Taking the cell by stealth or storm? Protein transduction domains (PTDs) as versatile vectors for delivery

Biochemical and functional analysis of TIR domain containing protein from Brucella melitensis

Toll/interleukin-1 like receptors are evolutionarily conserved proteins in eukaryotes that play crucial role in pathogen recognition and innate immune responses. Brucella are facultative intracellular bacterial pathogens causing brucellosis in animal and human hosts. Brucella behave as a stealthy pathogen by evading the immune recognition or suppressing the TLR signaling cascades. Brucella encode a TIR domain containing protein, TcpB, which suppresses NF-kappaB activation as well as pro-inflammatory cytokine secretion mediated by TLR2 and TLR4 receptors. TcpB targets the TIRAP mediated pathway to suppress TLR signaling. With the objective of detailed characterization, we have over expressed and purified TcpB from Brucella melitensis in native condition. The purified protein exhibited lipid-binding properties and cell permeability. NF-kappaB inhibition property of endogenous TcpB has also been demonstrated. The data provide insight into the mechanism of action of TcpB in the intracellular niche of Brucella.

Cell penetrating peptide inhibitors of nuclear factor-kappa B

The nuclear factor kappa B (NF-kappaB) transcription factors are activated by a range of stimuli including pro-inflammatory cytokines. Active NF-kappaB regulates the expression of genes involved in inflammation and cell survival and aberrant NF-kappaB activity plays pathological roles in certain types of cancer and diseases characterized by chronic inflammation. NF-kappaB signaling is an attractive target for the development of novel anti-inflammatory or anti-cancer drugs and we discuss here how the method of peptide transduction has been used to specifically target NF-kappaB. Peptide transduction relies on the ability of certain small cell-penetrating peptides (CPPs) to enter cells, and a panel of CPP-linked inhibitors (CPP-Is) has been developed to directly inhibit NF-kappaB signaling. Remarkably, several of these NF-kappaB-targeting CPP-Is are effective in vivo and therefore offer exciting potential in the clinical setting.

CNS delivery via adsorptive transcytosis

Adsorptive-mediated transcytosis (AMT) provides a means for brain delivery of medicines across the blood-brain barrier (BBB). The BBB is readily equipped for the AMT process: it provides both the potential for binding and uptake of cationic molecules to the luminal surface of endothelial cells, and then for exocytosis at the abluminal surface. The transcytotic pathways present at the BBB and its morphological and enzymatic properties provide the means for movement of the molecules through the endothelial cytoplasm. AMT-based drug delivery to the brain was performed using cationic proteins and cell-penetrating peptides (CPPs). Protein cationization using either synthetic or natural polyamines is discussed and some examples of diamine/polyamine modified proteins that cross BBB are described. Two main families of CPPs belonging to the Tat-derived peptides and Syn-B vectors have been extensively used in CPP vector-mediated strategies allowing delivery of a large variety of small molecules as well as proteins across cell membranes in vitro and the BBB in vivo. CPP strategy suffers from several limitations such as toxicity and immunogenicity--like the cationization strategy--as well as the instability of peptide vectors in biological media. The review concludes by stressing the need to improve the understanding of AMT mechanisms at BBB and the effectiveness of cationized proteins and CPP-vectorized proteins as neurotherapeutics.

Quantum dots – Nano-sized probes for the exploration of cellular and intracellular targeting

Nanoparticles emerged as promising tool in drug targeting, since, after appropriate modification, they are able to deliver their payload to specific sites, like tissues, cells, or even certain cellular organelles. In this context, the delivery of nanoparticles from the circulation into the target cells represents a crucial step. Here, model drug delivery systems such as quantum dots are ideal candidates to elucidate this process in more detail, since they provide outstanding features like a small and uniform size, unique optical properties for most sensitive detection and modifiable surfaces. Recent progress in the surface chemistry of quantum dots expanded their use in biological applications, reduced their cytotoxicity and rendered quantum dots a powerful tool for the investigation of distinct cellular processes, like uptake, receptor trafficking and intracellular delivery. In this review, we will not only describe the ideal attributes of QDs for biological applications and imaging but also their distinct specific and non-specific pathways into the cells as well as their intracellular fate.

Functional cell permeable motifs within medically relevant proteins

Increasing experimental evidence indicates that short polybasic peptides are able to translocate across the membrane of living cells. However, these peptides, often derived from viruses and insects, may induce unspecific effects that could mask the action of their cargoes. Here, we show that a panel of lysine and/or arginine-rich peptides, derived from human proteins involved in cell signalling pathways leading to inflammation, possess the intrinsic ability to cross intact cellular membranes. These peptides are also capable of carrying a biologically active cargo. One of these peptides, encompassing the cell permeable sequence of the Toll-receptor 4 (TLR4) adaptor protein (TIRAP) and modified to carry a dominant-negative domain of the same TIRAP protein, selectively inhibited the production of pro-inflammatory cytokines upon LPS challenge, in in vitro, ex vivo and in vivo experiments. Docking studies indicated that this inhibition might be mediated by the disruption of the recruitment of downstream effector molecules. These results show for the first time the potential of using for therapy cell permeable peptides derived from human proteins involved in disease.

Design and mechanism of action of a novel bacteria-selective antimicrobial peptide from the cell-penetrating peptide Pep-1

Here, we report the successful design of a novel bacteria-selective antimicrobial peptide, Pep-1-K (KKTWWKTWWTKWSQPKKKRKV). Pep-1-K was designed by replacing Glu-2, Glu-6, and Glu-11 in the cell-penetrating peptide Pep-1 with Lys. Pep-1-K showed strong antibacterial activity against reference strains (MIC = 1-2 microM) of Gram-positive and Gram-negative bacteria as well as against clinical isolates (MIC = 1-8 microM) of methicillin-resistant Staphylococcus aureus and multidrug-resistant Pseudomonas aeruginosa. In contrast, Pep-1-K did not cause hemolysis of human erythrocytes even at 200 microM. These results indicate that Pep-1-K may be a good candidate for antimicrobial drug development, especially as a topical agent against antibiotic-resistant microorganisms. Tryptophan fluorescence studies indicated that the lack of hemolytic activity of Pep-1-K correlated with its weak ability to penetrate zwitterionic phosphatidylcholine/cholesterol (10:1, w/w) vesicles, which mimic eukaryotic membranes. Furthermore, Pep-1-K caused little or no dye leakage from negatively charged phosphatidylethanolamine/phosphatidylglycerol (7:3, w/w) vesicles, which mimic bacterial membranes but had a potent ability to cause depolarization of the cytoplasmic membrane potential of intact S. aureus cells. These results suggested that Pep-1-K kills microorganisms by not the membrane-disrupting mode but the formation of small channels that permit transit of ions or protons but not molecules as large as calcein.

Re-evaluating the role of strongly charged sequences in amphipathic cell-penetrating peptides: a fluorescence study using Pep-1

Cell-penetrating peptides (CPPs) are able to translocate across biological membranes and deliver bioactive proteins. Cellular uptake and intracellular distribution of CPPs is commonly evaluated with fluorescent labels, which can alter peptide properties. The effect of carboxyfluorescein label in the Lys-rich domain of the amphipathic CPP pep-1, was evaluated and compared with non-labelled pep-1 in vitro and in vivo. A reduced membrane affinity and an endosomal-dependent translocation mechanism, at variance with non-labelled pep-1, were detected. Therefore, the charged domain is not a mere enabler of peptide adsorption but has a crucial role in the translocation pathway of non-labelled pep-1.

Peptide inhibitors of protein kinases-discovery, characterisation and use

Protein kinases are now the second largest group of drug targets, and most protein kinase inhibitors in clinical development are directed towards the ATP-binding site. However, these inhibitors must compete with high intracellular ATP concentrations and they must discriminate between the ATP-binding sites of all protein kinases as well the other proteins that also utilise ATP. It would therefore be beneficial to target sites on protein kinases other than the ATP-binding site. This review describes the discovery, characterisation and use of peptide inhibitors of protein kinases. In many cases, the development of these peptides has resulted from an understanding of the specific protein-binding partners for a particular protein kinase. In addition, novel peptide sequences have been discovered in library screening approaches and have provided new leads in the discovery and/or design of peptide inhibitors of protein kinases. These approaches are therefore providing exciting new opportunities in the development of ATP non-competitive inhibitors of protein kinases.

Environmental factors that enhance the action of the cell penetrating peptide pep-1

Pep-1 is a cell penetrating peptide (CPP) derived from the nuclear localization sequence of Simian Virus 40 large antigen T and from reverse transcriptase of Human Immunodeficiency Virus. Although it has been successfully used to transport proteins into cells, its action at the molecular level is not yet clear, mainly the local environmental factors that condition partition and translocation. Characterization in aqueous medium and quantification of partition into bilayers were carried out. Dynamic light scattering studies show that pep-1 self-associates in aqueous medium. The role of the bilayer phase, anionic lipids, ionic strength of the medium, reducing agents and pep-1 concentration on the extent and kinetics of partition were studied. Unlike others cationic CPP (e.g. penetratin) pep-1 has a high affinity to neutral vesicles (Kp = 2.8 x 10(3)), which is enhanced by anionic lipids. In a reduction environment partition is strongly inhibited (Kp = 2.2 x 10(2)), which might be a key-feature in the biological action of pep-1. Peptide incorporation takes place in the millisecond time-range to the lipidic interfaces. These environmental factors are systematized to enlighten how they help cellular uptake.

Metabolic cleavage of cell-penetrating peptides in contact with epithelial models: human calcitonin (hCT)-derived peptides, Tat(47-57) and penetratin(43-58)

We assessed the metabolic degradation kinetics and cleavage patterns of some selected CPP (cell-penetrating peptides) after incubation with confluent epithelial models. Synthesis of N-terminal CF [5(6)-carboxyfluorescein]-labelled CPP, namely hCT (human calcitonin)-derived sequences, Tat(47-57) and penetratin(43-58), was through Fmoc (fluoren-9-ylmethoxycarbonyl) chemistry. Metabolic degradation kinetics of the tested CPP in contact with three cell-cultured epithelial models, MDCK (Madin-Darby canine kidney), Calu-3 and TR146, was evaluated by reversed-phase HPLC. Identification of the resulting metabolites of CF-hCT(9-32) was through reversed-phase HPLC fractionation and peak allocation by MALDI-TOF-MS (matrix-assisted laser-desorption ionization-time-of-flight mass spectrometry) or direct MALDI-TOF-MS of incubates. Levels of proteolytic activity varied highly between the investigated epithelial models and the CPP. The Calu-3 model exhibited the highest proteolytic activity. The patterns of metabolic cleavage of hCT(9-32) were similar in all three models. Initial cleavage of this peptide occurred at the N-terminal domain, possibly by endopeptidase activity yielding both the N- and the C-terminal counterparts. Further metabolic degradation was by aminopeptidase, endopeptidase and/or carboxypeptidase activities. In conclusion, when in contact with epithelial models, the studied CPP were subject to efficient metabolism, a prerequisite of cargo release on the one hand, but with potential for premature cleavage and loss of the cargo as well on the other. The results, particularly on hCT(9-32), may be used as a template to suggest structural modifications towards improved CPP performance.

Small G-protein Rho is involved in the maintenance of cardiac myocyte morphology

The use of small membrane-permeable sequences or protein transduction domains (PTDs) can facilitate the transport of proteins into many cell types. In preliminary studies with the application of three PTDs (penetratin, modified penetratin, and the HIV TAT transduction domains) to cardiac myocytes, we found that the TAT and penetratin sequences showed high efficiency of uptake and low toxicity. Rho has been previously shown to be an important regulator of cytoskeletal organization and morphology in other non-cardiac cell types. To evaluate a role for Rho in cardiac myocyte morphology, we used the TAT-PTD to deliver a RhoA-specific inhibitor, the C3 exoenzyme, to cultured cardiac myocytes. We showed that this incubation with TAT-C3 abolished the basal levels of RhoA activity, demonstrating the efficacy of this treatment. Incubation with TAT-C3 also altered cardiac myocyte morphology so that TAT-C3-treated cells produced multiple projections from the major cell body. This was accompanied by a statistically significant increase in cell size, albeit to a lesser extent than the changes accompanying exposure to the hypertrophic agent, endothelin-1. Furthermore, the change in size of TAT-C3-treated cells was not accompanied by the induction of atrial natriuretic factor (ANF) expression that accompanies the hypertrophy of cardiac myocytes. These results reveal a role for RhoA in the maintenance of normal myocyte morphology.

Counting on mitogen-activated protein kinases—ERKs 3, 4, 5, 6, 7 and 8

Signal transduction pathways in eukaryotic cells integrate diverse extracellular signals, and regulate complex biological responses such as growth, differentiation and death. One group of proline-directed Ser/Thr protein kinases, the mitogen-activated protein kinases (MAPKs), plays a central role in these signalling pathways. Much attention has focused in recent years on three subfamilies of MAPKs, the extracellular signal regulated kinases (ERKs), c-Jun N-terminal kinases (JNKs) and the p38 MAPKs. However, the ERK family is broader than the ERK1 and ERK2 proteins that have been the subject of most studies in this area. Here we overview the work on ERKs 3 to 8, emphasising where possible their biological activities as well as distinctive biochemical properties. It is clear from these studies that these additional ERKs show similarities to ERK1 and ERK2, but with some interesting differences that challenge the paradigm of the archetypical ERK1/2 MAPK pathway.

Chances and pitfalls of cell penetrating peptides for cellular drug delivery

Over the past decade, several classes and/or prototypes of cell penetrating peptides (CPP) have been identified and investigated in multiple aspects. CPP represent peptides, which show the ability to cross the plasma membrane of mammalian cells, and may thus give rise to the intracellular delivery of problematic therapeutic cargos, such as peptides, proteins, oligonucleotides, plasmids and even nanometer-sized particles, which otherwise cannot cross the plasma membrane. Most of the currently recognized CPP are of cationic nature and derived from viral, insect or mammalian proteins endowed with membrane translocation properties. The exact mechanisms underlying the translocation of CPP across the cellular membrane are still poorly understood. However, several similarities in translocation can be found. Early studies on CPP translocation mechanisms tended to suggest that the internalization of these peptides was neither significantly inhibited by low temperature, depletion of the cellular adenosine triphosphate (ATP) pool, nor by inhibitors of endocytosis. Moreover, chemical modification of the peptide sequence, such as the synthesis of retro-, enantio- or retroenantio-analogs, appeared not to affect the internalization properties. Therefore, translocation was concluded to result from direct, physical transfer through the lipid bilayer of the cell membrane. Later studies, however, showed convincing evidence for the involvement of endocytosis as the dominating mechanism for cellular internalization. In addition to describing the general properties of the commonly recognized classes of CPP, in this review we will also point out some limitations and typical pitfalls of CPP as carriers for therapeutics. In particular we will comment on emerging discrepancies with the current dogma, on cell-to-cell variability, biological barrier permeability, metabolic fate, toxicity and immunogenicity of CPP.

Biodistribution of intracellularly acting peptides conjugated reversibly to Tat

Intracellularly acting peptide modulators of signaling enzymes provide a powerful means to regulate signaling events. Delivery of peptides into cells is facilitated by conjugation to carrier peptides, such as Tat. When peptides are irreversibly conjugated to Tat, Tat-mediated subcellular localization may predominate, resulting in mislocalization of the peptide cargo. We have used intracellularly acting peptides, conjugated to Tat by a disulfide bond, to modulate protein kinase C (PKC) signaling; these PKC-modulating peptides are released from Tat upon intracellular delivery. Previously, the distribution of these peptides within tissue and throughout the body had not been demonstrated. We show here intravascular delivery of a PKC-peptide, reversibly conjugated to Tat, resulted in distribution throughout cardiac tissue. In addition, a single injection resulted in selective modulation of PKC activity in many organs. Therefore, intracellularly acting peptide modulators of signaling enzymes, reversibly conjugated to Tat, have extensive biodistribution and can be used to modulate signaling pathways in vivo.

Production of cell lines secreting TAT fusion proteins

Transduction of proteins and other macromolecules constitutes a potent technology to analyze cell functions and to achieve therapeutic interventions. In general, fusion proteins with protein transduction domains, such as TAT, are produced in a bacterial expression system. Here we describe the generation of a mammalian expression vector coding for TAT-EGFP fusion protein. Transfection of CHO-K1 cells by this vector and subsequent selection by Zeocin resulted in cell lines that express and secrete EGFP, a variant of the green fluorescent protein GFP. The ultimate cell line was produced by first cloning the stable integrants and subsequent selection of EGFP-expressing cells by flow cytometric sorting. In the resulting cell line approximately 98% of cells express EGFP. Using the same methodology, we generated cell lines that express DsRed fluorescent protein. The advantages of using such a mammalian expression system include the ease of generating TAT fusion proteins and the potential for sustained production of such proteins in vitro and, potentially, in vivo.

Targeting the JNK MAPK cascade for inhibition: basic science and therapeutic potential

The c-Jun N-terminal protein kinases (JNKs) form one subfamily of the mitogen-activated protein kinase (MAPK) group of serine/threonine protein kinases. The JNKs were first identified by their activation in response to a variety of extracellular stresses and their ability to phosphorylate the N-terminal transactivation domain of the transcription factor c-Jun. One approach to study the function of the JNKs has included in vivo gene knockouts of each of the three JNK genes. Whilst loss of either JNK1 or JNK2 alone appears to have no serious consequences, their combined knockout is embryonic lethal. In contrast, the loss of JNK3 is not embryonic lethal, but rather protects the adult brain from glutamate-induced excitotoxicity. This latter example has generated considerable enthusiasm with JNK3, considered an appropriate target for the treatment of diseases in which neuronal death should be prevented (e.g. stroke, Alzheimer's and Parkinson's diseases). More recently, these gene knockout animals have been used to demonstrate that JNK could provide a suitable target for the protection against obesity and diabetes and that JNKs may act as tumour suppressors. Considerable effort is being directed to the development of chemical inhibitors of the activators of JNKs (e.g. CEP-1347, an inhibitor of the MLK family of JNK pathway activators) or of the JNKs themselves (e.g. SP600125, a direct inhibitor of JNK activity). These most commonly used inhibitors have demonstrated efficacy for use in vivo, with the successful intervention to decrease brain damage in animal models (CEP-1347) or to ameliorate some of the symptoms of arthritis in other animal models (SP600125). Alternative peptide-based inhibitors of JNKs are now also in development. The possible identification of allosteric modifiers rather than direct ATP competitors could lead to inhibitors of unprecedented specificity and efficacy.

Cytoplasmic and nuclear delivery of a TAT-derived peptide and a beta-peptide after endocytic uptake into HeLa cells

Several short, highly cationic peptides are able to enter the cytoplasm and nucleus of cells from the extracellular medium. The mechanism of entry is unknown. A number of fluorescence-based studies suggested that these molecules cross the plasma membrane by an energy-independent process, directly gaining access to the cytoplasm. Recent reports have questioned this conclusion, attributing the prior observations to artifacts resulting from fixation procedures used to prepare cells for fluorescence microscopy. These studies analyzed live cells and showed that the peptides entered through endocytosis and accumulated in endocytic vesicles, without necessarily entering the cytoplasm. To resolve this controversy and to extend the analyses to non-natural beta-peptide sequences, we studied the cytoplasmic and nuclear delivery of a fluorescein-labeled 9-residue sequence derived from the human immunodeficiency virus transactivator of transcription (TAT) peptide, TAT-(47-57), as well as a similarly labeled 12-residue beta-peptide, beta-(VRR)4, in live cells. Using fluorescence confocal microscopy, we show that when added to cells, both peptides are found in endocytic vesicles containing the transferrin receptor as well as in the cytoplasm and nucleus (TAT-(47-57)) or nucleolus (beta-(VRR)4). The cells were verified to be intact through all experimental procedures by demonstrating their ability to exclude propidium iodide. Endocytic entry of the peptides was blocked by the energy poisons sodium azide and 2-deoxyglucose, whereas staining of the nucleus (nucleolus), but not endocytic vesicles, was abrogated by treating the cells with ammonium chloride. Our observations are consistent with the proposal that TAT-(47-57) and beta-(VRR)4 enter cells by endocytosis and then exit an endosomal compartment to enter the cytoplasm by means of a mechanism requiring endosome acidification.

Membrane translocation of penetratin and its derivatives in different cell lines

The third helix of the homeodomain of the Antennapedia homeoprotein can translocate through the cell membrane into the nucleus and can be used as an intracellular vehicle for the delivery of oligopeptides and oligonucleotides. A 16-amino acid-long peptide fragment, called penetratin, is internalized by the cells in a specific, non-receptor-mediated manner. For a better understanding of the mechanism of the transfer, penetratin and two analogs were synthesized:The conformation of penetratin peptides 1-3 was examined in both extracellular matrix-mimetic and membrane-mimetic environments. (1)H-NMR and CD spectroscopic measurements were performed in mixtures of TFE/water with different ratios. Peptides 1-3 were labeled by reacting their N-terminal free amino group with fluorescein isothiocyanate (FITC). Membrane translocation of the labelled peptides was studied with cell cultures [WEHI 164 murine fibrosarcoma cells (WC/1); chicken fibroblast cells (CEC-32); chicken monocytic cells (HD-11); human fibroblast (SV 80) and human monocytic cells (MonoMac-6)]. Confocal laser scanning microscopy and flow cytometry assay were used to study membrane translocation. Amphiphilicity was calculated for each peptide. In our experiments all the penetratin peptides penetrated into the cells. Helical conformation and membrane translocation ability showed little correlation: substitution of the two Trp with Phe increased the stability of helical conformation but decreased membrane translocation activity. The results of fluorescence microscopy and flow cytometry show that penetratin can be translocated into the cells by two mechanisms: endocytosis and direct transport through the cell membrane.