Fluorophore labeling of a cell-penetrating peptide induces differential effects on its cellular distribution and affects cell viability

No Evidence for Plasma Membrane Potential-Independent Cell Penetrating Peptide Direct Translocation

Cell-penetrating peptides (CPPs) are small peptides that can carry bioactive cargoes into cells. CPPs access the cell's cytosol via direct translocation across the plasma membrane. We and others have shown that direct translocation of CPPs occurs through water pores that are formed upon hyperpolarization of the cell's membrane. Direct translocation through water pores can therefore be blocked by depolarizing the plasma membrane. Other direct translocation mechanisms have been proposed that would not rely on membrane hyperpolarization. It has been reported, for example, that in HEK cells, CPP translocation occurs in a plasma membrane potential-independent manner, in contrast to HeLa cells, where CPP access to the cytosol required plasma membrane hyperpolarization. To address these apparent discrepant data, we have tested the requirement of plasma membrane hyperpolarization in a series of cell lines, including HEK and HeLa cells, for CPP direct translocation. Our data, obtained from a wide range of CPP concentrations, show that efficient direct translocation always requires plasma membrane hyperpolarization. We discuss the possible reasons why earlier studies have not evidenced the importance of the plasma membrane potential in the cytosolic uptake of CPPs in some cell lines.

An antisense oligomer conjugate with unpredicted bactericidal activity against Fusobacterium nucleatum

Fusobacteria are commensal members of the oral microbiome that can spread from their primary niche and colonize distal sites in the human body. Their enrichment in colorectal and breast cancer tissue has been associated with tumor growth, metastasis, and chemotherapeutic resistance. The use of non-selective antibiotics to remove fusobacteria impairs tumor progression, but prolonged application causes side effects, such as gastrointestinal problems and dysbiosis. Species-specific antisense antibiotics based on peptide nucleic acid (PNA) have shown efficacy in many gram-negative species, suggesting that antisense PNAs may also enable a tailored depletion of fusobacteria. Here, we have investigated the antibacterial potential of cell-penetrating peptide (CPP)-PNA conjugates targeting the mRNA of putative essential genes in Fusobacterium nucleatum. Unexpectedly, we observed no growth inhibition with any of the target-specific PNAs but identified a non-targeting control CPP-PNA [FUS79, (RXR)4XB-GACATAATTGT] as a potent growth inhibitor of F. nucleatum. Our data suggest that the CPP and specific sequence features of FUS79 are responsible for its activity, rather than an antisense effect. Interestingly, FUS79 also inhibits the growth of five additional fusobacterial strains but not of F. nucleatum subsp. vincentii (FNV). RNA-seq analysis indicates that FUS79 induces a membrane stress response in a vulnerable F. nucleatum strain but not in FNV. Collectively, our attempt at developing antisense antibiotics for fusobacteria discovers a potent growth inhibitor, whose bactericidal effect appears independent of target-specific mRNA inhibition.IMPORTANCEEnrichment of F. nucleatum at cancer sites is associated with increased tumor growth and metastasis. Antibiotic treatment to remove the bacteria was shown to change the course of cancer progression. Here, we explore first steps to establish peptide nucleic acids (PNAs) as specific antisense antibiotics, thereby laying the foundation for further development of antisense technology in fusobacteria. Although the CPP-PNA FUS79 was initially designed as a control, we observed that the compound was bactericidal for specific fusobacterial strains. Our results suggest that FUS79 might be able to selectively deplete fusobacterial strains from bacterial communities, offering a new perspective on fusobacterial removal at the tumor site.

Graft-then-Shrink Polymer Coatings for Localized Surface Plasmon Resonance Active Interfaces

Increasing the polymer content on biosensors is important to improve sensor function by altering surface properties and increasing the number of capture sites for analytes. Grafting-to methods are often employed but may be limited by insufficient polymer immobilization. Herein, we have utilized Graft-then-Shrink (GtS) to simultaneously increase polymer content on grafting-to surfaces and produce low-cost, local surface plasmon resonance (LSPR) Au biosensors. The biosensors were incorporated within microwell plates, where the translocation of materials across biological barriers can be tracked by visible light absorbance shifts as a platform for biological barrier crossing molecules. Biosensors were constructed by coating a flat Au layer on stretched polystyrene (PS) with thiol-terminated polymers that, upon heating, produced LSPR active wrinkled Au layers with ∼78% greater polymer content and lower water contact angles (WCA; ∼15°) compared to Shrink-then-Graft (StG) controls (∼55°) for PEG2MA coatings. To demonstrate translocation detection, 48-well microplates were 3D printed for GtS biosensor incorporation in the presence of a phospholipid bilayer. Using visible light to track LSPR peak shifts, cell penetrating peptides (CPPs) were screened for bilayer translocation and rate kinetics. GtS offers a simple method to increase the polymer content within coatings and an LSPR fabrication platform to track biomolecule translocation.

Late-Stage Minimal Labeling of Peptides and Proteins for Real-Time Imaging of Cellular Trafficking

The cellular uptake routes of peptides and proteins are complex and diverse, often handicapping therapeutic success. Understanding their mechanisms of internalization requires chemical derivatization with approaches that are compatible with wash-free and real-time imaging. In this work, we developed a new late-stage labeling strategy for unprotected peptides and proteins, which retains their biological activity while enabling live-cell imaging of uptake and intracellular trafficking. Benzo-2,1,3-thiadiazoles were selectively incorporated into Cys residues of both linear and cyclic peptides via Pd-mediated arylation with good yields and high purities. The resulting labeled peptides are chemically stable under physiological conditions and display strong fluorogenic character for wash-free imaging studies. We utilized this approach to prepare native-like analogues of cell-penetrating peptides and performed time-course analysis of their internalization routes in live cells by fluorescence lifetime imaging. Furthermore, we applied our strategy to label the chemokine protein mCCL2 and monitor its internalization via receptor-mediated endocytosis in live macrophages. This study provides a straightforward strategy for late-stage fluorogenic labeling of intact peptides and small proteins and direct visualization of dynamic intracellular events.

Guanidines Conjugated with Cell-Penetrating Peptides: A New Approach for the Development of Antileishmanial Molecules

Leishmaniasis is a neglected tropical disease caused by a protozoan of the genus Leishmania, which has visceral and cutaneous forms. The symptoms of leishmaniasis include high fever and weakness, and the cutaneous infection also causes lesions under the skin. The drugs used to treat leishmaniasis have become less effective due to the resistance mechanisms of the protozoa. In addition, the current compounds have low selectivity for the pathogen, leading to various side effects, which results in lower adherence to treatment. Various strategies were developed to solve this problem. The bioconjugation between natural compounds with antimicrobial activity and cell-penetrating peptides could alleviate the resistance and toxicity of current treatments. This work aims to conjugate the cell penetration peptide TAT to the guanidine GVL1. The GVL1-TAT bioconjugate exhibited leishmanicidal activity against Leishmania amazonensis and Leishmania infantum with a high selectivity index. In addition, the bioconjugate was more active against the intracellular enzyme CPP than the individual compounds. This target is very important for the viability and virulence of the parasite within the host cell. Docking studies confirmed the higher interaction of the conjugate with CPP and suggested that other proteins, such as trypanothione reductase, could be targeted. Thus, the data indicated that guanidines conjugated with cell-penetrating peptides could be a good approach for developing antileishmanial molecules.

Development of a dextrin-vitamin D3 micelle nanocarrier for the antimicrobial peptide LLKKK18 as a potential therapeutic agent for bone infections

In this work, an expedite synthesis was developed for a self-assembled micelle carrier for the antimicrobial peptide LL18. Covalent one-pot functionalization of dextrin with succinylated vitamin D3 and succinic anhydride produced an amphiphilic material that undergoes self-assembly into micelles in aqueous medium. Succinylated dextrin-vitamin D3 micelles were efficiently loaded with LL18 by electrostatic and hydrophobic interactions. Remarkably, the LL18-loaded micelle formulation dramatically improves the antibacterial activity of free LL18 against S. aureus, completely abrogates its severe hemolytic activity, redirects the internalization of LL18 from the perinuclear region of osteoblasts to the lysosomes and reduces cellular toxicity towards osteoblasts and macrophages. Overall, this work demonstrates that self-assembled micelle formulations based on dextrin, vitamin D3 and antimicrobial peptides, are promising platforms to develop multifunctional antibiotic-independent antimicrobial agents, not prone to the development of bacterial resistance, to treat bone infections.

Computational investigation of the effect of BODIPY labelling on peptide-membrane interaction

Optical monitoring of peptide binding to live cells is hampered by the abundance of naturally occurring fluorophores such as tryptophan. Unnatural amino acids incorporating synthetic fluorophores such as BODIPY overcome these optical limitations. A drawback to using fluorophores in lipid binding peptide design is their propensity to override other interactions, potentially causing the peptides to lose their binding selectivity. Here, the binding strength of a selection of peptides incorporating a variety of BODIPY derivatized amino acids has been studied via molecular dynamics simulations to quantify the impact of BODIPY incorporation on peptide-membrane binding behaviour.

Quantitative analysis of electroporation-mediated intracellular delivery via bioorthogonal luminescent reaction

Efficient intracellular delivery is crucial for biotherapeutics, such as proteins, oligonucleotides, and CRISPR/Cas9 gene-editing systems, to achieve their efficacy. Despite the great efforts of developing new intracellular delivery carriers, the lack of straightforward methods for intracellular delivery quantification limits further development in this area. Herein, we designed a simple and versatile bioorthogonal luminescent reaction (BioLure assay) to analyze intracellular delivery. Our results suggest that BioLure can be used to estimate the amount of intracellularly delivered molecules after electroporation, and the estimation by BioLure is in good correlation with the results from complementary methods. Furthermore, we used BioLure assay to correlate the intracellularly-delivered RNase A amount with its tumoricidal activity. Overall, BioLure is a versatile tool for understanding the intracellular delivery process on live cells, and establishing the link between the cytosolic concentration of intracellularly-delivered biotherapeutics and their therapeutic efficacy.

Improving corneal permeability of dexamethasone using penetration enhancing agents: First step towards achieving topical drug delivery to the retina

With an ever-increasing burden of vision loss caused by diseases of the posterior ocular segment, there is an unmet clinical need for non-invasive treatment strategies. Topical drug application using eye drops suffers from low to negligible bioavailability to the posterior segment as a result of static and dynamic defensive ocular barriers to penetration, while invasive delivery systems are expensive to administer and suffer potentially severe complications. As the cornea is the main anatomical barrier to uptake of topically applied drugs from the ocular surface, we present an approach to increase corneal permeability of a corticosteroid, dexamethasone sodium-phosphate (DSP), using a novel penetration enhancing agent (PEA). We synthesised a novel polyacetylene (pAc) polymer and compared its activity to two previously described cell penetrating peptide (CPP) based PEAs, TAT and penetratin, with respect to increasing transcorneal permeability of DSP in a rapid ex-vivo porcine corneal assay over 60 min. The transcorneal apparent permeability coefficients (Papp) for diffusion of pAc, and fluorescein isothiocyanate (FITC) conjugated TAT and penetratin were up to 5 times higher (p < 0.001), when compared to controls. When pAc was used in formulation with DSP, an almost 5-fold significant increase was observed in Papp of DSP across the cornea (p = 0.0130), a significant 6-fold increase with TAT (p = 0.0377), and almost 7-fold mean increase with penetratin (p = 0.9540). Furthermore, we investigated whether the PEAs caused any irreversible damage to the barrier integrity of the corneal epithelium by measuring transepithelial electrical resistance (TEER) and immunostaining of tight junction proteins using zonula occludens-1 (ZO-1) and occludin antibodies. There was no damage or structural toxicity, and the barrier integrity was preserved after PEA application. Finally, an in-vitro cytotoxicity assessment of all PEAs in human retinal pigment epithelium cells (ARPE-19) demonstrated that all PEAs were very well-tolerated, with IC50 values of 64.79 mM for pAc and 1335.45 µM and 87.26 µM for TAT and penetratin, respectively. Our results suggest that this drug delivery technology could potentially be used to achieve a significantly higher intraocular therapeutic bioavailability after topical eye drop administration, than currently afforded.

Evaluation of the Cytosolic Uptake of HaloTag Using a pH-Sensitive Dye

The efficient cytosolic delivery of proteins is critical for advancing novel therapeutic strategies. Current delivery methods are severely limited by endosomal entrapment, and detection methods lack sophistication in tracking the fate of delivered protein cargo. HaloTag, a commonly used protein in chemical biology and a challenging delivery target, is an exceptional model system for understanding and exploiting cellular delivery. Here, we employed a combinatorial strategy to direct HaloTag to the cytosol. We established the use of Virginia Orange, a pH-sensitive fluorophore, and Janelia Fluor 585, a similar but pH-agnostic fluorophore, in a fluorogenic assay to ascertain protein localization within human cells. Using this assay, we investigated HaloTag delivery upon modification with cell-penetrating peptides, carboxyl group esterification, and cotreatment with an endosomolytic agent. We found efficacious cytosolic entry with two distinct delivery methods. This study expands the toolkit for detecting the cytosolic access of proteins and highlights that multiple intracellular delivery strategies can be used synergistically to effect cytosolic access. Moreover, HaloTag is poised to serve as a platform for the delivery of varied cargo into human cells.

Polymyxins with Potent Antibacterial Activity against Colistin-Resistant Pathogens: Fine-Tuning Hydrophobicity with Unnatural Amino Acids

In view of the increased prevalence of antimicrobial resistance among human pathogens, antibiotics against multidrug-resistant (MDR) bacteria are in urgent demand. In particular, the rapidly emerging resistance to last-resort antibiotic colistin, used for severe Gram-negative MDR infections, is critical. Here, a series of polymyxins containing unnatural amino acids were explored, and some analogues exhibited excellent antibacterial activity against Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa. Hydrophobicity of the compounds within this series (as measured by retention in reversed-phase analytical HPLC) exhibited a discernible correlation with their antimicrobial activity. This trend was particularly pronounced for colistin-resistant pathogens. The most active compounds demonstrated competitive activity against a panel of Gram-negative pathogens, while exhibiting low in vitro cytotoxicity. Importantly, most of these hits also retained (or even had increased) potency against colistin-susceptible strains. These findings infer that fine-tuning hydrophobicity may enable the design of polymyxin analogues with favorable activity profiles.

Cell Penetrating Peptides: Classification, Mechanisms, Methods of Study, and Applications

Cell-penetrating peptides (CPPs) encompass a class of peptides that possess the remarkable ability to cross cell membranes and deliver various types of cargoes, including drugs, nucleic acids, and proteins, into cells. For this reason, CPPs are largely investigated in drug delivery applications in the context of many diseases, such as cancer, diabetes, and genetic disorders. While sharing this functionality and some common structural features, such as a high content of positively charged amino acids, CPPs represent an extremely diverse group of elements, which can differentiate under many aspects. In this review, we summarize the most common characteristics of CPPs, introduce their main distinctive features, mechanistic aspects that drive their function, and outline the most widely used techniques for their structural and functional studies. We highlight current gaps and future perspectives in this field, which have the potential to significantly impact the future field of drug delivery and therapeutics.

Delivered complementation in planta (DCIP) enables measurement of peptide-mediated protein delivery efficiency in plants

Using a fluorescence complementation assay, Delivered Complementation in Planta (DCIP), we demonstrate cell-penetrating peptide-mediated cytosolic delivery of peptides and recombinant proteins in Nicotiana benthamiana. We show that DCIP enables quantitative measurement of protein delivery efficiency and enables functional screening of cell-penetrating peptides for in-planta protein delivery. Finally, we demonstrate that DCIP detects cell-penetrating peptide-mediated delivery of recombinantly expressed proteins such as mCherry and Lifeact into intact leaves. We also demonstrate delivery of a recombinant plant transcription factor, WUSCHEL (AtWUS), into N. benthamiana. RT-qPCR analysis of AtWUS delivery in Arabidopsis seedlings also suggests delivered WUS can recapitulate transcriptional changes induced by overexpression of AtWUS. Taken together, our findings demonstrate that DCIP offers a new and powerful tool for interrogating cytosolic delivery of proteins in plants and highlights future avenues for engineering plant physiology.

Designing Fluorescent Nuclear Permeable Peptidomimetics to Target Proliferating Cell Nuclear Antigen

Human proliferating cell nuclear antigen (PCNA) is a critical mediator of DNA replication and repair, acting as a docking platform for replication proteins. Disrupting these interactions with a peptidomimetic agent presents as a promising avenue to limit proliferation of cancerous cells. Here, a p21-derived peptide was employed as a starting scaffold to design a modular peptidomimetic that interacts with PCNA and is cellular and nuclear permeable. Ultimately, a peptidomimetic was produced which met these criteria, consisting of a fluorescein tag and SV40 nuclear localization signal conjugated to the N-terminus of a p21 macrocycle derivative. Attachment of the fluorescein tag was found to directly affect cellular uptake of the peptidomimetic, with fluorescein being requisite for nuclear permeability. This work provides an important step forward in the development of PCNA targeting peptidomimetics for use as anti-cancer agents or as cancer diagnostics.

Stereoisomer-Dependent Membrane Association and Capacity for Insulin Delivery Facilitated by Penetratin

Cell-penetrating peptides (CPPs), such as penetratin, are often investigated as drug delivery vectors and incorporating d-amino acids, rather than the natural l-forms, to enhance proteolytic stability could improve their delivery efficiency. The present study aimed to compare membrane association, cellular uptake, and delivery capacity for all-l and all-d enantiomers of penetratin (PEN) by using different cell models and cargos. The enantiomers displayed widely different distribution patterns in the examined cell models, and in Caco-2 cells, quenchable membrane binding was evident for d-PEN in addition to vesicular intracellular localization for both enantiomers. The uptake of insulin in Caco-2 cells was equally mediated by the two enantiomers, and while l-PEN did not increase the transepithelial permeation of any of the investigated cargo peptides, d-PEN increased the transepithelial delivery of vancomycin five-fold and approximately four-fold for insulin at an extracellular apical pH of 6.5. Overall, while d-PEN was associated with the plasma membrane to a larger extent and was superior in mediating the transepithelial delivery of hydrophilic peptide cargoes compared to l-PEN across Caco-2 epithelium, no enhanced delivery of the hydrophobic cyclosporin was observed, and intracellular insulin uptake was induced to a similar degree by the two enantiomers.

Selenomethionine as alternative label to the fluorophore TAMRA when exploiting cell-penetrating peptides as blood-brain barrier shuttles to better mimic the physicochemical properties of the non-labelled peptides

The cell-penetrating peptides (CPPs) Tat and penetratin are frequently explored as shuttles for drug delivery across the blood-brain barrier (BBB). CPPs are often labelled with fluorophores for analytical purposes, with 5(6)-carboxytetramethylrhodamine (TAMRA) being a popular choice. However, TAMRA labelling affects the physicochemical properties of the resulting fluorophore-CPP construct when compared to the CPP alone. Selenomethionine (M^Se) may be introduced as alternative label, which, due to its small size and amino acid nature, likely results in minimal alterations of the peptide physicochemical properties. With this study we compared, head-to-head, the effect of M^Se and TAMRA labelling of Tat and penetratin with respect to their physicochemical properties, and investigated effects hereof on brain capillary endothelial cell (BCEC) models. TAMRA labelling positively affected the ability of the peptides to adhere to the cell membranes as well being internalized into the BCECs when compared to M^Se labelling. TAMRA labelling of penetratin added toxicity to the BCECs to a higher extent than TAMRA labelling of Tat, whereas M^Se labelling did not affect the cellular viability. Both TAMRA and M^Se labelling of penetratin decreased the barrier integrity of BCEC monolayers, but not to an extent that improved transport of the paracellular marker ¹⁴C-mannitol. In conclusion, M^Se labelling of Tat and penetratin adds minimal alterations to the physicochemical properties of these CPPs and their resulting effects on BCECs, and thereby represents a preferred alternative to TAMRA for peptide quantification purposes.

Plasma membrane depolarization reveals endosomal escape incapacity of cell-penetrating peptides

Cell-penetrating peptides (CPPs) are short (<30 amino acids), generally cationic, peptides that deliver diverse cargos into cells. CPPs access the cytosol either by direct translocation through the plasma membrane or via endocytosis followed by endosomal escape. Both direct translocation and endosomal escape can occur simultaneously, making it non-trivial to specifically study endosomal escape alone. Here we depolarize the plasma membrane and showed that it inhibits the direct translocation of several CPPs but does not affect their uptake into endosomes. Despite good endocytic uptake many CPPs previously considered to access the cytosol via endosomal escape, failed to access the cytosol once direct translocation was abrogated. Even CPPs designed for enhanced endosomal escape actually showed negligible endosomal escape into the cytosol. Our data reveal that cytosolic localization of CPPs occurs mainly by direct translocation across the plasma membrane. Cell depolarization represents a simple manipulation to stringently test the endosomal escape capacity of CPPs.

Antimicrobial and Cell-Penetrating Peptides: Understanding Penetration for the Design of Novel Conjugate Antibiotics

Antimicrobial peptides (AMPs) are short oligopeptides that can penetrate the bacterial inner and outer membranes. Together with cell-penetrating peptides (CPPs), they are called membrane active peptides; peptides which can translocate across biological membranes. Over the last fifty years, attempts have been made to understand the molecular features that drive the interactions of membranes with membrane active peptides. This review examines the features of a membrane these peptides exploit for translocation, as well as the physicochemical characteristics of membrane active peptides which are important for translocation. Moreover, it presents examples of how these features have been used in recent years to create conjugates consisting of a membrane active peptide, called a "vector", attached to either a current or novel antibiotic, called a "cargo" or "payload". In addition, the review discusses what properties may contribute to an ideal peptide vector able to deliver cargoes across the bacterial outer membrane as the rising issue of antimicrobial resistance demands new strategies to be employed to combat this global public health threat.

Approaches for evaluation of novel CPP-based cargo delivery systems

Cell penetrating peptides (CPPs) can be broadly defined as relatively short synthetic, protein derived or chimeric peptides. Their most remarkable property is their ability to cross cell barriers and facilitate the translocation of cargo, such as drugs, nucleic acids, peptides, small molecules, dyes, and many others across the plasma membrane. Over the years there have been several approaches used, adapted, and developed for the evaluation of CPP efficacies as delivery systems, with the fluorophore attachment as the most widely used approach. It has become progressively evident, that the evaluation method, in order to lead to successful outcome, should concede with the specialties of the delivery. For characterization and assessment of CPP-cargo a combination of research tools of chemistry, physics, molecular biology, engineering, and other fields have been applied. In this review, we summarize the diverse, in silico, in vitro and in vivo approaches used for evaluation and characterization of CPP-based cargo delivery systems.

Pulmonary surfactant and drug delivery: vehiculization of a tryptophan-tagged antimicrobial peptide over the air-liquid interfacial highway

This work evaluates interaction of pulmonary surfactant (PS) and antimicrobial peptides (AMPs) in order to investigate (i) if PS can be used to transport AMPs, and (ii) to what extent PS interferes with AMP function and vice versa. This, in turn, is motivated by a need to find new strategies to treat bacterial infections in the airways. Low respiratory tract infections (LRTIs) are a leading cause of illness and death worldwide that, together with the problem of multidrug-resistant (MDR) bacteria, bring to light the necessity of developing effective therapies that ensure high bioavailability of the drug at the site of infection and display a potent antimicrobial effect. Here, we propose the combination of AMPs with PS to improve their delivery, exemplified for the hydrophobically end-tagged AMP, GRR10W4 (GRRPRPRPRPWWWW-NH₂), with previously demonstrated potent antimicrobial activity against a broad spectrum of bacteria under various conditions. Experiments using model systems emulating the respiratory interface and an operating alveolus, based on surface balances and bubble surfactometry, served to demonstrate that a fluorescently labelled version of GRR10W4 (GRR10W4-F), was able to interact and insert into PS membranes without affecting its biophysical function. Therefore, vehiculization of the peptide along air-liquid interfaces was enabled, even for interfaces previously occupied by surfactants layers. Furthermore, breathing-like compression-expansion dynamics promoted the interfacial release of GRR10W4-F after its delivery, which could further allow the peptide to perform its antimicrobial function. PS/GRR10W4-F formulations displayed greater antimicrobial effects and reduced toxicity on cultured airway epithelial cells compared to that of the peptide alone. Taken together, these results open the door to the development of novel delivery strategies for AMPs in order to increase the bioavailability of these molecules at the infection site via inhaled therapies.

Chiroptical detection of self-aggregating fluorescent rhodamine conjugates: mistakes and prospects

Fluorescent conjugation can be considered as the chromophoric derivatization of the target and as such it may provide additional, structure-related information quickly available by using circular dichroism (CD) spectroscopy. In this essay, peculiar CD spectroscopic data reported earlier for thyroid hormone-rhodamine conjugates have been re-evaluated. Contrary to the original proposal of the intramolecular folding of the labeled hormone, the bisignate motif in the CD spectrum is a clear evidence of dye-dye, intermolecular chiral exciton coupling indicating supramolecular self-association of the conjugates. This anomalous solution behaviour undermines the credibility of experimental results reported with such conjugates still being used in the laboratory practice. The extension of routine far-UV CD spectroscopic scans of chiral fluorophore conjugates into the near-UV and visible spectral region is strongly recommended.

Studies of cell-penetrating peptides by biophysical methods

Biophysical studies have a very high impact on the understanding of internalization, molecular mechanisms, interactions, and localization of CPPs and CPP/cargo conjugates in live cells or in vivo. Biophysical studies are often first carried out in test-tube set-ups or in vitro, leading to the complicated in vivo systems. This review describes recent studies of CPP internalization, mechanisms, and localization. The multiple methods in these studies reveal different novel and important aspects and define the rules for CPP mechanisms, hopefully leading to their improved applicability to novel and safe therapies.

Far-UV circular dichroism signatures indicate fluorophore labeling induced conformational changes of penetratin

Fluorescent labeling is a broadly utilized approach to assess in vitro and in vivo behavior of biologically active, especially cell-penetrating and antimicrobial peptides. In this communication, far-UV circular dichroism (CD) spectra of penetratin (PEN) fluorophore conjugates reported previously have been re-evaluated. Compared to the intrinsically disordered native peptide, rhodamine B and carboxyfluorescein derivatives of free and membrane-bound PEN exhibit extrinsic CD features. Potential sources of these signals displayed above 220 nm are discussed suggesting the contributions of both intra- and intermolecular chiral exciton coupling mechanisms. Careful evaluation of the CD spectra of fluorophore-labeled peptides is a valuable tool for early detection of labeling-provoked structural alterations which in turn may modify the membrane binding and cellular uptake compared to the unconjugated form.

Multi-dimensional-double-spiral (MDDS) inertial microfluidic platform for sperm isolation directly from the raw semen sample

Here, we propose a fully-automated platform using a spiral inertial microfluidic device for standardized semen preparation that can process patient-derived semen samples with diverse fluidic conditions without any pre-washing steps. We utilized the multi-dimensional double spiral (MDDS) device to effectively isolate sperm cells from other non-sperm seminal cells (e.g., leukocytes) in the semen sample. The recirculation platform was employed to minimize sample dependency and achieve highly purified and concentrated (up to tenfold) sperm cells in a rapid and fully-automated manner (~ 10 min processing time for 50 mL of diluted semen sample). The clinical (raw) semen samples obtained from healthy donors were directly used without any pre-washing step to evaluate the developed separation platform, which showed excellent performance with ~ 80% of sperm cell recovery, and > 99.95% and > 98% removal of 10-μm beads (a surrogate for leukocytes) from low-viscosity and high-viscosity semen samples, respectively. We expect that the novel platform will be an efficient and automated tool to achieve purified sperm cells directly from raw semen samples for assisted reproductive technologies (ARTs) as an alternative to density centrifugation or swim-up methods, which often suffer from the low recovery of sperm cells and labor-intensive steps.

Novel Lysine-Rich Delivery Peptides of Plant Origin ERD and Human S100: The Effect of Carboxyfluorescein Conjugation, Influence of Aromatic and Proline Residues, Cellular Internalization, and Penetration Ability

The need for novel drug delivery peptides is an important issue of the modern pharmaceutical research. Here, we test K-rich peptides from plant dehydrin ERD14 (ERD-A, ERD-B, and ERD-C) and the C-terminal CPP-resembling region of S100A4 (S100) using the 5(6)-carboxyfluorescein (Cf) tag at the N-terminus. Via a combined pH-dependent NMR and fluorescence study, we analyze the effect of the Cf conjugation/modification on the structural behavior, separately investigating the (5)-Cf and (6)-Cf forms. Flow cytometry results show that all peptides internalize; however, there is a slight difference between the cellular internalization of (5)- and (6)-Cf-peptides. We indicate the possible importance of residues with an aromatic sidechain and proline. We prove that ERD-A localizes mostly in the cytosol, ERD-B and S100 have partial colocalization with lysosomal staining, and ERD-C mainly localizes within vesicle-like compartments, while the uptake mechanism mainly occurs through energy-dependent paths.

Improving Membrane Activity and Cargo Delivery Efficacy of a Cell-Penetrating Peptide by Loading with Carboranes

Cell-penetrating peptides (CPPs) have emerged as versatile tools to increase the intracellular accumulation of different kinds of cargoes. For an efficient cellular uptake and drug delivery, their organization into a distinct and stable secondary structure at the outer surface of the plasma membrane is a hallmark and supports optimal lipid-peptide interactions. Incorporation of hydrophobic moieties, such as carboranes (CBs), has the potential to increase the lipophilicity of peptides, and thus, to facilitate the formation of secondary structures. Herein, we present synthesis and biophysical as well as biological characterization of carborane-CPP conjugates having incorporated one or more CB clusters. Our results highlight the possibility to modulate the secondary structure of CPPs by the addition of CB's leading to constructs with altered membrane activity and promising use in terms of nucleic acid delivery.

Molecular photoswitches in aqueous environments

Molecular photoswitches enable dynamic control of processes with high spatiotemporal precision, using light as external stimulus, and hence are ideal tools for different research areas spanning from chemical biology to smart materials. Photoswitches are typically organic molecules that feature extended aromatic systems to make them responsive to (visible) light. However, this renders them inherently lipophilic, while water-solubility is of crucial importance to apply photoswitchable organic molecules in biological systems, like in the rapidly emerging field of photopharmacology. Several strategies for solubilizing organic molecules in water are known, but there are not yet clear rules for applying them to photoswitchable molecules. Importantly, rendering photoswitches water-soluble has a serious impact on both their photophysical and biological properties, which must be taken into consideration when designing new systems. Altogether, these aspects pose considerable challenges for successfully applying molecular photoswitches in aqueous systems, and in particular in biologically relevant media. In this review, we focus on fully water-soluble photoswitches, such as those used in biological environments, in both in vitro and in vivo studies. We discuss the design principles and prospects for water-soluble photoswitches to inspire and enable their future applications.

Challenges and Methods for the Study of CPP Translocation Mechanisms

Fluorescence-based methods are widely used to detect crossing of peptides across model or biological membranes. For membrane-active peptides, i.e., peptides that have strong membrane tropism, fluorescence experiments must be accompanied by relevant controls, otherwise they can lead to inconsistent interpretation and underestimation of their limitations. Here we describe how to prepare samples to study fluorescent peptide crossing droplet interface bilayer (model membrane) or cell membrane (biological membrane) and the pitfalls that can affect observational qualitative and quantitative data.

Fluorescent Amino Acid Initiated de novo Cyclic Peptides for the Label-Free Assessment of Cell Permeability*

The major obstacle in applying peptides to intracellular targets is their low inherent cell permeability. Standard approaches to attach a fluorophore (e. g. FITC, TAMRA) can change the physicochemical properties of the parent peptide and influence their ability to penetrate and localize in cells. We report a label-free strategy for evaluating the cell permeability of cyclic peptide leads. Fluorescent tryptophan analogues 4-cyanotryptophan (4CNW) and β-(1-azulenyl)-L-alanine (AzAla) were incorporated into in vitro translated macrocyclic peptides by initiator reprogramming. We then demonstrate these efficient blue fluorescent emitters are good tools for monitoring peptide penetration into cells.

Imaging therapeutic peptide transport across intestinal barriers

Oral delivery is a highly preferred method for drug administration due to high patient compliance. However, oral administration is intrinsically challenging for pharmacologically interesting drug classes, in particular pharmaceutical peptides, due to the biological barriers associated with the gastrointestinal tract. In this review, we start by summarizing the pharmacological performance of several clinically relevant orally administrated therapeutic peptides, highlighting their low bioavailabilities. Thus, there is a strong need to increase the transport of peptide drugs across the intestinal barrier to realize future treatment needs and further development in the field. Currently, progress is hampered by a lack of understanding of transport mechanisms that govern intestinal absorption and transport of peptide drugs, including the effects of the permeability enhancers commonly used to mediate uptake. We describe how, for the past decades, mechanistic insights have predominantly been gained using functional assays with end-point read-out capabilities, which only allow indirect study of peptide transport mechanisms. We then focus on fluorescence imaging that, on the other hand, provides opportunities to directly visualize and thus follow peptide transport at high spatiotemporal resolution. Consequently, it may provide new and detailed mechanistic understanding of the interplay between the physicochemical properties of peptides and cellular processes; an interplay that determines the efficiency of transport. We review current methodology and state of the art in the field of fluorescence imaging to study intestinal barrier transport of peptides, and provide a comprehensive overview of the imaging-compatible in vitro, ex vivo, and in vivo platforms that currently are being developed to accelerate this emerging field of research.

Development of novel GnRH and Tat48-60 based luminescent probes with enhanced cellular uptake and bioimaging profile

There is a clear need to develop photostable chromophores for bioimaging with respect to the classically utilized green fluorescent dye fluorescein. Along these lines, we utilized a phosphorescent carboxy-substituted ruthenium(ii) polypyridyl [Ru(bipy)2(mcb)]2+ (bipy = 2,2'-bipyridyl and mcb = 4-carboxy-4'-methyl-2,2'-bipyridyl) complex. We developed two luminescent peptide conjugates of the cell-penetrating peptide Tat48-60 consisting of either [Ru(bipy)2(mcb)]2+ or 5(6)-carboxyfluorescein (5(6)-FAM) tethered on the Lys50 of the peptide through amide bond. We confirmed the efficient cellular uptake of both bioconjugates in HeLa cells by confocal microscopy and flow cytometry and proved that the ruthenium-based chromophore possesses enhanced photostability compared to a 5(6)-FAM-based peptide, after continuous laser scanning. Furthermore, we designed and developed a luminescent agent with high photostability, based on the ruthenium core, that could be selectively localized in cancer cells overexpressing the GnRH receptor (GnRH-R). To achieve this, we took advantage of the tumor-homing character of d-Lys6-GnRH which selectively recognizes the GnRH-R. The [Ru(bipy)2(mcb)]2+-d-Lys6-GnRH peptide conjugate was synthesized, and its cellular uptake was evaluated through flow cytometric analysis and live-cell imaging in HeLa and T24 bladder cancer cells as negative and positive controls of GnRH-R, respectively. Besides the selective targeting that the specific conjugate could offer, we also recorded high internalization levels in T24 bladder cancer cells. The ruthenium(ii) polypyridyl peptide-based conjugates we developed is an intriguing approach that offers targeted cell imaging in the Near Infrared region, and simultaneously paves the way for further advancements in the dynamic studies on cellular imaging.

Translocation of the nonlabeled antimicrobial peptide PGLa across lipid bilayers and its entry into vesicle lumens without pore formation

Fluorescent-probe-labeled peptides are used to study the interactions of peptides with cells and lipid vesicles but labeling peptides with fluorescent probes can significantly change these interactions. We recently developed a new method to detect the entry of nonlabeled peptides into the lumen of single giant unilamellar vesicles (GUVs). Here we applied this method to examine the interaction of the antimicrobial peptide PGLa with single GUVs to elucidate whether PGLa can enter the GUV lumen without pore formation. First, we examined the interaction of nonlabeled PGLa with single GUVs comprising dioleoylphosphatidylglycerol (DOPG) and dioleoylphosphatidylcholine (DOPC) (4/6) whose lumens contain the fluorescent probe AF647 and DOPG/DOPC (8/2)-large unilamellar vesicles encapsulating a high concentration of calcein. After a large lag period from starting the interaction with PGLa, the fluorescence intensity of the GUV lumen due to calcein (I_calcein) increased gradually without leakage of AF647, indicating that PGLa enters the GUV lumen without pore formation in the GUV membrane. The fraction of entry of PGLa increased with increasing PGLa concentration. Simultaneous measurement of the fractional area change of the GUV membrane (δ) and PGLa-induced increase in I_calcein showed that the entry of PGLa occurs only during the second increase in δ, indicating that PGLa enters the lumen during its translocation from the outer leaflet to the inner leaflet. The fraction of entry of PGLa without pore formation increased with increasing membrane tension. Based on these results, we discuss the elementary processes and the mechanism of the entry of PGLa into the GUV lumen.

Approaches to Introduce Helical Structure in Cysteine-Containing Peptides with a Bimane Group

An i-i+4 or i-i+3 bimane-containing linker was introduced into a peptide known to target Estrogen Receptor alpha (ERα), in order to stabilise an α-helical geometry. These macrocycles were studied by CD and NMR to reveal the i-i+4 constrained peptide adopts a 3₁₀ -helical structure in solution, and an α-helical conformation on interaction with the ERα coactivator recruitment surface in silico. An acyclic bimane-modified peptide is also helical, when it includes a tryptophan or tyrosine residue; but is significantly less helical with a phenylalanine or alanine residue, which indicates such a bimane modification influences peptide structure in a sequence dependent manner. The fluorescence intensity of the bimane appears influenced by peptide conformation, where helical peptides displayed a fluorescence increase when TFE was added to phosphate buffer, compared to a decrease for less helical peptides. This study presents the bimane as a useful modification to influence peptide structure as an acyclic peptide modification, or as a side-chain constraint to give a macrocycle.

Nanoscale Visualization of Morphological Alteration of Live-Cell Membranes by the Interaction with Oligoarginine Cell-Penetrating Peptides

The interactions between the cell membrane and biomolecules remain poorly understood. For example, arginine-rich cell-penetrating peptides (CPPs), including octaarginines (R8), are internalized by interactions with cell membranes. However, during the internalization process, the exact membrane dynamics introduced by these CPPs are still unknown. Here, we visualize arginine-rich CPPs and cell-membrane interaction-induced morphological changes using a system that combines scanning ion-conductance microscopy and spinning-disk confocal microscopy, using fluorescently labeled R8. This system allows time-dependent, nanoscale visualization of structural dynamics in live-cell membranes. Various types of membrane remodeling caused by arginine-rich CPPs are thus observed. The induction of membrane ruffling and the cup closure are observed as a process of endocytic uptake of the peptide. Alternatively suggested is the concave structural formation accompanied by direct peptide translocation through cell membranes. Studies using R8 without fluorescent labeling also demonstrate a non-negligible effect of the fluorescent moiety on membrane structural alteration.

The Rhodamine B-encrypted vipericidin peptide, RhoB-Ctn[1-9], displays in vitro antimicrobial activity against opportunistic bacteria and yeasts

BACKGROUND

Crotalicidin (Ctn), a snake venom cathelicidin-related antimicrobial peptide, is a 34-residue-long linear lysine-rich vipericidin obtained from the South American rattlesnake, Crotalus durissus terrificus. Ctn contains tandem repeats of nine amino acid residues (1KRFKKFFKK9 and 16KRLKKIFKK24; consensus: 1KRhKKhFKK9, h = hydrophobic amino acid) as an integral part of its structure.

OBJECTIVE

The aim of this study was to evaluate the antimicrobial activity of the encrypted vipericidin nonapeptide KRFKKFFKK, designated as Ctn[1-9], and its structural analogue, rhodamine-B‒conjugated Ctn[1-9], designated as RhoB-Ctn[1-9].

METHOD

The susceptibility of representative pathogenic bacteria and yeasts to antimicrobial agents was determined using the broth microdilution minimum inhibitory concentration (MIC) method. Cytotoxicity was estimated using a hemolytic assay. The accumulation of RhoB-Ctn[1-9] in microbial cells was observed by fluorescence microscopy. The antimicrobial synergism of RhoB-Ctn[1-9] with antimicrobials was evaluated using a checkerboard analysis.

RESULTS

RhoB-conjugated Ctn[1-9] displayed selective antimicrobial activity against infectious gram-negative bacteria such as Escherichia coli, Pseudomonas aeruginosa, and pathogenic species of Candida with low hemolytic effects on human erythrocytes which was not observed with unconjugated Ctn[1-9]. RhoB-Ctn[1-9] could permeate cell membranes and accumulate intracellularly in microbial cells. RhoB-Ctn[1-9] exhibits synergistic effects when used with antibiotics or antifungal agents and reduced the MICs of the peptide and antimicrobials.

CONCLUSION

These findings indicate the potential of crotalicidin-related short peptides as structural motifs for the diversification of biological functionalities. Further, they set the stage to investigate the molecular mechanisms by which chemically modified vipericidin repeats modulate cell fate.

Cellular Internalization and Inhibition Capacity of New Anti-Glioma Peptide Conjugates: Physicochemical Characterization and Evaluation on Various Monolayer- and 3D-Spheroid-Based in Vitro Platforms

Most therapeutic agents used for treating brain malignancies face hindered transport through the blood-brain barrier (BBB) and poor tissue penetration. To overcome these problems, we developed peptide conjugates of conventional and experimental anticancer agents. SynB3 cell-penetrating peptide derivatives were applied that can cross the BBB. Tuftsin derivatives were used to target the neuropilin-1 transport system for selectivity and better tumor penetration. Moreover, SynB3-tuftsin tandem compounds were synthesized to combine the beneficial properties of these peptides. Most of the conjugates showed high and selective efficacy against glioblastoma cells. SynB3 and tandem derivatives demonstrated superior cellular internalization. The penetration profile of the conjugates was determined on a lipid monolayer and Transwell co-culture system with noncontact HUVEC-U87 monolayers as simple ex vivo and in vitro BBB models. Importantly, in 3D spheroids, daunomycin-peptide conjugates possessed a better tumor penetration ability than daunomycin. These conjugates are promising tools for the delivery systems with tunable features.

Antimicrobial Peptides and Copper(II) Ions: Novel Therapeutic Opportunities

The emergence of new pathogens and multidrug resistant bacteria is an important public health issue that requires the development of novel classes of antibiotics. Antimicrobial peptides (AMPs) are a promising platform with great potential for the identification of new lead compounds that can combat the aforementioned pathogens due to their broad-spectrum antimicrobial activity and relatively low rate of resistance emergence. AMPs of multicellular organisms made their debut four decades ago thanks to ingenious researchers who asked simple questions about the resistance to bacterial infections of insects. Questions such as "Do fruit flies ever get sick?", combined with pioneering studies, have led to an understanding of AMPs as universal weapons of the immune system. This review focuses on a subclass of AMPs that feature a metal binding motif known as the amino terminal copper and nickel (ATCUN) motif. One of the metal-based strategies of hosts facing a pathogen, it includes wielding the inherent toxicity of copper and deliberately trafficking this metal ion into sites of infection. The sudden increase in the concentration of copper ions in the presence of ATCUN-containing AMPs (ATCUN-AMPs) likely results in a synergistic interaction. Herein, we examine common structural features in ATCUN-AMPs that exist across species, and we highlight unique features that deserve additional attention. We also present the current state of knowledge about the molecular mechanisms behind their antimicrobial activity and the methods available to study this promising class of AMPs.

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.

A review on recent advances in amino acid and peptide-based fluorescence and its potential applications

Fluorescence is widely used to detect functional groups and ions, and peptides are used in various fields due to their excellent biological activity.

An Inherently Fluorescent Peptide Constraint to Define Secondary Structure: Moving Away from Auxiliary Tags

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A critical analysis of methods used to investigate the cellular uptake and subcellular localization of RNA therapeutics

RNA therapeutics are a promising strategy to treat genetic diseases caused by the overexpression or aberrant splicing of a specific protein. The field has seen major strides in the clinical efficacy of this class of molecules, largely due to chemical modifications and delivery strategies that improve nuclease resistance and enhance cell penetration. However, a major obstacle in the development of RNA therapeutics continues to be the imprecise, difficult, and often problematic nature of most methods used to measure cell penetration. Here, we review these methods and clearly distinguish between those that measure total cellular uptake of RNA therapeutics, which includes both productive and non-productive uptake, and those that measure cytosolic/nuclear penetration, which represents only productive uptake. We critically analyze the benefits and drawbacks of each method. Finally, we use key examples to illustrate how, despite rigorous experimentation and proper controls, our understanding of the mechanism of gymnotic uptake of RNA therapeutics remains limited by the methods commonly used to analyze RNA delivery.

To What Extent Do Fluorophores Bias the Biological Activity of Peptides? A Practical Approach Using Membrane-Active Peptides as Models

The characterization of biologically active peptides relies heavily on the study of their efficacy, toxicity, mechanism of action, cellular uptake, or intracellular location, using both in vitro and in vivo studies. These studies frequently depend on the use of fluorescence-based techniques. Since most peptides are not intrinsically fluorescent, they are conjugated to a fluorophore. The conjugation may interfere with peptide properties, thus biasing the results. The selection of the most suitable fluorophore is highly relevant. Here, a comprehensive study with blood-brain barrier (BBB) peptide shuttles (PepH3 and PepNeg) and antimicrobial peptides (AMPs) (vCPP2319 and Ctn[15-34]), tested as anticancer peptides (ACPs), having different fluorophores, namely 5(6)-carboxyfluorescein (CF), rhodamine B (RhB), quasar 570 (Q570), or tide fluor 3 (TF3) attached is presented. The goal is the evaluation of the impact of the selected fluorophores on peptide performance, applying routinely used techniques to assess cytotoxicity/toxicity, secondary structure, BBB translocation, and cellular internalization. Our results show that some fluorophores significantly modulate peptide activity when compared with unlabeled peptides, being more noticeable in hydrophobic and charged fluorophores. This study highlights the need for a careful experimental design for fluorescently labeled molecules, such as peptides.

Conjugation of Therapeutic PSD-95 Inhibitors to the Cell-Penetrating Peptide Tat Affects Blood-Brain Barrier Adherence, Uptake, and Permeation

Novel stroke therapies are needed. Inhibition of the interaction between the postsynaptic density-95 (PSD-95)/disc large/ZO-1 (PDZ) domains of PSD-95 and the N-methyl-D-aspartate (NMDA) receptor has been suggested as a strategy for relieving neuronal damage. The peptides NR2B9c and N-dimer have been designed to hinder this interaction; they are conjugated to the cell-penetrating peptide Tat to facilitate blood-brain barrier (BBB) permeation and neuronal uptake. Tat-N-dimer exhibits 1000-fold better target affinity than Tat-NR2B9c, but the same magnitude of improvement is not observed in terms of therapeutic effect. Differences in BBB permeation by Tat-NR2B9c and Tat-N-dimer may explain this difference, but studies providing a direct comparison of Tat-NR2B9c and Tat-N-dimer are lacking. The aim of the present study was therefore to compare the BBB uptake and permeation of Tat-NR2B9c and Tat-N-dimer. The peptides were conjugated to the fluorophore TAMRA and their chemical stability assessed. Endothelial membrane association and cell uptake, and transendothelial permeation were estimated using co-cultures of primary bovine brain capillary endothelial cells and rat astrocytes. In vivo BBB permeation was demonstrated in mice using two-photon microscopy imaging. Tissue distribution was evaluated in mice demonstrating brain accumulation of TAMRA-Tat (0.4% ID/g), TAMRA-Tat-NR2B9c (0.3% ID/g), and TAMRA-Tat-N-dimer (0.25% ID/g). In conclusion, we demonstrate that attachment of NR2B9c or N-dimer to Tat affects both the chemical stability and the ability of the resulting construct to interact with and permeate the BBB.

Quantitative measurement of cytosolic penetration using the chloroalkane penetration assay

A major barrier for drug development is ensuring molecules can access intracellular targets. This is especially true for biomolecules, which are notoriously difficult to deliver to the cytosol. Many current methods for measuring the internalization of therapeutic biomolecules are largely indirect and qualitative, and they do not offer information about subcellular localization. We recently reported a new assay, called the ChloroAlkane Penetration Assay (CAPA), that addresses some of the drawbacks of existing methods. CAPA is high-throughput, quantitative, and compartment-specific, and can be used to monitor cytosolic penetration over time and under a variety of culture conditions. We have used CAPA to investigate the cytosolic localization of peptides, proteins, and oligonucleotides. In this chapter, we discuss the materials, protocols, and troubleshooting necessary to perform CAPA and appropriately analyze the data. We end with a discussion about the applications and limitations of CAPA, and we speculate on the potential of the assay and its variations.

Fluorophore-Assisted Click Chemistry through Copper(I) Complexation

The copper-catalyzed alkyne-azide cycloaddition (CuAAC) is one of the most powerful chemical strategies for selective fluorescent labeling of biomolecules in in vitro or biological systems. In order to accelerate the ligation process and ensure efficient formation of conjugates under diluted conditions, external copper(I) ligands or sophisticated copper(I)-chelating azides are used. This latter strategy, however, increases the bulkiness of the triazole linkage, thus perturbing the biological function or dynamic behavior of the conjugates. In a proof-of-concept study, we investigated the use of an extremely compact fluorophore-based copper(I) chelating azide in order to accelerate the CuAAC with concomitant fluorescence labeling; in our strategy, the fluorophore is able to complex copper(I) species while retaining its photophysical properties. It is believed that this unprecedented approach which was applied for the labeling of a short peptide molecule and the fluorescent labeling of live cells, could be extended to other families of nitrogen-based fluorophores in order to tune both the reaction rate and photophysical characteristics.

EJP18 peptide derived from the juxtamembrane domain of epidermal growth factor receptor represents a novel membrane-active cell-penetrating peptide

Membrane-active peptides have been extensively studied to probe protein–membrane interactions, to act as antimicrobial agents and cell-penetrating peptides (CPPs) for the delivery of therapeutic agents to cells. Hundreds of membrane-active sequences acting as CPPs have now been described including bioportides that serve as single entity modifiers of cell physiology at the intracellular level. Translation of promising CPPs in pre-clinical studies have, however, been disappointing as only few identified delivery systems have progressed to clinical trials. To search for novel membrane-active peptides a sequence from the EGFR juxtamembrane region was identified (named EJP18), synthesised, and examined in its L- and D-form for its ability to mediate the delivery of a small fluorophore and whole proteins to cancer cell lines. Initial studies identified the peptide as being highly membrane-active causing extensive and rapid plasma membrane reorganisation, blebbing, and toxicity. At lower, non-toxic concentrations the peptides outperformed the well-characterised CPP octaarginine in cellular delivery capacity for a fluorophore or proteins that were associated with the peptide covalently or via ionic interactions. EJP18 thus represents a novel membrane-active peptide that may be used as a naturally derived model for biophysical protein–membrane interactions or for delivery of cargo into cells for therapeutic or diagnostic applications.