Membrane interaction and secondary structure of de novo designed arginine-and tryptophan peptides with dual function

Arginine (315) is required for the PLIN2-CGI-58 interface and plays a functional role in regulating nascent LDs formation in bovine adipocytes

Perilipin-2 (PLIN2) can anchor to lipid droplets (LDs) and play a crucial role in regulating nascent LDs formation. Bimolecular fluorescence complementation (BiFC) and flow cytometry were examined to verify the PLIN2-CGI-58 interaction efficiency in bovine adipocytes. GST-Pulldown assay was used to detect the key site arginine315 function in PLIN2-CGI-58 interaction. Experiments were also examined to research these mutations function of PLIN2 in LDs formation during adipocytes differentiation, LDs were measured after staining by BODIPY, lipogenesis-related genes were also detected. Results showed that Leucine (L371A, L311A) and glycine (G369A, G376A) mutations reduced interaction efficiencies. Serine (S367A) mutations enhanced the interaction efficiency. Arginine (R315A) mutations resulted in loss of fluorescence in the cytoplasm and disrupted the interaction with CGI-58, as verified by pulldown assay. R315W mutations resulted in a significant increase in the number of LDs compared with wild-type (WT) PLIN2 or the R315A mutations. Lipogenesis-related genes were either up- or downregulated when mutated PLIN2 interacted with CGI-58. Arginine315 in PLIN2 is required for the PLIN2-CGI-58 interface and could regulate nascent LD formation and lipogenesis. This study is the first to study amino acids on the PLIN2 interface during interaction with CGI-58 in bovine and highlight the role played by PLIN2 in the regulation of bovine adipocyte lipogenesis.

Tryptophan, more than just an interfacial amino acid in the membrane activity of cationic cell-penetrating and antimicrobial peptides

Trp is unique among the amino acids since it is involved in many different types of noncovalent interactions such as electrostatic and hydrophobic ones, but also in π-π, π-cation, π-anion and π-ion pair interactions. In membranotropic peptides and proteins, Trp locates preferentially at the water-membrane interface. In antimicrobial or cell-penetrating peptides (AMPs and CPPs respectively), Trp is well-known for its strong role in the capacity of these peptides to interact and affect the membrane organisation of both bacteria and animal cells at the level of the lipid bilayer. This essential amino acid can however be involved in other types of interactions, not only with lipids, but also with other membrane partners, that are crucial to understand the functional roles of membranotropic peptides. This review is focused on this latter less known role of Trp and describes in details, both in qualitative and quantitative ways: (i) the physico-chemical properties of Trp; (ii) its effect in CPP internalisation; (iii) its importance in AMP activity; (iv) its role in the interaction of AMPs with glycoconjugates or lipids in bacteria membranes and the consequences on the activity of the peptides; (v) its role in the interaction of CPPs with negatively charged polysaccharides or lipids of animal membranes and the consequences on the activity of the peptides. We intend to bring highlights of the physico-chemical properties of Trp and describe its extensive possibilities of interactions, not only at the well-known level of the lipid bilayer, but with other less considered cell membrane components, such as carbohydrates and the extracellular matrix. The focus on these interactions will allow the reader to reevaluate reported studies. Altogether, our review gathers dedicated studies to show how unique are Trp properties, which should be taken into account to design future membranotropic peptides with expected antimicrobial or cell-penetrating activity.

Highly cationic cell-penetrating peptides affect the barrier integrity and facilitates mannitol permeation in a human stem cell-based blood-brain barrier model

The blood-brain barrier (BBB) allows passive permeation of only a limited number of, primarily lipophilic, low-molecular weight drugs that obey the so-called "rule of CNS likeness". Therefore, novel strategies to facilitate drug delivery across the BBB are needed. Cell-penetrating peptides (CPPs) enable delivery of various therapeutic cargoes into cells and may potentially serve as shuttles for delivery of brain-specific drugs across the BBB. The CPPs Tat_47-57 and penetratin are prototypical cationic CPPs, whereas apidaecin and oncocin belong to the group of proline-rich cationic antimicrobial peptides displaying CPP-like properties. The aim of the present study was to investigate the potential of Tat_47-57, penetratin, apidaecin, and oncocin for interaction with and permeation of the BBB in vitro. We also studied whether the CPPs facilitated permeation of the paracellular flux marker mannitol as well as the transcellular flux marker propranolol. The peptides were labelled with the fluorophore 6-TAMRA (T) for visualization and quantification purposes. CPP membrane-adherence, membrane-embedding, and cellular uptake as well as barrier-permeation were evaluated in murine brain capillary endothelial cells (bEND3) and human induced pluripotent stem cell-derived (Bioni-010c) brain capillary endothelial-like monolayers. The cationic and the proline-rich cationic CPPs were taken up into the Bioni-010c monolayers. T-Tat_47-57, T-apidaecin, and T-oncocin also permeated Bioni-010c monolayers, whereas T-penetratin did not. However, both T-Tat_47-57 and T-penetratin affected the barrier integrity to a degree that facilitated permeation of ¹⁴C-mannitol. These results may therefore pave the way for future CPP-mediated brain delivery of small drugs that do not obey the "rule of CNS likeness".

PEP27-2, a Potent Antimicrobial Cell-Penetrating Peptide, Reduces Skin Abscess Formation during Staphylococcus aureus Infections in Mouse When Used in Combination with Antibiotics

PEP27, a 27-amino acid (aa) peptide secreted by Streptococcus pneumoniae, is an autolytic peptide that functions as a major virulence factor. To develop a clinically applicable antimicrobial peptide (AMP), we designed PEP27 analogs with Trp substitutions to enhance its antimicrobial activity compared to that of PEP27. Particularly, PEP27-2 showed strong antimicrobial activity against a wide variety of bacteria, including multidrug-resistant (MDR) bacteria. It was found that the antimicrobial activity of PEP27-2 was increased by substituting Trp for the aa at the middle position of PEP27. We found that PEP27-2 acts as an effective cell-penetrating peptide in bacterial and mammalian cells. Here, we proved that subcutaneous infection with MDR Staphylococcus aureus induced skin lesions such as skeletal muscle damage, deep inflammation, and necrosis of the overlaying dermis in mice. Combination treatment with antibiotics revealed synergistic effects, remarkably reducing abscess size and improving the bacteria removal rate from the infection site. Moreover, PEP27-2-antibiotic combination treatment reduced inflammation, lowering levels of tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), IL-6, inducible NO synthase (iNOS), and cyclooxygenase (COX-2) in skin abscess tissue. The results suggest that the PEP27-2 peptide is a promising therapeutic option for combating MDR bacterial strains by enhancing antibiotic penetration and protecting against MDR bacteria.

Non-Covalent Carrier Hydrophobicity as a Universal Predictor of Intracellular Protein Activity

Over the past decade, extensive optimization of polymeric cell-penetrating peptide (CPP) mimics (CPPMs) by our group has generated a substantial library of broadly effective carriers which circumvent the need for covalent conjugation often required by CPPs. In this study, design rules learned from CPPM development were applied to reverse-engineer the first library of simple amphiphilic block copolypeptides for non-covalent protein delivery, namely, poly(alanine-block-arginine), poly(phenylalanine-block-arginine), and poly(tryptophan-block-arginine). This new CPP library was screened for enhanced green fluorescent protein and Cre recombinase delivery alongside a library of CPPMs featuring equivalent side-chain configurations. Due to the added hydrophobicity imparted by the polymer backbone as compared to the polypeptide backbone, side-chain functionality was not a universal predictor of carrier performance. Rather, overall carrier hydrophobicity predicted the top performers for both internalization and activity of protein cargoes, regardless of backbone identity. Furthermore, comparison of protein uptake and function revealed carriers which facilitated high gene recombination despite remarkably low Cre internalization, leading us to formalize the concept of intracellular availability (IA) of the delivered cargo. IA, a measure of cargo activity per quantity of cargo internalized, provides valuable insight into the physical relationship between cellular internalization and bioavailability, which can be affected by bottlenecks such as endosomal escape and cargo release. Importantly, carriers with maximal IA existed within a narrow hydrophobicity window, more hydrophilic than those exhibiting maximal cargo uptake. Hydrophobicity may be used as a scaffold-independent predictor of protein uptake, function, and IA, enabling identification of new, effective carriers which would be overlooked by uptake-based screening methods.

Challenge to overcome current limitations of cell-penetrating peptides

The penetration of biological membranes is a prime obstacle for the delivery of pharmaceutical drugs. Cell-penetrating peptide (CPP) is an efficient vehicle that can deliver various cargos across the biological membranes. Since the discovery, CPPs have been rigorously studied to unveil the underlying penetrating mechanism as well as to exploit CPPs for various biomedical applications. This review will focus on the various strategies to overcome current limitations regarding stability, selectivity, and efficacy of CPPs.

Increased Carrier Peptide Stability through pH Adjustment Improves Insulin and PTH(1-34) Delivery In Vitro and In Vivo Rather than by Enforced Carrier Peptide-Cargo Complexation

Oral delivery of therapeutic peptides is hampered by their large molecular size and labile nature, thus limiting their permeation across the intestinal epithelium. Promising approaches to overcome the latter include co-administration with carrier peptides. In this study, the cell-penetrating peptide penetratin was employed to investigate effects of co-administration with insulin and the pharmacologically active part of parathyroid hormone (PTH(1-34)) at pH 5, 6.5, and 7.4 with respect to complexation, enzymatic stability, and transepithelial permeation of the therapeutic peptide in vitro and in vivo. Complex formation between insulin or PTH(1-34) and penetratin was pH-dependent. Micron-sized complexes dominated in the samples prepared at pH-values at which penetratin interacts electrostatically with the therapeutic peptide. The association efficiency was more pronounced between insulin and penetratin than between PTH(1-34) and penetratin. Despite the high degree of complexation, penetratin retained its membrane activity when applied to liposomal structures. The enzymatic stability of penetratin during incubation on polarized Caco-2 cell monolayers was pH-dependent with a prolonged half-live determined at pH 5 when compared to pH 6.5 and 7.4. Also, the penetratin-mediated transepithelial permeation of insulin and PTH(1-34) was increased in vitro and in vivo upon lowering the sample pH from 7.4 or 6.5 to 5. Thus, the formation of penetratin-cargo complexes with several molecular entities is not prerequisite for penetratin-mediated transepithelial permeation a therapeutic peptide. Rather, a sample pH, which improves the penetratin stability, appears to optimize the penetratin-mediated transepithelial permeation of insulin and PTH(1-34).

The Critical Role of Tryptophan in the Antimicrobial Activity and Cell Toxicity of the Duck Antimicrobial Peptide DCATH

Antimicrobial peptides (AMPs) have attracted more attention for their potential candidates for new antibiotic drugs. As a novel identified cathelicidin AMP from duck, dCATH owns broad-spectrum antimicrobial activities but with a noticeable toxicity. To explore dCATH-derived AMPs with reduced cell toxicity and improved cell selectivity, a series of truncated and tryptophan-replaced peptides of dCATH were designed. Two truncated peptides containing one of the two tryptophan (Trp) residues at the positions of 4 and 17 (W4 and W17) of dCATH, dCATH(1-16) and dCATH(5-20), showed strong antibacterial activity, but didn't show obvious hemolytic activity and cytotoxicity. The derived peptides not containing Trp didn't possess obvious antimicrobial activity, and their hemolytic and cytotoxic effect was also diminished. Also as evidence by Trp fluorescence experiment that existence of W4 and W17 was crucially important to the antimicrobial activity, hemolysis and cytotoxicity of dCATH, and one of the two Trp residues was competent and necessary to retain its antimicrobial activity. Antibacterial mechanism analysis showed that dCATH(1-16) and dCATH(5-20) killed bacterial cells by increasing permeability and causing a loss of membrane integrity. dCATH(1-16) and dCATH(5-20) possessed insignificant inhibitory activity against levels of IL-6, TNF-α, and NO in RAW 264.7 cells treated with LPS. In vivo, intraperitoneal administration of the two peptides significantly decreased mortality and provided protection against LPS-induced inflammation in mice challenged with lethal dose of LPS. The two peptides, dCATH(1-16) and dCATH(5-20), which possessed high antibacterial activity and cell selectivity, may herald development prospects as new antibacterial agents in the future.

Novel Miniature Membrane Active Lipopeptidomimetics against Planktonic and Biofilm Embedded Methicillin-Resistant Staphylococcus aureus

Escalating multidrug resistance and highly evolved virulence mechanisms have aggravated the clinical menace of methicillin-resistant Staphylococcus aureus (MRSA) infections. Towards development of economically viable staphylocidal agents here we report eight structurally novel tryptophan-arginine template based peptidomimetics. Out of the designed molecules, three lipopeptidomimetics (S-6, S-7 and S-8) containing 12-amino dodecanoic acid exhibited cell selectivity and good to potent activity against clinically relevant pathogens MRSA, methicillin-resistant Staphylococcus epidermidis and vancomycin-resistant Enterococcus faecium (MIC: 1.4–22.7 μg/mL). Mechanistically, the active peptidomimetics dissipated membrane potential and caused massive permeabilization on MRSA concomitant with loss of viability. Against stationary phase MRSA under nutrient-depleted conditions, active peptidomimetics S-7 and S-8 achieved > 6 log reduction in viability upon 24 h incubation while both S-7 (at 226 μg/mL) and S-8 (at 28 μg/mL) also destroyed 48 h mature MRSA biofilm causing significant decrease in viability (p < 0.05). Encouragingly, most active peptidomimetic S-8 maintained efficacy against MRSA in presence of serum/plasma while exhibiting no increase in MIC over 17 serial passages at sub-MIC concentrations implying resistance development to be less likely. Therefore, we envisage that the current template warrants further optimization towards the development of cell selective peptidomimetics for the treatment of device associated MRSA infections.

Peptide drug stability: The anti-inflammatory drugs Pep19-2.5 and Pep19-4LF in cream formulation

In previous years, we developed anti-infective drugs based on antimicrobial peptides (AMPs), which have been shown to effectively block severe infections and inflammation in vitro as well as in vivo. Besides systemic application, the occurrence of severe local infections necessitates a topical application for example in the case of severe skin and soft tissue infections (SSTI). Recent investigations show that the synthetic anti-lipopolysaccharide peptide (SALP) Pep19-2.5 (Aspidasept® I) and a variant called Pep19-4LF (Aspidasept® II) are able to supress inflammation reactions also in keratinocytes, Langerhans cells, and dendritic cells from the skin. For topical application, a possible formulation represents the drug dispersed into a pharmaceutical cream (DAC base cream). Here, we present investigations on the stability of the peptides using this formulation in dependence on time, which includes the evaluation of the extraction procedure, the quantitative analysis of the peptides after extraction, its sensitivity to protease degradation and its ability to maintain activity against LPS-induced inflammation in vitro. We have developed an extraction procedure for the peptides with an optimum yield and showed that Pep19-2.5 is present as a dimer after extraction from the cream, whereas Pep19-4LF retains its monomeric form. Both peptides show no degradation by chymotrypsin after extraction for at least 1 h, which is indicative for an attachment of constituents of the base cream, inhibiting the cutting into peptidic part structures. The extracted peptides and in particular the dimeric Pep19-2.5 are still able to inhibit the LPS-induced inflammation reaction in human mononuclear cells.

Novel Design of Heptad Amphiphiles To Enhance Cell Selectivity, Salt Resistance, Antibiofilm Properties and Their Membrane-Disruptive Mechanism

Coiled-coil, a basic folding pattern of native proteins, was previously demonstrated to be associated with the specific spatial recognition, association, and dissociation of proteins and can be used to perfect engineering peptide model. Thus, in this study, a series of amphiphiles composed of heptads repeats with coiled-coil structures was constructed, and the designed peptides exhibited a broad spectrum of antimicrobial activities. Circular dichroism and biological assays showed that the heptad repeats and length of the linker between the heptads largely influenced the amphiphile's helical propensity and cell selectivity. The engineered amphiphiles were also found to efficiently reduce sessile P. aeruginosa biofilm biomass, neutralize endotoxins, inhibit the inflammatory response, and remain active under physiological salt concentrations. In summary, these findings are helpful for short AMP design with a highly therapeutic index to treat bacteria-induced infection.

LL-37-derived membrane-active FK-13 analogs possessing cell selectivity, anti-biofilm activity and synergy with chloramphenicol and anti-inflammatory activity

Although the human-derived antimicrobial peptide (AMP) LL-37 has potent antimicrobial and anti-inflammatory activities, its therapeutic application is limited by its low cell selectivity and high production cost due to its large size. To overcome these problems, we tried to develop novel LL-37-derived short α-helical AMPs with improved cell selectivity and without a significant loss of anti-inflammatory activity relative to that of parental LL-37. Using amino acid substitution, we designed and synthesized a series of FK13 analogs based on the sequence of the 13-meric short FK13 peptide (residues 17-29 of LL-37) that has been identified as the region responsible for the antimicrobial activity of LL-37. Among the designed FK13 analogs, FK-13-a1 and FK-13-a7 showed high cell selectivity and retained the anti-inflammatory activity. The therapeutic index (a measure of cell selectivity) of FK-13-a1 and FK-13-a7 was 6.3- and 2.3-fold that of parental LL-37, respectively. Furthermore, FK-13-a1 and FK-13-a7 displayed more potent antimicrobial activity against antibiotic-resistant bacteria including MRSA, MDRPA, and VREF, than did LL-37. In addition, FK-13-a1 and FK-13-a7 exhibited greater synergistic effects with chloramphenicol against MRSA and MDRPA and were more effective anti-biofilm agents against MDRPA than LL-37 was. Moreover, FK-13-a1 and FK-13-a7 maintained their activities in the presence of physiological salts and human serum. SYTOX green uptake, membrane depolarization and killing kinetics revealed that FK13-a1 and FK13-a7 kills microbial cells by permeabilizing the cell membrane and damaging membrane integrity. Taken together, our results suggest that FK13-a1 and FK13-a7 can be developed as novel antimicrobial/anti-inflammatory agents.

Effect of poly-glutamate on uptake efficiency and cytotoxicity of cell penetrating peptides

Cell penetrating peptides (CPPs) were developed as vehicles for efficient delivery of various molecules. An ideal CPP-peptide should not display any toxicity against cancer cells as well as healthy cells and efficiently enter into the cell. Because of the cationic nature and the intrinsic vector capabilities, these peptides can cause cytotoxicity. One of the possible reasons for toxicity of CPPs is direct translocation and consequently, pore formation on the plasma membrane. In this study it was demonstrated that interaction of poly-glutamate with CPP considerably reduced their cytotoxicity in A549 cell. This strategy could be useful for efficient drug delivery mediated by CPP.

Cell-penetrating peptides as tools to enhance non-injectable delivery of biopharmaceuticals

Non-injectable delivery of peptide and protein drugs is hampered by their labile nature, hydrophilicity, and large molecular size; thus limiting their permeation across mucosae, which represent major biochemical and physical barriers to drugs administered via e.g. the oral, nasal, and pulmonary routes. However, in recent years cell-penetrating peptides (CPP) have emerged as promising tools to enhance mucosal delivery of co-administered or conjugated peptide and protein cargo and more advanced CPP-cargo formulations are emerging. CPPs act as transepithelial delivery vectors, but the mechanism(s) by which CPPs mediate cargo translocation across an epithelium is so far poorly understood; both due to the fact that multiple factors influence the resulting uptake and trafficking mechanisms as well as to the complicated nature of sensitive studies of this. In addition to a proper mechanistic understanding, documentation of CPP-mediated delivery in higher animal species than rodent as well as extensive toxicological studies are necessary for CPP-containing non-injectable DDSs to reach the clinic.

Applications and Challenges for Use of Cell-Penetrating Peptides as Delivery Vectors for Peptide and Protein Cargos

The hydrophilic nature of peptides and proteins renders them impermeable to cell membranes. Thus, in order to successfully deliver peptide and protein-based therapeutics across the plasma membrane or epithelial and endothelial barriers, a permeation enhancing strategy must be employed. Cell-penetrating peptides (CPPs) constitute a promising tool and have shown applications for peptide and protein delivery into cells as well as across various epithelia and the blood-brain barrier (BBB). CPP-mediated delivery of peptides and proteins may be pursued via covalent conjugation of the CPP to the cargo peptide or protein or via physical complexation obtained by simple bulk-mixing of the CPP with its cargo. Both approaches have their pros and cons, and which is the better choice likely relates to the physicochemical properties of the CPP and its cargo as well as the route of administration, the specific barrier and the target cell. Besides the physical barrier, a metabolic barrier must be taken into consideration when applying peptide-based delivery vectors, such as the CPPs, and stability-enhancing strategies are commonly employed to prolong the CPP half-life. The mechanisms by which CPPs translocate cell membranes are believed to involve both endocytosis and direct translocation, but are still widely investigated and discussed. The fact that multiple factors influence the mechanisms responsible for cellular CPP internalization and the lack of sensitive methods for detection of the CPP, and in some cases the cargo, further complicates the design and conduction of conclusive mechanistic studies.

Archetypal tryptophan-rich antimicrobial peptides: properties and applications

Drug-resistant microorganisms ('superbugs') present a serious challenge to the success of antimicrobial treatments. Subsequently, there is a crucial need for novel bio-control agents. Many antimicrobial peptides (AMPs) show a broad-spectrum activity against bacteria, fungi or viruses and are strong candidates to complement or substitute current antimicrobial agents. Some AMPs are also effective against protozoa or cancer cells. The tryptophan (Trp)-rich peptides (TRPs) are a subset of AMPs that display potent antimicrobial activity, credited to the unique biochemical properties of tryptophan that allow it to insert into biological membranes. Further, many Trp-rich AMPs cross bacterial membranes without compromising their integrity and act intracellularly, suggesting interactions with nucleic acids and enzymes. In this work, we overview some archetypal TRPs derived from natural sources, i.e., indolicidin, tritrpticin and lactoferricin, summarising their biochemical properties, structures, antimicrobial activities, mechanistic studies and potential applications.

Antimicrobial peptides: an alternative for innovative medicines?

Antimicrobial peptides are small molecules with activity against bacteria, yeasts, fungi, viruses, bacteria, and even tumor cells that make these molecules attractive as therapeutic agents. Due to the alarming increase of antimicrobial resistance, interest in alternative antimicrobial agents has led to the exploitation of antimicrobial peptides, both synthetic and from natural sources. Thus, many peptide-based drugs are currently commercially available for the treatment of numerous ailments, such as hepatitis C, myeloma, skin infections, and diabetes. Initial barriers are being increasingly overcome with the development of cost-effective, more stable peptides. Herein, we review the available strategies for their synthesis, bioinformatics tools for the rational design of antimicrobial peptides with enhanced therapeutic indices, hurdles and shortcomings limiting the large-scale production of AMPs, as well as the challenges that the pharmaceutical industry faces on their use as therapeutic agents.

Importance of Tryptophan in Transforming an Amphipathic Peptide into a Pseudomonas aeruginosa-Targeted Antimicrobial Peptide

Here, we found that simple substitution of amino acids in the middle position of the hydrophobic face of an amphipathic peptide RI16 with tryptophan (T9W) considerably transformed into an antimicrobial peptide specifically targeting Pseudomonas aeruginosa. Minimal inhibitory concentration (MIC) results demonstrated that T9W had a strong and specifically antimicrobial activity against P. aeruginosa, including antibiotic-resistant strains, but was not active against Escherichia coli, Salmonella typhimurium, Staphylococcus aureus and Staphyfococcus epidermidis. Fluorescent spectroscopic assays indicated that T9W interacted with the membrane of P. aeruginosa, depolarizing the outer and the inner membrane of bacterial cells. Salt susceptibility assay showed that T9W still maintained its strong anti-pseudomonas activity in the presence of salts at physiological concentrations, and in hemolytic and MTT assays T9W also showed no toxicity against human blood cells and macrophages. In vivo assay demonstrated that T9W also displayed no toxicity to Chinese Kun Ming (KM) mice. Furthermore, the strong antibiofilm activity was also observed with the peptide T9W, which decreased the percentage of biomass formation in a dose-dependent manner. Overall, these findings indicated that design of single-pathogen antimicrobial agents can be achieved by simple amino acid mutation in naturally occurring peptide sequences and this study suggested a model of optimization/design of anti-pseudomonas drugs in which the tryptophan residue was a conserved element.

Peptide-membrane interactions of arginine-tryptophan peptides probed using quartz crystal microbalance with dissipation monitoring

Membrane-active peptides include peptides that can cross cellular membranes and deliver macromolecular cargo as well as peptides that inhibit bacterial growth. Some of these peptides can act as both transporters and antibacterial agents. It is desirable to combine the knowledge from these two different fields of membrane-active peptides into design of new peptides with tailored actions, as transporters of cargo or as antibacterial substances, targeting specific membranes. We have previously shown that the position of the amino acid tryptophan in the peptide sequence of three arginine-tryptophan peptides affects their uptake and intracellular localization in live mammalian cells, as well as their ability to inhibit bacterial growth. Here, we use quartz crystal microbalance with dissipation monitoring to assess the induced changes caused by binding of the three peptides to supported model membranes composed of POPC, POPC/POPG, POPC/POPG/cholesterol or POPC/lactosyl PE. Our results indicate that the tryptophan position in the peptide sequence affects the way these peptides interact with the different model membranes and that the presence of cholesterol in particular seems to affect the membrane interaction of the peptide with an even distribution of tryptophans in the peptide sequence. These results give mechanistic insight into the function of these peptides and may aid in the design of membrane-active peptides with specified cellular targets and actions.

Characterization of a novel cell penetrating peptide derived from human Oct4

Background

Oct4 is a transcription factor that plays a major role for the preservation of the pluripotent state in embryonic stem cells as well as for efficient reprogramming of somatic cells to induced pluripotent stem cells (iPSC) or other progenitors. Protein-based reprogramming methods mainly rely on the addition of a fused cell penetrating peptide. This study describes that Oct4 inherently carries a protein transduction domain, which can translocate into human and mouse cells.

Results

A 16 amino acid peptide representing the third helix of the human Oct4 homeodomain, referred to as Oct4 protein transduction domain (Oct4-PTD), can internalize in mammalian cells upon conjugation to a fluorescence moiety thereby acting as a cell penetrating peptide (CPP). The cellular distribution of Oct4-PTD shows diffuse cytosolic and nuclear staining, whereas penetratin is strictly localized to a punctuate pattern in the cytoplasm. By using a Cre/loxP-based reporter system, we show that this peptide also drives translocation of a functionally active Oct4-PTD-Cre-fusion protein. We further provide evidence for translocation of full length Oct4 into human and mouse cell lines without the addition of any kind of cationic fusion tag. Finally, physico-chemical properties of the novel CPP are characterized, showing that in contrast to penetratin a helical structure of Oct4-PTD is only observed if the FITC label is present on the N-terminus of the peptide.

Conclusions

Oct4 is a key transcription factor in stem cell research and cellular reprogramming. Since it has been shown that recombinant Oct4 fused to a cationic fusion tag can drive generation of iPSCs, our finding might contribute to further development of protein-based methods to generate iPSCs.

Moreover, our data support the idea that transcription factors might be part of an alternative paracrine signalling pathway, where the proteins are transferred to neighbouring cells thereby actively changing the behaviour of the recipient cell.

Electronic supplementary material

The online version of this article (doi: 10.1186/2045-9769-3-2) contains supplementary material, which is available to authorized users.