple antigen peptide (MAP) incorporating the immunodominant CD8 epitope of ovalbumin, SIINFEKL (MAPOVACD8). We demonstrate that penetratin linked to the MAPOVACD8 construct either by a disulfide (SS) or thioether (SC) linkage promotes the uptake, cross presentation and subsequent in vivo proliferation and generation of OVACD8 (SIINFEKL)-specific T cells. The MAPOVACD8 construct without penetratin is not presented by MHC class I molecules nor does it generate an in vivo IFN-γ response in C57BL/6 mice. Moreover, we clearly define the uptake and intracellular processing pathways of AntpMAPOVACD8 SS and SC revealing the majority of AntpMAPOVACD8 is taken up by DC via an endocytic, proteasome and tapasin independent mechanism. We also show that the uptake mechanism of AntpMAPOVACD8 is dose dependent and uptake or intracellular processing is not altered by the type of chemical linkage.

Inhibition of HIV-1 integrase nuclear import and replication by a peptide bearing integrase putative nuclear localization signal

Background

The integrase (IN) of human immunodeficiency virus type 1 (HIV-1) has been implicated in different steps during viral replication, including nuclear import of the viral pre-integration complex. The exact mechanisms underlying the nuclear import of IN and especially the question of whether it bears a functional nuclear localization signal (NLS) remain controversial.

Results

Here, we studied the nuclear import pathway of IN by using multiple in vivo and in vitro systems. Nuclear import was not observed in an importin α temperature-sensitive yeast mutant, indicating an importin α-mediated process. Direct interaction between the full-length IN and importin α was demonstrated in vivo using bimolecular fluorescence complementation assay (BiFC). Nuclear import studies in yeast cells, with permeabilized mammalian cells, or microinjected cultured mammalian cells strongly suggest that the IN bears a NLS domain located between residues 161 and 173. A peptide bearing this sequence -NLS-IN peptide- inhibited nuclear accumulation of IN in transfected cell-cycle arrested cells. Integration of viral cDNA as well as HIV-1 replication in viral cell-cycle arrested infected cells were blocked by the NLS-IN peptide.

Conclusion

Our present findings support the view that nuclear import of IN occurs via the importin α pathway and is promoted by a specific NLS domain. This import could be blocked by NLS-IN peptide, resulting in inhibition of viral infection, confirming the view that nuclear import of the viral pre-integration complex is mediated by viral IN.

ICAM-2 expression mediates a membrane-actin link, confers a nonmetastatic phenotype and reflects favorable tumor stage or histology in neuroblastoma

The actin cytoskeleton is a primary determinant of tumor cell motility and metastatic potential. Motility and metastasis are thought to be regulated, in large part, by the interaction of membrane proteins with cytoplasmic linker proteins and of these linker proteins, in turn, with actin. However, complete membrane-to-actin linkages have been difficult to identify. We used co-immunoprecipitation and competitive peptide assays to show that intercellular adhesion molecule-2 (ICAM-2)/alpha-actinin/actin may comprise such a linkage in neuroblastoma cells. ICAM-2 expression limited the motility of these cells and redistributed actin fibers in vitro, and suppressed development of disseminated tumors in an in vivo model of metastatic neuroblastoma. Consistent with these observations, immunohistochemical analysis demonstrated ICAM-2 expression in primary neuroblastoma tumors exhibiting features that are associated with limited metastatic disease and more favorable clinical outcome. In neuroblastoma cell lines, ICAM-2 expression did not affect AKT activation, tumorigenic potential or chemosensitivity, as has been reported for some types of transfected cells. The observed ICAM-2-mediated suppression of metastatic phenotype is a novel function for this protein, and the interaction of ICAM-2/alpha-actinin/actin represents the first complete membrane-linker protein-actin linkage to impact tumor cell motility in vitro and metastatic potential in an in vivo model. Current work focuses on identifying specific protein domains critical to the regulation of neuroblastoma cell motility and metastasis and on determining if these domains represent exploitable therapeutic targets.

Penetratin tandemly linked to a CTL peptide induces anti-tumour T-cell responses via a cross-presentation pathway

Recently there has been increasing evidence to suggest that membrane translocating peptides enter cells by a receptor-dependent pathway. There have been some studies on the mechanism of major histocompatibility complex (MHC) class I presentation of membrane translocating peptides incorporating cytotoxic T lymphocyte epitopes. However, these have been on different cell lines and only a limited number of inhibitors of the antigen presentation pathway were used. Herein, we demonstrate a comprehensive study utilizing a full spectrum of inhibitors to various pathways of MHC class I to elucidate the mechanism of the membrane translocating peptide, penetratin from Antennapedia (Int). It is clear that Int, RQIKIWFQNRRMKWKK when tandemly linked to a cytotoxic T lymphocyte peptide of ovalbumin, SIINFEKL (IntSIIN) is endocytosed via phagocytosis or macropinocytosis by dendritic cells in an ATP-dependent manner and is processed by a proteasome- and tapasin-independent pathway for presentation and loading to MHC class I molecules. In addition, the majority of antigen is taken up by negatively charged receptors. IntSIIN activates T cells in vitro and in vivo and protects mice against challenge with an ovalbumin-expressing tumour.

Pharmacokinetics and biodistribution of novel aptamer compositions

PURPOSE

Aptamers are highly selective nucleic acid-based drugs that are currently being developed for numerous therapeutic indications. Here, we determine plasma pharmacokinetics and tissue distribution in rat of several novel aptamer compositions, including fully 2'-O-methylated oligonucleotides and conjugates bearing high-molecular weight polyethylene glycol (PEG) polymers, cell-permeating peptides, and cholesterol.

METHODS

Levels of aptamer conjugates in biological samples were quantified radiometrically and by a hybridization-based dual probe capture assay with enzyme-linked fluorescent readout. Intact aptamer in urine was detected by capillary gel electrophoresis and matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF).

RESULTS

Aptamer compositions examined exhibited a wide range of mean residence times in circulation (0.6-16 h) and significant variation in distribution levels among organs and tissues. Among the conjugates tested, in vivo properties of aptamers were altered most profoundly by conjugation with PEG groups. Complexation with a 20 kDa PEG polymer proved nearly as effective as a 40 kDa PEG polymer in preventing renal clearance of aptamers. Conjugation with 20 kDa PEG prolonged aptamer circulatory half-life, while reducing both the extent of aptamer distribution to the kidneys and the rate of urinary elimination. In contrast, the fully 2'-O-Me aptamer composition showed rapid clearance from circulation, and elimination with intact aptamer detectable in urine at 48 h post-administration.

CONCLUSIONS

We find that conjugation and chemical composition can alter fundamental aspects of aptamer residence in circulation and distribution to tissues. Though the primary effect of PEGylation was on aptamer clearance, the prolonged systemic exposure afforded by presence of the 20 kDa moiety appeared to facilitate distribution of aptamer to tissues, particularly those of highly perfused organs.

Cell-penetrating peptides: mechanism and kinetics of cargo delivery

Cell-penetrating peptides (CPPs) are short peptides of less than 30 amino acids that are able to penetrate cell membranes and translocate different cargoes into cells. The only common feature of these peptides appears to be that they are amphipathic and net positively charged. The mechanism of cell translocation is not known but it is apparently receptor and energy independent although, in certain cases, translocation can be partially mediated by endocytosis. Cargoes that are successfully internalized by CPPs range from small molecules to proteins and supramolecular particles. Most CPPs are inert or have very limited side effects. Their penetration into cells is rapid and initially first-order, with half-times from 5 to 20 min. The size of smaller cargoes does not affect the rate of internalization, but with larger cargoes, the rate is substantially decreased. CPPs are novel vehicles for the translocation of cargo into cells, whose properties make them potential drug delivery agents, of interest for future use.

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.

The use of cell-penetrating peptides for drug delivery

In the past decade, several peptides that can translocate cell membranes have been identified. Some of these peptides, which can be divided into different families, have short amino acid sequences (10-27 residues in length) and enter the cell by a receptor-independent mechanism. Furthermore, these peptides are capable of internalizing hydrophilic cargoes. Although the detailed mechanism by which these molecules enter cells is poorly understood, their ability to traverse the membrane into the cytoplasm has provided a new and powerful biological tool for transporting drugs across cell membranes.

Bordetella pertussis adenylate cyclase delivers chemically coupled CD8+ T-cell epitopes to dendritic cells and elicits CTL in vivo

The adenylate cyclase (CyaA) produced by Bordetella pertussis is able to deliver CD8+ and CD4+ T-cell epitopes genetically grafted within the catalytic domain of the molecule into antigen presenting cells in vivo. We develop now a new approach in which peptides containing CD8+ epitopes are chemically linked to CyaA. We show that CTL responses were induced in mice immunized with CyaA bearing these CD8+ epitopes. Moreover, we demonstrate that the OVA257-264 CD8+ epitope chemically grafted to CyaA is presented to CD8+ T cells by a mechanism requiring (1) proteasome processing, (2) TAP and (3) neosynthesis of MHC class I molecules. Thus, this novel strategy represents a very versatile system as a single CyaA carrier protein could be easily and rapidly coupled to any desired synthetic peptide.

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.

Transduction peptides: from technology to physiology

During the past fifteen years, a variety of peptides have been characterized for their ability to translocate into live cells. Most are efficient vectors that can internalize hydrophilic cargoes, and so provide a valuable biological (and potentially therapeutic) tool for targeting proteins into cells. Furthermore, translocation of cell-permeable peptides across the plasma membrane and their subsequent access to the cytosol, even when fused to large hydrophilic proteins, is challenging the perception of the plasma membrane as an impermeable barrier.

A Stepwise Dissection of the Intracellular Fate of Cationic Cell-penetrating Peptides

The role of endosomal acidification and retrograde transport for the uptake of the highly basic cell-penetrating peptides penetratin, Tat, and oligoarginine was investigated. The effect of a panel of drugs that interfere with discrete steps of endocytosis or Golgi-mediated transport on uptake and cellular distribution of fluorescein-labeled peptide analogues was probed by confocal microscopy, flow cytometry, and fluorescence spectroscopy of whole cell lysates. The analyses were carried out in MC57 fibrosarcoma cells and in HeLa cells. While MC57 fibrosarcoma cells showed some vesicular fluorescence and a pronounced cytoplasmic fluorescence, in HeLa cells little cytoplasmic fluorescence was observed. In MC57 cells the inhibitors of endosomal acidification chloroquine and bafilomycin A1 abolished the release of the peptides into the cytoplasm. Release into the cytosol preserved endosomal integrity. In addition, cellular uptake of the peptides was inhibited by brefeldin A, a compound interfering with trafficking in the trans-Golgi network. In contrast, nordihydroguaiaretic acid, a drug that stimulates the rapid retrograde movement of both Golgi stacks and trans-Golgi network to the endoplasmic reticulum, promoted a cytoplasmic localization of Tat peptides in peptide-pulsed HeLa cells. The effects of these drugs on trafficking shared characteristics with those reported for the trafficking of plant and bacterial toxins, such as cholera toxin, which reach the cytoplasm by means of retrograde transport. A sequence comparison revealed a common stretch of 8-10 amino acids with high sequence homology to the Tat peptide. The structural and functional data therefore strongly suggest a common mechanism of import for cationic cell-penetrating peptides and the toxins.

The cationic region from HIV tat enhances the cell-surface expression of epitope/MHC class I complexes

The potential of genetic immunization has been acknowledged for almost a decade, but disappointing immunogenicity in humans has delayed its introduction into the clinical arena. To try to increase the potency of genetic immunization, we and others have evaluated 'translocatory' proteins, which are thought to exit living cells by an uncharacterized pathway, and enter neighboring cells in an energy-independent manner. Several laboratories, including our own, have begun to question these remarkable properties. Our previous studies showed that the ability of an epitope to induce major histocompatibility complex (MHC) class I restricted CD8(+) T cells was, indeed, enhanced by its being attached to the proposed translocatory sequence of the HIV-1 tat protein. However, we found little evidence that the increased immunogenicity resulted from transfer of the fusion peptide between living cells, and we proposed that it resulted instead from an increased epitope/MHC expression on the surface of transfected cells. Here, we directly test this hypothesis. We show that cells cotransfected with plasmids encoding an epitope, and the relevant MHC class I allele, can stimulate epitope-specific T cells, and that attachment of the epitope to a putative translocatory sequence - which we term herein an 'integral cationic region' (ICR) - leads to a marked increase in stimulatory activity. This elevated stimulatory capacity does not result from a nonspecific increase in MHC class I expression. We use a high-affinity T-cell receptor (TcR) specific for the epitope/MHC combination to quantitate directly the cell-surface expression of the immunogenic complex, and we show that the attachment of the tat ICR to an epitope results in a substantial enhancement of its cell-surface presentation. These data suggest an alternative explanation for the immune enhancement seen with ICRs.

"Translocatory proteins" and "protein transduction domains": a critical analysis of their biological effects and the underlying mechanisms

It has been suggested that several proteins, termed "translocatory" or "messenger" proteins, can move between living cells-exiting the cell of synthesis via an uncharacterized secretory pathway and entering adjacent cells by a nonendocytic mechanism that is active even at 4 degrees C. These activities, which have been mapped to short, highly basic regions termed "protein transduction domains" (PTDs), have engendered considerable interest in the gene therapy and vaccine research communities. If these proteins, and PTDs, are to be used in human or veterinary medicine, it is vital that the mechanisms underlying their effects be understood. This article presents a critical evaluation of the current literature and describes recent findings that indicate that the effects of these sequences might be explained by well-established biological principles.

Cell surface adherence and endocytosis of protein transduction domains

Protein transduction domains (PTD), such as the HIV TAT and the herpes simplex virus VP22 proteins, are reported to translocate across the membranes of mammalian cells. The mechanism of PTD membrane translocation has largely remained elusive, but recent studies suggest that the reported PTD translocation is due to a fixation artifact. We have constructed and expressed the PTDs VP22, TAT, polyarginine, and polylysine fused to the green fluorescent protein to visualize these proteins in both living and fixed cells. The investigated PTDs strongly adhered to the surface of living cells and were internalized by constitutive endocytosis. No cytosolic or nuclear import of the proteins was detected. In contrast, the PTD-GFP fusion proteins were redistributed to the cytosol and nucleus directly after fixation. Our findings suggest that the PTDs only mediate cell surface adherence, a property shared with many other positively charged macromolecules. The cell surface adherence results in endocytosis and accumulation of proteins in endosomes. We suggest that the biological effects observed for PTD fusion proteins are due to cell surface interactions and internalization of the proteins into cells by classical endocytosis.

A novel TAT-mitochondrial signal sequence fusion protein is processed, stays in mitochondria, and crosses the placenta

Mutations in nuclear and mitochondrial genomes can lead to defects in mitochondrial function. To date, repair of these defects with exogenous proteins or gene transfer has been difficult with either viral or nonviral vectors. We hypothesized that TAT fusion proteins would cross both mitochondrial membranes and that incorporation of a mitochondrial signal sequence into a TAT fusion protein would allow processing and localization of exogenous proteins in mitochondria. A TAT-mitochondrial malate dehydrogenase signal sequence (mMDH)-enhanced green fluorescent protein (eGFP) fusion protein was constructed. TAT-mMDH-eGFP allowed rapid transduction and localization of fusion protein into mitochondria of multiple cell types. In contrast, TAT-GFP, without a mitochondrial signal sequence, rapidly transduced into cells and mitochondria, displayed pseudo-first-order kinetics, but did not remain there. Mice injected 5 days prior with TAT-mMDH-eGFP had detectable eGFP activity in multiple tissue types. Western blotting of cytosolic and mitochondrial fractions isolated from their livers confirmed eGFP localization to mitochondria and that the mMDH transit peptide was recognized and processed. Furthermore, TAT-mMDH-eGFP fusion protein injected into pregnant mice crossed the placenta and was detectable in both the fetus and the newborn pups. TAT fusion proteins containing a mitochondrial signal sequence are a viable method to localize proteins to mitochondria.

Induction of antigen-specific CTL responses using antigens conjugated to short peptide vectors

Linear peptides (SynB vectors) with specific sequence motifs have been identified that are capable of enhancing the transport of a wide range of molecules into cells. These peptide vectors have been used to deliver exogenous peptides and protein Ags across the cell membrane and into the cytoplasm of cells. Specifically, in vitro analysis indicated that these SynB peptides enhanced the uptake of two 9-mer peptide Ags, NP(147-155) and Mtb(250-258) (T cell epitopes of influenza nucleoprotein and Mycobacterium tuberculosis, respectively) and the M. tuberculosis Ag Mtb8.4 protein, into K562 cells when covalently linked to the respective Ags. Furthermore, selected SynB vectors, when conjugated to these same Ags and used as immunogens, resulted in considerably enhanced Ag-specific CTL responses. Several SynB vectors were tested and resulted in varying levels of cellular uptake. The efficiency of uptake correlated with the ability of the SynB construct to deliver each epitope in vivo and induce specific CTL responses in mice. These data suggest that peptide vectors, such as SynB that transport target Ags across the cell membrane in a highly efficient manner, have significant potential for vaccine delivery.

A quantitative validation of fluorophore-labelled cell-permeable peptide conjugates: fluorophore and cargo dependence of import

Cell-permeable peptides were evaluated for a quantitatively controlled import of small molecules. The dependence of the import efficiency on the fluorophore, on the position of the fluorophore as well as on the nature of the cargo were addressed. Cellular uptake was quantitated by flow cytometry and fluorescence correlation microscopy (FCM). Fluorophores with different spectral characteristics, covering the whole visible spectral range, were selected in order to enable the simultaneous detection of several cell-permeable peptide constructs. The transcytosis sequences were based either on the sequence of the Antennapedia homeodomain protein (AntpHD)-derived penetratin peptide or the Kaposi fibroblast growth factor (FGF)-derived membrane translocating sequence (MTS)-peptide. In general, the AntpHD-derived peptides had a three- to fourfold higher import efficiency than the MTS-derived peptides. In spite of the very different physicochemical characteristics of the fluorophores, the import efficiencies for analogues labelled at different positions within the sequence of the import peptides showed a strong positive correlation. However, even for peptide cargos of very similar size, pronounced differences in import efficiency were observed. The use of cell-permeable peptide/cargo constructs for intracellular analyses of structure-function relationships therefore requires the determination of the intracellular concentrations for each construct individually.

Positively charged DNA-binding proteins cause apparent cell membrane translocation

Several positively charged DNA-binding proteins such as the human immunodeficiency virus Tat protein, the Antennapedia (Antp) homeobox protein, and the herpes simplex virus VP22 protein have been reported to translocate across cell membranes and accumulate in cell nuclei. The import occurs by a poorly understood mechanism that appears to be receptor- and energy-independent. We showed that both VP22 and the positively charged histone H1 adhered to the cell membrane of living cells and were not removed by extensive washing. However, after fixation the proteins relocated to the cell nucleus. The nuclear accumulation of VP22 and histone H1 after fixation shows that positively charged proteins may appear to translocate across the cell membrane because of a fixation artifact. The majority of studies on "membrane permeable" proteins and peptides have been performed using fixation techniques, and our study shows that influx of these proteins may occur during fixation rather than in living cells.

Enhancing T cell activation and antiviral protection by introducing the HIV-1 protein transduction domain into a DNA vaccine

Protein transduction domains (PTD), which can transport proteins or peptides across biological membranes, have been identified in several proteins of viral, invertebrate, and vertebrate origin. Here, we evaluate the immunological and biological consequences of including PTD in synthetic peptides and in DNA vaccines that contain CD8(+) T cell epitopes from lymphocytic choriomeningitis virus (LCMV). Synthetic PTD-peptides did not induce detectable CD8(+) T cell responses. However, fusion of an open reading frame encoding a PTD to an epitope minigene caused transfected tissue culture cells to stimulate epitope-specific T cells much more effectively. Kinetic studies indicated that the epitope reached the surface of transfected cells more rapidly and that the number of transfected cells needed to stimulate T cell responses was reduced by 35- to 50-fold when compared to cells transfected with a standard minigene plasmid. The mechanism underlying the effect of PTD linkage is not clear, but transit of the PTD-attached epitope from transfected cells to nontransfected cells (cross presentation) seemed to play, at most, a minimal role. Mice immunized once with the plasmid encoding the PTD-linked epitope showed a markedly accelerated CD8(+) T cell response and, unlike mice immunized with a standard plasmid, were completely protected against a normally lethal LCMV challenge administered only 8 days post-immunization.