Metabolic energy-independent mechanism of internalization for the cell penetrating peptide penetratin

A universal method to analyze cellular internalization mechanisms via endocytosis without non-specific cross-effects

Endocytosis is an essential biological process for nutrient absorption and intercellular communication; it can also be used to accelerate the cellular internalization of drug delivery carriers. Clarifying the cellular uptake mechanisms of unidentified endogenous and exogenous molecules and designing new effective drug delivery systems require an accurate, specific endocytosis analysis methodology. Therefore, we developed a method to specifically evaluate cellular internalization via three main endocytic pathways: clathrin- and caveolae-mediated endocytosis, and macropinocytosis. We first revealed that most known endocytosis inhibitors had no specific inhibitory effect or were cytotoxic. Second, we successfully established an alternative method using small interfering RNA to knock down dynamin-2 and caveolin-1, which are necessary for clathrin- and caveolae-mediated endocytosis, in HeLa cells. Third, we established another method to specifically analyze macropinocytosis using rottlerin on A431 cells. Finally, we validated the proposed methods by testing the cellular internalization of a biological molecule (insulin) and carriers (nanoparticles and cell-penetrating peptides). Through this study, we established versatile methods to precisely and specifically evaluate endocytosis of newly developed biopharmaceuticals or drug delivery systems.

Role of PI(4,5)P2 and Cholesterol in Unconventional Protein Secretion

Besides its protective role in the maintenance of cell homeostasis, the plasma membrane is the site of exchanges between the cell interior and the extracellular medium. To circumvent the hydrophobic barrier formed by the acyl chains of the lipid bilayer, protein channels and transporters are key players in the exchange of small hydrophilic compounds such as ions or nutrients, but they hardly account for the transport of larger biological molecules. Exchange of proteins usually relies on membrane-fusion events between vesicles and the plasma membrane. In recent years, several alternative unconventional protein secretion (UPS) pathways across the plasma membrane have been characterised for a specific set of secreted substrates, some of them excluding any membrane-fusion events (Dimou and Nickel, Curr Biol 28:R406-R410, 2018). One of thesbe pathways, referred as type I UPS, relies on the direct translocation of the protein across the plasma membrane and not surprisingly, lipids are essential players in this process. In this chapter, we discuss the roles of phosphatidylinositol(4,5)bisphosphate (PI(4,5)P2) and cholesterol in unconventional pathways involving Engrailed-2 homeoprotein and fibroblast growth factor 2.

The Membrane Activity of the Amphibian Temporin B Peptide Analog TB_KKG6K Sheds Light on the Mechanism That Kills Candida albicans

Temporin B (TB) is a 13-amino-acid-long, cationic peptide secreted by the granular glands of the European frog Rana temporaria. We recently showed that the modified TB peptide analog TB_KKG6K rapidly killed planktonic and sessile Candida albicans at low micromolar concentrations and was neither hemolytic nor cytotoxic to mammalian cells in vitro. The present study aimed to shed light into its mechanism of action, with a focus on its fungal cell membrane activity. We utilized different fluorescent dyes to prove that it rapidly induces membrane depolarization and permeabilization. Studies on model membrane systems revealed that the TB analog undergoes hydrophobic and electrostatic membrane interactions, showing a preference for anionic lipids, and identified phosphatidylinositol and cardiolipin as possible peptide targets. Fluorescence microscopy using fluorescein isothiocyanate-labeled TB_KKG6K in the presence of the lipophilic dye FM4-64 indicated that the peptide compromises membrane integrity and rapidly enters C. albicans cells in an energy-independent manner. Peptide-treated cells analyzed by cryo-based electron microscopy exhibited no signs of cell lysis; however, subcellular structures had disintegrated, suggesting that intracellular activity may form part of the killing mechanism of the peptide. Taken together, this study proved that TB_KKG6K compromises C. albicans membrane function, which explains the previously observed rapid, fungicidal mode of action and supports its great potential as a future anti-Candida therapeutic. IMPORTANCE Fungal infections with the opportunistic human pathogen C. albicans are associated with high mortality rates in immunocompromised patients. This is partly due to the yeast's ability to rapidly develop resistance toward currently available antifungals. Small, cationic, membrane-active peptides are promising compounds to fight against resistance development, as many of them effectuate rapid fungal cell death. This fast killing is believed to hamper the development of resistance, as the fungi do not have sufficient time to adapt to the antifungal compound. We previously reported that the synthetic variant of the amphibian TB peptide, TB_KKG6K, rapidly kills C. albicans. In the current study, the mechanism of action of the TB analog was investigated. We show that this TB analog is membrane-active and impairs cell membrane function, highlighting its potential to be developed as an attractive alternative anti-C. albicans therapeutic that may hinder the development of resistance.

Sterolight as imaging tool to study sterol uptake, trafficking and efflux in living cells

Information about cholesterol subcellular localization and transport pathways inside cells is essential for understanding and treatment of cholesterol-related diseases. However, there is a lack of reliable tools to monitor it. This work follows the fate of Sterolight, a BODIPY-labelled sterol, within the cell and demonstrates it as a suitable probe for visualization of sterol/lipid trafficking. Sterolight enters cells through an energy-independent process and knockdown experiments suggest caveolin-1 as its potential cellular carrier. Intracellular transport of Sterolight is a rapid process, and transfer from ER and mitochondria to lysosomes and later to lipid droplets requires the participation of active microtubules, as it can be inhibited by the microtubule disruptor nocodazole. Excess of the probe is actively exported from cells, in addition to being stored in lipid droplets, to re-establish the sterol balance. Efflux occurs through a mechanism requiring energy and may be selectively poisoned with verapamil or blocked in cells with mutated cholesterol transporter NPC1. Sterolight is efficiently transferred within and between different cell populations, making it suitable for monitoring numerous aspects of sterol biology, including the live tracking and visualization of intracellular and intercellular transport.

How Do Biomolecules Cross the Cell Membrane?

Biomolecules such as peptides, proteins, and nucleic acids generally cannot cross a cell membrane by passive diffusion. Nevertheless, cell-penetrating peptides (CPPs), bacterial protein toxins, certain eukaryotic proteins, viruses, and many synthetic drug delivery vehicles have been shown to enter the cytosol of eukaryotic cells with varying efficiencies. They generally enter the cell by one or more of the endocytic mechanisms and are initially localized inside the endosomes. But how they cross the endosomal membrane to reach the cytosol (i.e., endosomal escape) has been a mystery for decades, and this knowledge gap has been a major bottleneck for the development of efficient drug delivery systems. In addition, many bacterial and eukaryotic proteins are transported across the plasma membrane in their native states into the periplasmic/extracellular space through the twin-arginine translocation (TAT) and unconventional protein secretion (UPS) systems, respectively. Again, the mechanisms underpinning these protein export systems remain unclear.In this Account, I introduce a previously unrecognized, fundamental membrane translocation mechanism which we have termed the vesicle budding-and-collapse (VBC) mechanism. Through VBC, biomolecules of diverse sizes and physicochemical properties autonomously translocate across cell membranes topologically (i.e., from one side to the other side of the membrane) but not physically (i.e., without going through the membrane). We have demonstrated that CPPs and bacterial protein toxins escape the endosome by the VBC mechanism in giant unilamellar vesicles as well as live mammalian cells. This advance resulted from studies in which we labeled the biomolecules with a pH-sensitive, red-colored dye (pHAb) and phosphatidylserine with a pH-insensitive green dye (TopFluor) and monitored the intracellular trafficking of the biomolecules in real time by confocal microscopy. In addition, by enlarging the endosomes with a kinase inhibitor, we were able to visualize the structural changes of the endosomes (i.e., endosomal escape intermediates) as they went through the VBC process. I postulate that bacterial/viral/eukaryotic proteins, nonenveloped viruses, and synthetic drug delivery vehicles (e.g., polyplexes, lipoplexes, and lipid nanoparticles) may also escape the endosome by inducing VBC. Furthermore, I propose that VBC may be the mechanism that drives the bacterial TAT and eukaryotic UPS systems. Our findings fill a long-standing gap in cell biology and provide guiding principles for designing more efficient drug delivery vehicles.

Extracellular Secretagogin is internalized into the cells through endocytosis

Secretagogin (SCGN) is a calcium-sensor protein with a regulatory role in glucose metabolism and the secretion of several peptide hormones. Many, but not all, functions of SCGN can be explained by its intracellular manifestation. Despite early data on SCGN secretion, the secretory mechanism, biological fate, physiological implications, and trans-cellular signaling of extracellular SCGN remain unknown. We here report that extracellular SCGN is readily internalized into the C2C12 cells in an energy-dependent manner. Using endocytosis inhibitors, we demonstrate that SCGN internalizes via clathrin-mediated endocytosis, following which, SCGN localizes largely in the cytosol. Exogenous SCGN treatment induces a global proteomic reprogramming in C2C12 cells. Gene ontology search suggests that SCGN-induced proteomic reprogramming favors protein synthesis and cellular growth. We thus validated the cell proliferative action of SCGN using C2C12, HepG2, and NIH-3T3 cell lines. Based on the data, we propose that circulatory SCGN is internalized into the target cells and modulates the expression of cell growth-related proteins. The work suggests that extracellular SCGN is a functional protein, which communicates with specific cell types and directly modulates cell proliferation.

Surface functionalization of zeolite-based drug delivery systems enhances their antitumoral activity in vivo

Zeolites have attractive features making them suitable carriers for drug delivery systems (DDS). As such, we loaded the anticancer drug 5-fluorouracil (5-FU), into two different zeolite structures, faujasite (NaY) and Linde Type L (LTL), to obtain different DDS. The prepared DDS were tested in vitro using breast cancer, colorectal carcinoma, and melanoma cell lines and in vivo using the chick embryo chorioallantoic membrane model (CAM). Both assays showed the best results for the Hs578T breast cancer cells, with a higher potentiation for 5-FU encapsulated in the zeolite LTL. To unveil the endocytic mechanisms involved in the internalization of the zeolite nanoparticles, endocytosis was inhibited pharmacologically in breast cancer and epithelial mammary human cells. The results suggest that a caveolin-mediated process was responsible for the internalized zeolite nanoparticles. Aiming to boost the DDS efficacy, the disc-shaped zeolite LTL outer surface was functionalized using amino (NH₂) or carboxylic acid (COOH) groups and coated with poly-l-lysine (PLL). Positively functionalized surface LTL nanoparticles revealed to be non-toxic to human cells and, importantly, their internalization was faster and led to a higher tumor reduction in vivo. Overall, our results provide further insights into the mechanisms of interaction between zeolite-based DDS and cancer cells, and pave the way for future studies aiming to improve DDS anticancer activity.

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 novel progress of drug delivery system for organelle targeting in tumour cells

At present, malignant tumours have become one of the most serious diseases that endanger human health. According to a survey on causes of death in Chinese population in early 1990s, the malignant tumours were the second leading cause of death. In the treatment of tumours, the ideal situation is that drugs should target and accumulate at tumour sites and destroy tumour cells specifically, without affecting normal cells and stem cells with regenerative capacity. This requires drugs to be specifically transported to the target organs, tissues, cells, and even specific organelles, like mitochondria, nuclei, lysosomes, endoplasmic reticulum (ER), and Golgi apparatus (GA). The nano drug delivery system can not only protect drugs from degradation but also facilitate functional modification and targeted drug delivery to the tumour site. This article mainly reviews the targeting of nano drug delivery systems to tumour cytoplasmic matrix, nucleus, mitochondria, ER, and lysosomes. Organelle-specific drug delivery system will be a major mean of targeting drug delivery with lower toxicity, less dosage and higher drug concentration in tumour cells.

Cucurbit[7]uril-conjugated dyes as live cell imaging probes: investigation on their cellular uptake and excretion pathways

Here we report the endocytosis and excretion pathways of two different dye-conjugated cucurbit[7]urils, (cyanine 3-conjugated CB[7] and rhodamine X-conjugated CB[7]), which have great potential as molecular probes for live cell imaging. The dye-CB[7]s are translocated into live cells (human breast carcinoma cells, MCF-7) via multiple pathways, predominantly by clathrin-mediated endocytosis, and excreted from cells via lysosome-associated exocytosis. Interestingly, the CB[7] moiety has a substantial influence on the uptake and excretion pathways. These findings may widen the applications of the dyes conjugated to CB[7] and assist in the design of new molecular probes for live cell imaging.

Annexin A2 expression and partners during epithelial cell differentiation

The members of the annexin family of calcium- and phospholipid-binding proteins participate in different cellular processes. Annexin A2 binds to S100A10, forming a functional heterotetrameric protein that has been involved in many cellular functions, such as exocytosis, endocytosis, cell junction formation, and actin cytoskeleton dynamics. Herein, we studied annexin A2 cellular movements and looked for its partners during epithelial cell differentiation. By using immunofluorescence, mass spectrometry (MS), and western blot analyses after S100A10 affinity column separation, we identified several annexin A2–S100A10 partner candidates. The association of putative annexin A2–S100A10 partner candidates obtained by MS after column affinity was validated by immunofluorescence and sucrose density gradient separation. The results show that three proteins are clearly associated with annexin A2: E-cadherin, actin, and caveolin 1. Overall, the data show that annexin A2 can associate with molecular complexes containing actin, caveolin 1, and flotillin 2 before epithelial differentiation and with complexes containing E-cadherin, actin, and caveolin 1, but not flotillin 2 after cell differentiation. The results indicate that actin, caveolin 1, and E-cadherin are the principal protein partners of annexin A2 in epithelial cells and that the serine phosphorylation of the N-terminal domain does not play an essential role during epithelial cell differentiation.

Cholesterol re-organisation and lipid de-packing by arginine-rich cell penetrating peptides: Role in membrane translocation

Cell penetrating peptides (CPPs) are able to transport hydrophilic molecules inside cells. To reach the cytosol, the peptide associated with a cargo must cross the plasma or the endosomal membrane. Different molecular mechanisms for peptide internalisation into cells have been proposed and it is becoming clear that the cellular internalisation mechanisms are different depending on the peptide sequence and structure and the target membrane. Herein, the penetration of three peptides into large unilamellar vesicles were studied: the homeodomain derived 16-residues penetratin, nona-arginine (R9), and a small peptide containing 6 arginine and 3 tryptophan residues (RW9). The membrane models were composed of phospholipids from natural sources containing different molecular species. We observed that among the three peptides, only the amphipathic peptide RW9 was able to cross the membrane vesicles in the liquid disordered state. The changes in the distribution of the previously characterized cholesterol-pyrene probe show that cholesterol-pyrene molecules dissociate from clusters upon membrane interaction with the three peptides and that the cholesterol environment becomes more disordered in the presence of RW9. Finally, we studied the effect of the peptides on lipid ordering on giant plasma membrane vesicles. The amphipathic peptides RW9 and its longer homologue RW16 induced lipid de-packing in plasma membrane vesicles. Overall, the data suggest that a disordered membrane favours the translocation of RW9, that the membrane cholesterol is redistributed during peptide interaction, and that the peptide amphipathic character is important to increase membrane fluidity and peptide membrane translocation.

Membrane potential drives direct translocation of cell-penetrating peptides

Cell-penetrating peptides (CPPs) are frequently employed as drug delivery agents with rapid cellular uptake, however, the uptake mechanism and the detailed translocation pathway are at present not completely understood. Both endocytosis and direct translocation through membrane pores have been observed in experiments and simulations under different conditions. Here we report the molecular dynamics simulations providing evidence for the direct translocation of CPPs across the membrane driven by the membrane electrostatic potential. The local membrane potential can be produced by the ion concentration imbalance across the membrane, which is ubiquitous in biological environments. Moreover, if positively charged CPPs are adsorbed on the membrane, this further enhances the membrane potential, opening membrane pores through which CPPs can be instantly transported in a chain-like configuration. The classical nucleation theory is applied to estimate the translocation time by calculating the changes in the free energy upon transferring CPPs across the membrane at different potentials, showing good agreement with available experimental measurements. The revealed CPP translocation mechanism can be broadly relevant for cellular processes in biology.

DNA-histone complexes as ligands amplify cell penetration and nuclear targeting of anti-DNA antibodies via energy-independent mechanisms

We have generated three monoclonal cell-penetrating antibodies (CPAbs) from a non-immunized lupus-prone (NZB × NZW)F1 mouse that exhibited high anti-DNA serum titres. These CPAbs are polyreactive because they bind to DNA and other cellular components, and localize mainly in the nucleus of HeLa cells, albeit with a distinct nuclear labelling profile. Herein, we have examined whether DNA-histone complexes (DHC) binding to CPAbs, before cell entry, could modify the cell penetration of CPAbs or their nuclear staining properties. By applying confocal microscopy and image analysis, we found that extracellular binding of purified CPAbs to DHC significantly enhanced their subsequent cell-entry, both in terms of percentages of positively labelled cells and fluorescence intensity (internalized CPAb amount), whereas there was a variable effect on their nuclear staining profile. Internalization of CPAbs, either alone or bound to DHC, remained unaltered after the addition of endocytosis-specific inhibitors at 37° or assay performance at 4°, suggesting the involvement of energy-independent mechanisms in the internalization process. These findings assign to CPAbs a more complex pathogenetic role in systemic lupus erythematosus where both CPAbs and nuclear components are abundant.

Interaction of amphiphilic α-helical cell-penetrating peptides with heparan sulfate

Cell-penetrating peptides (CPPs) are able to be taken up by cells and can deliver macromolecular cargos. However, the mechanism of this internalization is not yet fully understood. Recent theories suggest that the binding of cationic CPPs to negatively charged extracellular glycosaminoglycans, such as heparan sulfate (HS), is a possible mechanism of cellular uptake (CU). Our group has screened the CU activities of 54 systematically designed amphiphilic α-helical peptides in HeLa cells. Notably, a mutation in even a single residue significantly alters the CU ability of a peptide. To determine the structure-CU activity relationship of CPPs, four peptides, which contain a difference in one or two amino acids (i.e., Arg/Glu and Ala/Phe), were chosen from our CPP library to examine their interactions with HS. Fluorescence spectroscopy, isothermal titration calorimetry (ITC) and dynamic light scattering analysis indicated that the HS-binding affinities and HS-clustering abilities of the four CPPs correlated well with their CU activities in HeLa and A549 cells. The heat capacities of the CPPs, determined using ITC and binding free energy decomposition analyses in molecular dynamics simulations, revealed that electrostatic interactions were more dominant in the HS-binding processes of Arg-containing peptides in comparison to Glu-containing peptides, whereas hydrophobic contributions were the primary mode of interaction of Phe-containing peptides in comparison to Ala-containing peptides. Furthermore, it was implied that hydrophobic interactions may be more favourable than electrostatic interactions during the CU process.

Rapid Endolysosomal Escape and Controlled Intracellular Trafficking of Cell Surface Mimetic Quantum-Dots-Anchored Peptides and Glycopeptides

A novel strategy for the development of a high performance nanoparticules platform was established by means of cell surface mimetic quantum-dots (QDs)-anchored peptides/glycopeptides, which was developed as a model system for nanoparticle-based drug delivery (NDD) vehicles with defined functions helping the specific intracellular trafficking after initial endocytosis. In this paper, we proposed a standardized protocol for the preparation of multifunctional QDs that allows for efficient cellular uptake and rapid escaping from the endolysosomal system and subsequent cytoplasmic molecular delivery to the target cellular compartment. Chemoselective ligation of the ketone-functionalized hexahistidine derivative facilitated both efficient endocytic entry and rapid endolysosomal escape of the aminooxy/phosphorylcholine self-assembled monolayer-coated QDs (AO/PCSAM-QDs) to the cytosol in various cell lines such as human normal and cancer cells, while modifications of these QDs with cell-penetrating arginine-rich peptides showed poor cellular uptake and induced self-aggregation of AO/PCSAM-QDs. Combined use of hexahistidylated AO/PCSAM-QDs with serglycine-like glycopeptides, namely synthetic proteoglycan initiators (PGIs), elicited the entry and controlled intracellular trafficking, Golgi localization, and also excretion of these nanoparticles, which suggested that the present approach would provide an ideal platform for the design of high performance NDD systems.

Enabling cytoplasmic delivery and organelle targeting by surface modification of nanocarriers

Nanocarriers are designed to specifically accumulate in diseased tissues. In this context, targeting of intracellular compartments was shown to enhance the efficacy of many drugs and to offer new and more effective therapeutic approaches. This is especially true for therapies based on biologicals that must be encapsulated to favor cell internalization, and to avoid intracellular endosomal sequestration and degradation of the payload. In this review, we discuss specific surface modifications designed to achieve cell cytoplasm delivery and to improve targeting of major organelles; we also discuss the therapeutic applications of these approaches. Last, we describe some integrated strategies designed to sequentially overcome the biological barriers that separate the site of administration from the cell cytoplasm, which is the drug's site of action.

Cell Penetrating Peptide Adsorption on Magnetite and Silica Surfaces: A Computational Investigation

Magnetic nanoparticles (MNPs) represent one of the most promising materials as they can act as a versatile platform in the field of bionanotechnology for enhanced imaging, diagnosis, and treatment of various diseases. Silica is the most common compound for preparing coated iron oxide NPs since it improves colloidal stability and the binding affinity for various organic molecules. Biomolecules such as cell penetrating peptides (CPPs) might be employed to decorate MNPs, combining their promising physicochemical properties with a cell penetrating ability. In this work, a computational investigation on adsorption of Antennapedia homeodomain-derived penetrating peptide (pAntp) on silica and magnetite (MAG) surfaces is presented. By employing umbrella sampling molecular dynamics, we provided a quantitative estimation of the pAntp-surface adsorption free energy to highlight the influence of surface hydroxylation state on the adsorption mechanism. The interaction between peptide and surface has shown to be mainly driven by electrostatics. In case of MAG surface, also an important contribution of van der Waals (VdW) attraction was observed. Our data suggest that a competitive mechanism between MNPs and cell membrane might partially inhibit the CPP to carry out its membrane penetrating function.

Peptide-mediated delivery: an overview of pathways for efficient internalization

Poor cellular delivery and low bioavailability of novel potent therapeutic molecules continue to remain the bottleneck of modern cancer and gene therapy. Cell-penetrating peptides have provided immense opportunities for the intracellular delivery of bioactive cargos and have led to the first exciting successes in experimental therapy of muscular dystrophies. This review focuses on the mechanisms by which cell-penetrating peptides gain access to the cell interior and deliver cargos. Recent advances in augmenting delivery efficacy and facilitation of endosomal escape of cargo are presented, and the cell-penetrating peptide-mediated delivery of two of the most popular classes of cargo molecules, oligonucleotides and proteins, is analyzed. The arsenal of tools for oligonucleotide delivery has dramatically expanded in the last decade enabling harnessing of cell-surface receptors for targeted delivery.

Intake of silica nanoparticles by giant lipid vesicles: influence of particle size and thermodynamic membrane state

The uptake of nanoparticles into cells often involves their engulfment by the plasma membrane and a fission of the latter. Understanding the physical mechanisms underlying these uptake processes may be achieved by the investigation of simple model systems that can be compared to theoretical models. Here, we present experiments on a massive uptake of silica nanoparticles by giant unilamellar lipid vesicles (GUVs). We find that this uptake process depends on the size of the particles as well as on the thermodynamic state of the lipid membrane. Our findings are discussed in the light of several theoretical models and indicate that these models have to be extended in order to capture the interaction between nanomaterials and biological membranes correctly.

Organelle targeting: third level of drug targeting

Drug discovery and drug delivery are two main aspects for treatment of a variety of disorders. However, the real bottleneck associated with systemic drug administration is the lack of target-specific affinity toward a pathological site, resulting in systemic toxicity and innumerable other side effects as well as higher dosage requirement for efficacy. An attractive strategy to increase the therapeutic index of a drug is to specifically deliver the therapeutic molecule in its active form, not only into target tissue, nor even to target cells, but more importantly, into the targeted organelle, ie, to its intracellular therapeutic active site. This would ensure improved efficacy and minimize toxicity. Cancer chemotherapy today faces the major challenge of delivering chemotherapeutic drugs exclusively to tumor cells, while sparing normal proliferating cells. Nanoparticles play a crucial role by acting as a vehicle for delivery of drugs to target sites inside tumor cells. In this review, we spotlight active and passive targeting, followed by discussion of the importance of targeting to specific cell organelles and the potential role of cell-penetrating peptides. Finally, the discussion will address the strategies for drug/DNA targeting to lysosomes, mitochondria, nuclei and Golgi/endoplasmic reticulum.

Therapeutic potential of cell-penetrating peptides

The ability of cell-penetrating peptides to cross plasma membranes has been used for various applications, including the delivery of bioactive molecules to inhibit disease-producing cellular mechanisms. Selective drug delivery into target cells improves drug distribution and decreases dosing and toxicity. In this review, the authors outline the main challenges in the field, namely clarification of mechanisms of entry into cells, as well as current and future perspectives regarding cell-penetrating peptides application for human therapeutics. Here, the authors discuss some of the factors that influence efficacy of delivery and review the current status of preclinical studies and clinical trials involving the use of cell-penetrating peptide-mediated delivery of therapeutics.

Early stages of interactions of cell-penetrating peptide penetratin with a DPPC bilayer

Cell-penetrating-peptides (CPPs) can deliver themselves together with a macromolecular cargo into cells and, hence, have promising applications in drug delivery. The detailed physical mechanisms that underlie and determine their cellular uptake remain unknown. We used molecular dynamics (MD) simulations to study the interaction of a well-known CPP, namely penetratin, with a zwitterionic di-palmitoyl-phosphatidyl-choline (DPPC) bilayer. Our study shows that the arginine and lysine residues play a crucial role in peptide-membrane binding through charge-pair and hydrogen bond interactions. We also characterize peptide conformation and show that it remains helical near the N-terminus but can fold into a variety of other conformations in the residues close to the C-terminus. The response of membrane to the peptide is also investigated.