Cellular internalization of a cargo complex with a novel peptide derived from the third helix of the islet-1 homeodomain. Comparison with the penetratin peptide

Nature-inspired peptide of MtDef4 C-terminus tail enables protein delivery in mammalian cells

Cell-penetrating peptides show promise as versatile tools for intracellular delivery of therapeutic agents. Various peptides have originated from natural proteins with antimicrobial activity. We investigated the mammalian cell-penetrating properties of a 16-residue peptide with the sequence GRCRGFRRRCFCTTHC from the C-terminus tail of the Medicago truncatula defensin MtDef4. We evaluated the peptide's ability to penetrate multiple cell types. Our results demonstrate that the peptide efficiently penetrates mammalian cells within minutes and at a micromolar concentration. Moreover, upon N-terminal fusion to the fluorescent protein GFP, the peptide efficiently delivers GFP into the cells. Despite its remarkable cellular permeability, the peptide has only a minor effect on cellular viability, making it a promising candidate for developing a cell-penetrating peptide with potential therapeutic applications.

In Silico Selection and Evaluation of Pugnins with Antibacterial and Anticancer Activity Using Skin Transcriptome of Treefrog (Boana pugnax)

In order to combat bacterial and cancer resistance, we identified peptides (pugnins) with dual antibacterial l-anticancer activity from the Boana pugnax (B. pugnax) skin transcriptome through in silico analysis. Pugnins A and B were selected owing to their high similarity to the DS4.3 peptide, which served as a template for their alignment to the B. pugnax transcriptome, as well as their function as part of a voltage-dependent potassium channel protein. The secondary peptide structure stability in aqueous medium was evaluated as well, and after interaction with the Escherichia coli (E. coli) membrane model using molecular dynamics. These pugnins were synthesized via solid-phase synthesis strategy and verified by Reverse phase high-performance liquid chromatography (RP-HPLC) and mass spectrometry. Subsequently, their alpha-helix structure was determined by circular dichroism, after which antibacterial tests were then performed to evaluate their antimicrobial activity. Cytotoxicity tests against cancer cells also showed selectivity of pugnin A toward breast cancer (MFC7) cells, and pugnin B toward prostate cancer (PC3) cells. Alternatively, flow cytometry revealed necrotic cell damage with a major cytotoxic effect on human keratinocytes (HaCaT) control cells. Therefore, the pugnins found in the transcriptome of B. pugnax present dual antibacterial-anticancer activity with reduced selectivity to normal eukaryotic cells.

Functional maturation of immature β cells: A roadblock for stem cell therapy for type 1 diabetes

Type 1 diabetes mellitus (T1DM) is a chronic autoimmune disease caused by the specific destruction of pancreatic islet β cells and is characterized as the absolute insufficiency of insulin secretion. Current insulin replacement therapy supplies insulin in a non-physiological way and is associated with devastating complications. Experimental islet transplantation therapy has been proven to restore glucose homeostasis in people with severe T1DM. However, it is restricted by many factors such as severe shortage of donor sources, progressive loss of donor cells, high cost, etc. As pluripotent stem cells have the potential to give rise to all cells including islet β cells in the body, stem cell therapy for diabetes has attracted great attention in the academic community and the general public. Transplantation of islet β-like cells differentiated from human pluripotent stem cells (hPSCs) has the potential to be an excellent alternative to islet transplantation. In stem cell therapy, obtaining β cells with complete insulin secretion in vitro is crucial. However, after much research, it has been found that the β-like cells obtained by in vitro differentiation still have many defects, including lack of adult-type glucose stimulated insulin secretion, and multi-hormonal secretion, suggesting that in vitro culture does not allows for obtaining fully mature β-like cells for transplantation. A large number of studies have found that many transcription factors play important roles in the process of transforming immature to mature human islet β cells. Furthermore, PDX1, NKX6.1, SOX9, NGN3, PAX4, etc., are important in inducing hPSC differentiation in vitro. The absent or deficient expression of any of these key factors may lead to the islet development defect in vivo and the failure of stem cells to differentiate into genuine functional β-like cells in vitro. This article reviews β cell maturation in vivo and in vitro and the vital roles of key molecules in this process, in order to explore the current problems in stem cell therapy for diabetes.

Self-assembly and intracellular delivery of DNA by a truncated fragment derived from the Trojan peptide Penetratin

Penetratin is a short Trojan peptide that attracts great interest in biomedical research for its capacity to translocate biological membranes. Herein, we study in detail both self-assembly and intracellular delivery of DNA by the heptamer KIWFQNR, a truncated peptide derived from Penetratin. This shortened sequence possesses a unique design with bolaamphiphilic characteristics that preserves the longest noncationic amino acid portion found in Penetratin. These features convey amphipathicity to assist self-assembly and make it a suitable model for exploring the role of hydrophobic residues for peptide interaction and cell uptake. We show that the fragment forms peptiplexes (i.e., peptide-DNA complexes), and aggregates into long nanofibers with clear β-sheet signature. The supramolecular structure of nanofibers is likely composed of DNA cores surrounded by a peptide shell to which the double helix behaves as a template and induces fibrillization. A nucleation and growth mechanism proceeding through liquid-liquid phase separation of coacervates is proposed for describing the self-assembly of peptiplexes. We also demonstrate that peptiplexes deliver double-stranded 200 bp DNA into HeLa cells, indicating its potential for preparing non-viral vectors for oligonucleotides through noncovalent strategies. Since the main structural features of native Penetratin are conserved in this simpler fragment, our findings also highlight the role of uncharged amino acids for structuration, and thus for the ability of Penetratin to cross cell membranes.

Stem Cell Therapy and Type 1 Diabetes Mellitus: Treatment Strategies and Future Perspectives

Type 1 diabetes mellitus (T1DM) is classified as an autoimmune disease which progressively results in the depletion of insulin-secreting β-cells. Consequently, the insulin secretion stops leading to hyperglycemic situations within the body. Under severe conditions, it also causes multi-organ diabetes-associated dysfunctionalities notably hypercoagulability, neuropathy, nephropathy, retinopathy, and sometimes organ failures. The prevalence of this disease has been noticed about 3% that has highlighted the serious concerns for healthcare professionals around the globe. For the treatment of this disease, the cell therapy is considered as an important therapeutic approach for the replacement of damaged β-cells. However, the development of autoantibodies unfortunately reduces their effectiveness with the passage of time and finally with the recurrence of diabetes mellitus. The development of new techniques for extraction and transplantation of islets failed to support this approach due to the issues related to major surgery and lifelong dependence on immunosuppression. For T1DM, such cells are supposed to produce, store, and supply insulin to maintain glucose homeostasis. The urgent need of much-anticipated substitute for insulin-secreting β-cells directed the researchers to focus on stem cells (SCs) to produce insulin-secreting β-cells. For being more specific and targeted therapeutic approaches, SC-based strategies opened up the new horizons to cure T1DM. This cell-based therapy aimed to produce functional insulin-secreting β-cells to cure diabetes on forever basis. The intrinsic regenerative potential along with immunomodulatory abilities of SCs highlights the therapeutic potential of SC-based strategies. In this article, we have comprehensively highlighted the role of SCs to treat diabetes mellitus.

Selected application of peptide molecules as pharmaceutical agents and in cosmeceuticals

Introduction: Peptide molecules are being vastly investigated as an emerging class of therapeutic molecules in recent years. Currently, 60 peptides have been approved by the US Food and Drug Administration (FDA), and more would enter the market in near future. Peptides have already opened their ways into cosmeceutical and food industries as well.Areas covered: Antibodies, vaccines, and antimicrobial agents are the major classes of therapeutic peptides. Additionally, peptides may be employed in drug development to support cell penetration or targeting. The interest in antimicrobial peptides is surging due to the increasing risk of antibiotic-resistant pathogens. Peptide vaccines with their significant advantages compared with traditional vaccines, are expected to find their place in coming years, especially for cancer, microbial and allergen-specific immunotherapy. The usage of peptides in cosmeceuticals is also growing rapidly.Expert opinion: Peptide synthesis has become accessible, and advances in peptide engineering, sequencing technologies, and structural bioinformatics have resulted in the rational designing of novel peptides. All these advancements would lead to the more prominent roles of peptides in the mentioned areas. In this review, we discuss applications of peptides in different fields including pharmaceuticals, cosmeceuticals, besides the critical factors in designing efficient peptide molecules.

Islet-1 promotes the proliferation and invasion, and inhibits the apoptosis of A375 human melanoma cells

The aim of this study was to examine the effects of the insulin gene enhancer-binding protein, islet-1 (ISL1), on the proliferation, invasion and apoptosis of the human melanoma cell line, A375. An ISL1 overexpression lentiviral vector was constructed and transfected into the A375 cells. The proliferation of the A375 cells transfected with the ISL1 vector (termed A375/ISL1 cells) was examined by MTT assay, flow cytometry and TUNEL assay, and cell invasion was examined by Transwell assay. The expression levels of matrix metalloproteinase (MMP)-2 and MMP-9 were measured by qPCR and western blot analysis; the expression levels of Akt and p-Akt were measured in the cells treated with vascular endothelial growth factor (VEGF) and the PI3K/Akt inhibitor, LY294002, by western blot analysis. The optical density value of the A375/ISL1 cells was increased after 12 h of culture (P<0.001), as shown by MTT assay. The ratio of apoptotic A375/ISL1 cells was significantly decreased (P<0.001), as shown by flow cytometry and TUNEL assay. In addition, the average penetration rate of the A375/ISL1 cells significantly increased (P<0.001), as shown by Transwell assay. The expression levels of MMP-2 and MMP-9 were significantly increased in the A375/ISL1 cells, as shown by qPCR and western blot analysis (P<0.001). Moreover, treatment of the A375/ISL1 cells with VEGF for 48 h increased the expression of Akt and p-Akt compared with the control cells transfected with A375/green fluorescent protein (GFP) (P<0.05; P<0.001, respectively). In addition, in the A375/ISL1 cells treated with the LY294002 inhibitor for 24 and 48 h, the level of Akt was also found to increase compared to the control A375/GFP cells (P<0.05). On the whole, the findings of this study indicate that the overexpression of ISL1 promotes the proliferation and invasion, and inhibits the apoptosis of A375 melanoma cells. ISL1 thus plays an important role in A375 cell survival, and these effects are possibly mediate via the PI3K/Akt signaling pathway.

Stem cell therapy emerging as the key player in treating type 1 diabetes mellitus

Type 1 diabetes mellitus (T1DM) is an autoimmune disease causing progressive destruction of pancreatic β cells, ultimately resulting in loss of insulin secretion producing hyperglycemia usually affecting children. Replacement of damaged β cells by cell therapy can treat it. Currently available strategies are insulin replacement and islet/pancreas transplantation. Unfortunately these offer rescue for variable duration due to development of autoantibodies. For pancreas/islet transplantation a deceased donor is required and various shortfalls of treatment include quantum, cumbersome technique, immune rejection and limited availability of donors. Stem cell therapy with assistance of cellular reprogramming and β-cell regeneration can open up new therapeutic modalities. The present review describes the history and current knowledge of T1DM, evolution of cell therapies and different cellular therapies to cure this condition.

Classes of Cell-Penetrating Peptides

During the three decades of cell-penetrating peptides era the superfamily of CPPs has rapidly expanded, and the quest for new sequences continues. CPPs have been well recognized by scientific community and they have been used for transduction of a wide variety of molecules and particles into cultured cells and in vivo. In parallel with application of CPPs for delivering of active payloads, the mechanisms that such peptides take advantage of for gaining access to cells' insides have been in the focus of intense studies. Although the common denominator "cell penetration" unites all CPPs, the interaction partners on the cell surface, evoked cellular responses and even the uptake mechanisms might greatly vary between different peptide types. Here we present some possibilities for classification of CPPs based on their type of origin, physical-chemical properties, and the extent of modifications and design efforts. We also briefly analyze the internalization mechanisms with regard to their classification into groups based on physical-chemical characteristics.

Investigation of homeodomain membrane translocation properties: insights from the structure determination of engrailed-2 homeodomain in aqueous and membrane-mimetic environments

In addition to their well-known DNA-binding properties, homeodomains have the ability to efficiently translocate across biological membranes through still poorly-characterized mechanisms. To date, most biophysical studies addressing the mechanisms of internalization have focused on small synthetic peptides rather than full-length globular homeodomains. In this work, we characterized the conformational properties of chicken Engrailed 2 homeodomain (En2HD) in aqueous solution and in membrane mimetic environments using circular dichroism, Trp fluorescence, and NMR spectroscopy. En2HD adopts a well-defined three-helical bundle fold in aqueous solution. The Trp-48 residue, which is critical for internalization, is fully buried in the hydrophobic core. Circular dichroism and fluorescence reveal that a conformational transition occurs in anionic lipid vesicles and in micelles. En2HD loses its native three-dimensional structure in micellar environments but, remarkably, near-native helical secondary structures are maintained. Long-range interactions could be detected using site-directed spin labels, indicating that the three helices do not adopt extended orientations. Noncovalent paramagnetic probes yielded information about helix positioning and unveiled the burial of critical aromatic and basic residues within the micelles. Our results suggest that electrostatic interactions with membranes may be determinant in inducing a conformational change enabling Trp-48 to insert into membranes.

Cell-penetrating peptides: classes, origin, and current landscape

With more than ten new FDA approvals since 2001, peptides are emerging as an important therapeutic alternative to small molecules. However, unlike small molecules, peptides on the market today are limited to extracellular targets. By contrast, cell-penetrating peptides (CPPs) can target intracellular proteins and also carry other cargoes (e.g. other peptides, small molecules or proteins) into the cell, thus offering great potential as future therapeutics. In this review I present a classification scheme for CPPs based on their physical-chemical properties and origin, and I provide a general framework for understanding and discovering new CPPs.

High-content screening of peptide-based non-viral gene delivery systems

High-content screening (HCS) uses high-capacity automated fluorescence imaging for the quantitative analysis of single cells and cell populations. Here, we developed an HCS assay for rapid screening of non-viral gene delivery systems as exemplified by the screening of a small library of peptide-based transfectants. These peptides were simultaneously screened for transfection efficiency, cytotoxicity, induction of cell permeability and the capacity to transfect non-dividing cells. We demonstrated that HCS is a valuable extension to the already existing screening methods for the in vitro evaluation of non-viral gene delivery systems with the added value that multiple parameters can be screened in parallel thereby obtaining more information from a single screening event, which will accelerate the development of novel gene delivery systems.

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.

Predicting cell-penetrating peptides

Possibility to predict short peptide sequences capable to penetrate the plasma membrane opens new opportunities for developing peptide based intracellular delivery vectors, called cell-penetrating peptides (CPPs). Predictions of CPPs, however are often based on trial and error and may not always lead to new potent sequences. In this review we discuss different problems associated with CPP prediction. Additionally, the used methods of CPP prediction are compared. Also, a few suggestions are made for designing new CPP sequences and improvement of predictions.

Mesenchymal stem cells: biology and clinical potential in type 1 diabetes therapy

Mesenchymal stem cells (MSCs) can be derived from adult bone marrow, fat and several foetal tissues. In vitro, MSCs have the capacity to differentiate into multiple mesodermal and non-mesodermal cell lineages. Besides, MSCs possess immunosuppressive effects by modulating the immune function of the major cell populations involved in alloantigen recognition and elimination. The intriguing biology of MSCs makes them strong candidates for cell-based therapy against various human diseases. Type 1 diabetes is caused by a cell-mediated autoimmune destruction of pancreatic β-cells. While insulin replacement remains the cornerstone treatment for type 1 diabetes, the transplantation of pancreatic islets of Langerhans provides a cure for this disorder. And yet, islet transplantation is limited by the lack of donor pancreas. Generation of insulin-producing cells (IPCs) from MSCs represents an attractive alternative. On the one hand, MSCs from pancreas, bone marrow, adipose tissue, umbilical cord blood and cord tissue have the potential to differentiate into IPCs by genetic modification and/or defined culture conditions In vitro. On the other hand, MSCs are able to serve as a cellular vehicle for the expression of human insulin gene. Moreover, protein transduction technology could offer a novel approach for generating IPCs from stem cells including MSCs. In this review, we first summarize the current knowledge on the biological characterization of MSCs. Next, we consider MSCs as surrogate β-cell source for islet transplantation, and present some basic requirements for these replacement cells. Finally, MSCs-mediated therapeutic neovascularization in type 1 diabetes is discussed.

On The Biomedical Promise of Cell Penetrating Peptides: Limits Versus Prospects

The cell membrane poses a substantial hurdle to the use of pharmacologically active biomacromolecules that are not per se actively translocated into cells. An appealing approach to deliver such molecules involves tethering or complexing them with so-called cell penetrating peptides (CPPs) that are able to cross the plasma membrane of mammalian cells. The CPP approach is currently a major avenue in engineering delivery systems that are hoped to mediate the non-invasive import of problematic cargos into cells. The large number of different cargo molecules that have been efficiently delivered by CPPs ranges from small molecules to proteins and even liposomes and particles. With respect to the involved mechanism(s) there is increasing evidence for endocytosis as a major route of entry. Moreover, in terms of intracellular trafficking, current data argues for the transport to acidic early endosomal compartments with cytosolic release mediated via retrograde delivery through the Golgi apparatus and the endoplasmic reticulum. The focus of this review is to revisit the performance of cell penetrating peptides for drug delivery. To this aim we cover both accomplishments and failures and report on new prospects of the CPP approach. Besides a selection of successful case histories of CPPs we also review the limitations of CPP mediated translocation. In particular, we comment on the impact of (i) metabolic degradation, (ii) the cell line and cellular differentiation state dependent uptake of CPPs, as well as (iii) the regulation of their endocytic traffic by Rho-family GTPases. Further on, we aim at the identification of promising niches for CPP application in drug delivery. In this context, as inspired by current literature, we focus on three principal areas: (i) the delivery of antineoplastic agents, (ii) the delivery of CPPs as antimicrobials, and (iii) the potential of CPPs to target inflammatory tissues.

Protein transduction technology offers a novel therapeutic approach for diabetes

Diabetes remains a major burden, with more than 200 million people affected worldwide, representing 6% of the population. New technology, known as protein transduction technology, has been recently developed. A variety of peptides, known as protein transduction domains or cell-penetrating peptides, have been characterized for their ability to translocate into live cells. There are numerous examples of biologically active full-length proteins and peptides that have been delivered to cells and tissues both in vitro and in vivo, suggesting new avenues for the treatment of several diseases. Some studies have shown that this technology is useful for the treatment of diabetes. In islet isolation and transplantation, cell-permeable peptides deliver anti-apoptotic molecules to protect islets. Another peptide provides immunosuppression for fully mismatched islet allografts in mice. These findings suggest that peptide drugs could lead to outcome improvement for pancreatic islet transplantation. In mice with type 2 diabetes, a cell-penetrating peptide markedly improves insulin resistance and ameliorates glucose tolerance. Moreover, the technology facilitates the differentiation of stem cells into insulin-producing cells. Protein transduction technology has opened up several possibilities for the development of new peptide/protein drugs for the treatment of diabetes.

Medium-sized peptides as built in carriers for biologically active compounds

A growing number of oligopeptides of natural and/or synthetic origin have been described and considered as targeting structures for delivery bioactive compounds into various cell types. This review will outline the discovery of peptide sequences and the corresponding mid-sized oligopeptides with membrane translocating properties and also summarize de novo designed structures possessing similar features. Conjugates and chimera constructs derived from these sequences with covalently attached bioactive peptide, epitope, oligonucleotide, PNA, drug, reporter molecule will be reviewed. A brief note will refer to the present understanding on the uptake mechanism at the end of each section.

The importance of valency in enhancing the import and cell routing potential of protein transduction domain-containing molecules

Protein transduction domains (PTDs) are peptides that afford the internalization of cargo macromolecules (including plasmid DNA, proteins, liposomes, and nanoparticles). In the case of polycationic peptides, the efficiency of PTDs to promote cellular uptake is directly related to their molecular mass or their polyvalent presentation. Similarly, the efficiency of routing to the nucleus increases with the number of nuclear localization signals (NLS) associated with a cargo. The quantitative enhancement, however, depends on the identity of the PTD sequence as well as the targeted cell type. Thus the choice and multivalent presentation of PTD and NLS sequences are important criteria guiding the design of macromolecules intended for specific intracellular localization. This review outlines synthetic and recombinant strategies whereby PTDs and signal sequences can be assembled into multivalent peptide dendrimers and promote the uptake and routing of their cargoes. In particular, the tetramerization domain of the tumour suppressor p53 (p53tet) is emerging as a useful scaffold to present multiple routing and targeting moieties. Short cationic peptides fused to the 31-residue long p53tet sequence resulted in tetramers displaying a significant enhancement (up to 1000 fold) in terms of their ability to be imported into cells and delivered to the cell nucleus in relation to their monomeric analogues. The design of future polycationic peptide dendrimers as effective delivering vehicles will need to incorporate selective cell targeting functions and provide solutions to the issue of endosomal entrapment.

Bilayer interaction and localization of cell penetrating peptides with model membranes: A comparative study of a human calcitonin (hCT)-derived peptide with pVEC and pAntp(43–58)

Cell-penetrating peptides (CPPs) are able to translocate problematic therapeutic cargoes across cellular membranes. The exact mechanisms of translocation are still under investigation. However, evidence for endocytic uptake is increasing. We investigated the interactions of CPPs with phospholipid bilayers as first step of translocation. To this purpose, we employed four independent techniques, comprising (i) liposome buffer equilibrium dialysis, (ii) Trp fluorescence quenching, (iii) fluorescence polarization, and (iv) determination of zeta-potentials. Using unilamellar vesicles (LUVs) of different phospholipid composition, we compared weakly cationic human calcitonin (hCT)-derived peptides with the oligocationic CPPs pVEC and penetratin (pAntp). Apparent partition coefficients of hCT-derived peptides in neutral POPC LUVs were dependent on amino acid composition and secondary structure; partitioning in negatively charged POPC/POPG (80:20) LUVs was increased and mainly governed by electrostatic interactions. For hCT(9-32) and its derivatives, D values raised from about 100-200 in POPC to about 1000 to 1500 when negatively charged lipids were present. Localization profiles of CPPs obtained by Trp fluorescence quenching were dependent on the charge density of LUVs. In POPC/POPG, hCT-derived CPPs were located on the bilayer surface, whereas pVEC and pAntp resided deeper in the membrane. In POPG LUVs, an increase of fluorescence polarization was observed for pVEC and pAntp but not for hCT-derived peptides. Generally, we found strong peptide-phospholipid interactions, especially when negatively charged lipids were present.

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.

A brief introduction to cell-penetrating peptides

Cell membranes act as protective walls to exclude most molecules that are not actively imported by living cells. This is an efficient way for a cell to prevent uncontrolled influx or efflux of solutes, which otherwise would be harmful to it. Only compounds within a narrow range of molecular size, polarity and net charge are able to diffuse effectively through cell membranes. In order to overcome this barrier for effective delivery of membrane-impermeable molecules, several chemical and physical methods have been developed. These methods, e.g. electroporation, and more recent methods as cationic lipids/liposomes, have been shown to be effective for delivering hydrophobic macromolecules. The drawbacks of these harsh methods are, primarily, the unwanted cellular effects exerted by them, and, secondly, their limitation to in vitro applications. The last decade's discovery of cell-penetrating peptides translocating themselves across cell membranes of various cell lines, along with a cargo 100-fold their own size, via a seemingly energy-independent process, opens up the possibility for efficient delivery of DNA, antisense peptide nucleic acids, oligonucleotides, proteins and small molecules into cells both in vitro and in vivo.

Making proteins into drugs: assisted delivery of proteins and peptides into living neurons

The introduction of exogenous proteins and peptides into cells is a valuable experimental approach. However, exogenous proteins are not internalized by living cells readily. This limitation has been overcome by the development of peptide-based methods that assist in the delivery of exogenous proteins into the cytoplasm and nucleus. These methods may facilitate the in vivo delivery of reagents in therapies aimed at neurodegenerative disorders and recovery from nervous system injury.

Orientation and Conformation of Cell-Penetrating Peptide Penetratin in Phospholipid Vesicle Membranes Determined by Polarized-Light Spectroscopy

The orientation and conformation of the cell-penetrating peptide "penetratin" associated with phospholipid vesicle membranes has been determined using polarized-light spectroscopy. The magnitude of orientation of penetratin is unprecedented for a solute in our membrane system, which we believe indicates a strong, specific interaction with the membrane. To validate the spectroscopic technique for studying the orientation of the two tryptophan residues of penetratin, we applied tryptophan octyl ester as a model compound. It is found to be incorporated in the membrane and preferentially oriented with its hydrophobic benzene edge of the indole chromophore pointing into the membrane and its hydrophilic groups oriented toward the water. For penetratin, the results indicate that a central alpha-helical part of the peptide is aligned parallel with the membrane surface, while the ends of the peptide adopt a planar structure. The planes of the two tryptophan side chains show a preferred orientation parallel with the membrane surface, indicating that they are not inserted into the membrane.

Nonclassical transport proteins and peptides: an alternative to classical macromolecule delivery systems

The number of peptides and proteins known to exhibit nonclassical transport activity has increased significantly in recent years. In most cases, these entities have been studied in relation to their ability to deliver high molecular weight compounds, including proteins and DNA, for the ultimate purpose of developing new drug delivery strategies. In this review, an overview of the various types of vectors is presented. The in vitro and in vivo delivery successes of this technology, as well as preliminary therapeutic efforts, are described. Although a comprehensive mechanism of nonclassical transport has not yet been clearly established, we propose a straightforward model based on the cationic nature of the vectors and the need for lack of highly organized structure. In this hypothesis we suggest that the movement of polycations is mediated by a network of extra- and intracellular polyanions while transport across the bilayer is facilitated by cation-pi interactions between the vectors' basic groups and aromatic amino acid side chains in the bilayer spanning helices of membrane proteins.

Comparison of the interaction, positioning, structure induction and membrane perturbation of cell-penetrating peptides and non-translocating variants with phospholipid vesicles

Cell-penetrating peptides (CPPs) are able to translocate and carry cargo molecules across cell membranes. Using fluorescence techniques (polarization and quenching) and CD spectroscopy we studied the interaction, conformation and topology of two such peptides, transportan and 'penetratin' (pAntp), and two variants of differing translocating abilities, with small phospholipid vesicles of varying charge density. The induced structure of transportan is always helical independent of vesicle surface charge. pAntp and its two variants interact significantly only with negatively charged vesicles. The induced secondary structure depends on membrane charge and lipid/peptide ratio. The degree of membrane perturbation, evidenced by fluorescence polarization, of pAntp and its variants is related to their secondary structure. In the helical state, the peptides have little effect on the membrane. Under conditions where pAntp and its variants are converted into beta-structures, they cause membrane perturbation. Oriented CD suggests that the two CPPs (pAntp and transportan) in their helical state lie along the vesicle surface, while the two pAntp variants appear to penetrate deeper into the membrane.

Plasmon-waveguide resonance and impedance spectroscopy studies of the interaction between penetratin and supported lipid bilayer membranes

The interaction between the cell-penetrating peptide, penetratin, and solid-supported lipid bilayer membranes consisting of either egg phosphatidylcholine (PC) or a 75/25 mol% mixture of egg PC and palmitoyloleylphosphatidylglycerol has been studied by simultaneously measuring plasmon-waveguide resonance (PWR) spectra and impedance spectra of lipid-peptide mixtures. When penetratin was incorporated into an egg PC + palmitoyloleylphosphatidylglycerol bilayer, PWR measurements showed a hyperbolic increase in the average refractive index and the refractive index anisotropy, with no change in membrane thickness, over a concentration range between 0 and 2 micro M peptide. In the case of an egg PC bilayer, a biphasic dependence was observed, with a decrease in average refractive index and anisotropy and no thickness change occurring between 0 and 5 micro M peptide, and an increase in membrane thickness occurring between 5 and 15 micro M peptide with no further change in the refractive index parameters. For both membranes, the impedance spectroscopy measurements demonstrated that the electrical resistance was not altered by peptide incorporation, whereas a decrease in membrane capacitance occurred with the same concentration dependence as observed in the PWR experiments, although for the PC membrane no further changes in electrical properties were observed in the higher concentration range. A structural interpretation of these results is described, in which the peptide binds electrostatically within the headgroup region of the bilayer and influences the headgroup conformation, amount of bound water, and the lipid-packing density, without perturbing the hydrocarbon core of the bilayer.

Cell membrane translocation of the N-terminal (1-28) part of the prion protein

The N-terminal (1-28) part of the mouse prion protein (PrP) is a cell penetrating peptide, capable of transporting large hydrophilic cargoes through a cell membrane. Confocal fluorescence microscopy shows that it transports the protein avidin (67kDa) into several cell lines. The (1-28) peptide has a strong tendency for aggregation and beta-structure formation, particularly in interaction with negatively charged phospholipid membranes. The findings have implications for how prion proteins with uncleaved signal peptides in the N-termini may enter into cells, which is important for infection. The secondary structure conversion into beta-structure may be relevant as a seed for the conversion into the scrapie (PrP(Sc)) form of the protein and its amyloidic transformation.

Identification of the ability of highly charged nanomolar inhibitors of protein kinases to cross plasma membranes and carry a protein into cells

A fluorescently labeled adenosine-oligoarginine conjugate (ARC), nanomolar bisubstrate analogue-type inhibitor of basophilic protein kinases PKA and PKC, readily enters cells of different origin and localizes into cytoplasm and nucleus. Moreover, the biotinylated derivative of ARC is able to deliver avidin, a non-covalently attached protein cargo, into cells.

Conformational states of the cell-penetrating peptide penetratin when interacting with phospholipid vesicles: effects of surface charge and peptide concentration

The most commonly studied of the cell-penetrating peptides (CPP) is "penetratin" (pAntp), which functions as a carrier (vector), even for large hydrophilic (cargo) molecules. pAntp originates from the third helix of the Antennapedia homeodomain protein. The peptide is known to interact with negatively charged phospholipid vesicles, which leads to induction of secondary structure. In the present study, circular dichroism (CD) spectroscopy has been used to characterize the different secondary structures induced upon interaction with small unilamellar vesicles (SUVs) from mixtures of zwitterionic 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) and negatively charged 1-palmitoyl-2-oleoyl-phosphatidylglycerol (POPG). The interaction was monitored using an electron paramagnetic resonance (EPR) spin probe attached to the peptide, and the intrinsic fluorophore (tryptophan). We measured the secondary structure as a function of surface charge density, total lipid-to-peptide (L/P) molar ratio, and salt concentration, for completely bound peptide. With vesicles from POPG/POPC in a molar ratio below 30:70, at a high L/P, the peptide adopts a mainly helical conformation. Increasing the charge density, at the same L/P, promotes a higher degree of beta-structure. At a fixed charge density, reducing the L/P also results in an alpha-->beta structure conversion. Hence, low membrane surface charge density and low pAntp concentration both favor a mainly helical conformation, while high charge density and pAntp concentration promote a dominating beta-structure. We conclude that pAntp, when residing at the surface of a membrane, is chameleon-like in terms of its induced structure.