Protein transport across membranes: Comparison between lysine and guanidinium-rich carriers

Cell-Penetrating Peptide-Mediated Biomolecule Transportation in Artificial Lipid Vesicles and Living Cells

Signal transduction and homeostasis are regulated by complex protein interactions in the intracellular environment. Therefore, the transportation of impermeable macromolecules (nucleic acids, proteins, and drugs) that control protein interactions is essential for modulating cell functions and therapeutic applications. However, macromolecule transportation across the cell membrane is not easy because the cell membrane separates the intra/extracellular environments, and the types of molecular transportation are regulated by membrane proteins. Cell-penetrating peptides (CPPs) are expected to be carriers for molecular transport. CPPs can transport macromolecules into cells through endocytosis and direct translocation. The transport mechanism remains largely unclear owing to several possibilities. In this review, we describe the methods for investigating CPP conformation, translocation, and cargo transportation using artificial membranes. We also investigated biomolecular transport across living cell membranes via CPPs. Subsequently, we show not only the biochemical applications but also the synthetic biological applications of CPPs. Finally, recent progress in biomolecule and nanoparticle transportation via CPPs into specific tissues is described from the viewpoint of drug delivery. This review provides the opportunity to discuss the mechanism of biomolecule transportation through these two platforms.

Ocular Delivery of Therapeutic Agents by Cell-Penetrating Peptides

Cell-penetrating peptides (CPPs) are short peptides with the ability to translocate through the cell membrane to facilitate their cellular uptake. CPPs can be used as drug-delivery systems for molecules that are difficult to uptake. Ocular drug delivery is challenging due to the structural and physiological complexity of the eye. CPPs may be tailored to overcome this challenge, facilitating cellular uptake and delivery to the targeted area. Retinal diseases occur at the posterior pole of the eye; thus, intravitreal injections are needed to deliver drugs at an effective concentration in situ. However, frequent injections have risks of causing vision-threatening complications. Recent investigations have focused on developing long-acting drugs and drug delivery systems to reduce the frequency of injections. In fact, conjugation with CPP could deliver FDA-approved drugs to the back of the eye, as seen by topical application in animal models. This review summarizes recent advances in CPPs, protein/peptide-based drugs for eye diseases, and the use of CPPs for drug delivery based on systematic searches in PubMed and clinical trials. We highlight targeted therapies and explore the potential of CPPs and peptide-based drugs for eye diseases.

Impact of Cyclization and Methylation on Peptide Penetration through Droplet Interface Bilayers

Cell-penetrating peptides enter cells via diverse mechanisms, such as endocytosis, active transport, or direct translocation. For the design of orally delivered cell-penetrating peptides, it is crucial to know the contribution of these different mechanisms. In particular, the ability of a peptide to translocate through a lipid bilayer remains a key parameter for the delivery of cargos. However, existing approaches used to assess translocation often provide discrepant results probably because they have different sensitivities to the distinct translocation mechanisms. Here, we focus on the passive permeation of a range of hydrophobic cyclic peptides inspired by somatostatin, a somatotropin release-inhibiting factor. Using droplet interface bilayers (DIB), we assess the passive membrane permeability of these peptides and study the impact of the peptide cyclization and backbone methylation on translocation rates. Cyclization systematically improved the permeability of the tested peptides while methylation did not. By studying the interaction of the peptides with the DIB interfaces, we found membrane insertion and peptide intrinsic diffusion to be two independent factors of permeability. Compared to the industrial gold standard Caco-2 and parallel artificial membrane permeability assay (PAMPA) models, DIBs provide intermediate membrane permeability values, closer to Caco-2. Even for conditions where Caco-2 and PAMPA are discrepant, the DIB approach also gives results closer to Caco-2. Thereupon, DIBs represent a robust alternative to the PAMPA approach for predicting the permeability of peptides, even if the latter present extremely small structural differences.

Effect of Fe3+ on the sludge properties and microbial community structure in a lab-scale A2O process

During biological wastewater treatment, ferric salt (Fe³⁺) usually serves as an inorganic flocculant to improve the agglomeration and sedimentation of suspended solids, and thus the removal efficiency of pollutants to meet the increasing strictly regulated wastewater discharge standards. In this study, we investigated the effects of Fe³⁺ on the removal efficiencies of pollutants, sludge properties, dominant flora and metabolic pathways of bacterial community in a classical anaerobic-anoxic-oxic (A²O) process. The results showed that a Fe³⁺ concentration lower than 10 mg·L^-1 could improve the removal efficiencies of chemical oxygen demand (COD) and total nitrogen (TN), while an inhibition effect was exerted at concentration higher than 10 mg·L^-1. The maximum removal efficiencies of COD and TN were 97% and 89%, respectively, under the critical Fe³⁺ concentration of 10 mg·L^-1. Total phosphorous (TP) removal was constantly positively correlated with Fe³⁺ concentration, due to the enhanced adsorption of phosphorus on activated sludge with the increase of surface roughness. Thauera displayed the highest relative abundance, and certain bacteria in Proteobacteria, Dehloromonas and Candidatus-Competibacter exhibited good adaptability to high concentration of Fe³⁺. In the context of metabolic collaterals, the most abundant functional gene families were identified to be Carbohydrate Metabolism, Amino Acid Metabolism, Cell Motility, Membrane Transport, and Replication and Repair. This study provides an extensive mechanistic insight into the impact of Fe³⁺ on the A²O process, which is of fundamental significance to exploit the contributions of inorganic salts to biological wastewater treatment.

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.

Simultaneous Monitoring of Multi-Enzyme Activity and Concentration in Tumor Using a Triply Labeled Fluorescent In Vivo Imaging Probe

The use of fluorescent imaging probes that monitor the activity of proteases that experience an increase in expression and activity in tumors is well established. These probes can be conjugated to nanoparticles of iron oxide, creating a multimodal probe serving as both a magnetic resonance imaging (MRI) agent and an indicator of local protease activity. Previous works describe probes for cathepsin D (CatD) and metalloproteinase-2 (MMP2) protease activity grafted to cross-linked iron oxide nanoparticles (CLIO). Herein, we have synthesized a triply labeled fluorescent iron oxide nanoparticle molecular imaging (MI) probe, including an AF750 substrate concentration reporter along with probes for cathepsin B (CatB) sand MMP2 protease activity. The reporter provides a baseline signal from which to compare the activity of the two proteases. The activity of the MI probe was verified through incubation with the proteases and tested in vitro using the human HT29 tumor cell line and in vivo using female nude mice injected with HT29 cells. We found the MI probe had the appropriate specificity to the activity of their respective proteases, and the reporter dye did not activate when incubated in the presence of only MMP2 and CatB. Probe fluorescent activity was confirmed in vitro, and reporter signal activation was also noted. The fluorescent activity was also visible in vivo, with injected HT29 cells exhibiting fluorescence, distinguishing them from the rest of the animal. The reporter signal was also observable in vivo, which allowed the signal intensities of the protease probes to be corrected; this is a unique feature of this MI probe design.

Cell penetrating peptides in ocular drug delivery: State of the art

Despite the increasing number of effective therapeutics for eye diseases, their treatment is still challenging due to the presence of effective barriers protecting eye tissues. Cell Penetrating Peptides (CPPs), synthetic and natural short amino acid sequences able to cross cellular membrane thanks to a transduction domain, have been proposed as possible enhancing strategies for ophthalmic delivery. In this review, a general description of CPPs classes, design approaches and proposed cellular uptake mechanisms will be provided to the reader as an introduction to ocular CPPs application, together with an overview of the main problems related to ocular administration. The results obtained with CPPs for the treatment of anterior and posterior segment eye diseases will be then introduced, with a focus on non-invasive or minimally invasive administration, shifting from CPPs capability to obtain intracellular delivery to their ability to cross biological barriers. The problems related to in vitro, ex vivo and in vivo models used to investigate CPPs mediated ocular delivery will be also addressed together with potential ocular toxicity issues.

Reconstitution and functional studies of hamster P-glycoprotein in giant liposomes

This paper describes the preparation of giant unilamellar vesicles with reconstituted hamster P-glycoprotein (Pgp, ABCB1) for studying the transport activity of this efflux pump in individual liposomes using optical microscopy. Pgp, a member of ABC (ATP-binding cassette) transporter family, is known to contribute to the cellular multidrug resistance (MDR) against variety of drugs. The efficacy of many therapeutics is, thus, hampered by this efflux pump, leading to a high demand for simple and effective strategies to monitor the interactions of candidate drugs with this protein. Here, we applied small Pgp proteoliposomes to prepare giant Pgp-bearing liposomes via modified electroformation techniques. The presence of Pgp in the membrane of giant proteoliposomes was confirmed using immunohistochemistry. Assessment of Pgp ATPase activity suggested that this transporter retained its activity upon reconstitution into giant liposomes, with an ATPase specific activity of 439 ± 103 nmol/mg protein/min. For further confirmation, we assessed the transport activity of Pgp in these proteoliposomes by monitoring the translocation of rhodamine 123 (Rho123) across the membrane using confocal microscopy at various ATP concentrations (0-2 mM) and in the presence of Pgp inhibitors. Rate of change in Rho123 concentration inside the liposomal lumen was used to estimate the Rho123 transport rates (1/s) for various ATP concentrations, which were then applied to retrieve the Michaelis-Menten constant (Km) of ATP in Rho123 transport (0.42 ± 0.75 mM). Similarly, inhibitory effects of verapamil, colchicine, and cyclosporin A on Pgp were studied in this system and the IC50 values for these Pgp inhibitors were found 26.6 ± 6.1 μM, 94.6 ± 47.6 μM, and 0.21 ± 0.07 μM, respectively. We further analyzed the transport data using a kinetic model that enabled dissecting the passive diffusion of Rho123 from its Pgp-mediated transport across the membrane. Based on this model, the permeability coefficient of Rho123 across the liposomal membrane was approximately 1.25×10-7 cm/s. Comparing the membrane permeability in liposomes with and without Pgp revealed that the presence of this protein did not have a significant impact on membrane integrity and permeability. Furthermore, we used this model to obtain transport rate constants for the Pgp-mediated transport of Rho123 (m3/mol/s) at various ATP and inhibitor concentrations, which were then applied to estimate values of 0.53 ± 0.66 mM for Km of ATP and 25.2 ± 5.0 μM for verapamil IC50, 61.8 ± 34.8 μM for colchicine IC50, and 0.23 ± 0.09 μM for cyclosporin A IC50. The kinetic parameters obtained from the two analyses were comparable, suggesting a minimal contribution from the passive Rho123 diffusion across the membrane. This approach may, therefore, be applied for screening the transport activity of Pgp against potential drug candidates.

Functional aqueous droplet networks

Droplet interface bilayers (DIBs), comprising individual lipid bilayers between pairs of aqueous droplets in an oil, are proving to be a useful tool for studying membrane proteins. Recently, attention has turned to the elaboration of networks of aqueous droplets, connected through functionalized interface bilayers, with collective properties unachievable in droplet pairs. Small 2D collections of droplets have been formed into soft biodevices, which can act as electronic components, light-sensors and batteries. A substantial breakthrough has been the development of a droplet printer, which can create patterned 3D droplet networks of hundreds to thousands of connected droplets. The 3D networks can change shape, or carry electrical signals through defined pathways, or express proteins in response to patterned illumination. We envisage using functional 3D droplet networks as autonomous synthetic tissues or coupling them with cells to repair or enhance the properties of living tissues.

Cell-Penetrating Peptides: Design Strategies beyond Primary Structure and Amphipathicity

Efficient intracellular drug delivery and target specificity are often hampered by the presence of biological barriers. Thus, compounds that efficiently cross cell membranes are the key to improving the therapeutic value and on-target specificity of non-permeable drugs. The discovery of cell-penetrating peptides (CPPs) and the early design approaches through mimicking the natural penetration domains used by viruses have led to greater efficiency of intracellular delivery. Following these nature-inspired examples, a number of rationally designed CPPs has been developed. In this review, a variety of CPP designs will be described, including linear and flexible, positively charged and often amphipathic CPPs, and more rigid versions comprising cyclic, stapled, or dimeric and/or multivalent, self-assembled peptides or peptido-mimetics. The application of distinct design strategies to known physico-chemical properties of CPPs offers the opportunity to improve their penetration efficiency and/or internalization kinetics. This led to increased design complexity of new CPPs that does not always result in greater CPP activity. Therefore, the transition of CPPs to a clinical setting remains a challenge also due to the concomitant involvement of various internalization routes and heterogeneity of cells used in the in vitro studies.