Deciphering Structural Determinants Distinguishing Active from Inactive Cell-Penetrating Peptides for Cytosolic mRNA Delivery

The formation of noncovalent complexes by mixing of positively charged polymers with negatively charged oligonucleotides (ONs) is a widely explored concept in nanomedicine to achieve cellular delivery of ONs. Uptake of ON complexes occurs through endocytosis, which then requires release of ON from endosomes. As one type of polymer, cell-penetrating peptides (CPPs) are being used which are peptides of about 8-30 amino acids in length. However, only a few CPPs yield effective cytosolic ON delivery and activity. Several strategies have been devised to increase cellular uptake and enhance endosomal release, among which an increase of osmotic pressure through the so-called proton sponge effect, disruption of membrane integrity through membrane activity, and disulfide-mediated polymerization. Here, we address the relevance of these concepts for mRNA delivery by incorporating structural features into the human lactoferrin-derived CPP, which shows uptake but not delivery. The incorporation of histidines was explored to address osmotic pressure and structural motifs of the delivery-active CPP PepFect14 (PF14) to address membrane disturbance, and finally, the impact of polymerization was explored. Whereas oligomerization increased the stability of polyplexes against heparin-induced decomplexation, neither this approach nor the incorporation of histidine residues to promote a proton-sponge effect yielded activity. Also, the replacement of arginine residues with lysine or ornithine residues, as in PF14, was without effect, even though all polyplexes showed cellular uptake. Ultimately, sufficient activity could only be achieved by transferring amphipathic sequence motifs from PF14 into the hLF context with some benefit of oligomerization demonstrating overarching principles of delivery for CPPs, lipid nanoparticles, and other types of delivery polymers.

Imaging of CPP Delivery Mechanisms of Oligonucleotides

Cationic cell-penetrating peptides spontaneously associate with negatively charged oligonucleotides to form submicron nanoparticles, so-called polyplexes. Contact with cells leads to endosomal uptake of these nanoparticles. Oligonucleotide activity critically depends on endosomal release and finally dissociation of polyplexes. Fluorescence provides a highly powerful means to follow the spatial dynamics of oligonucleotide uptake, trafficking and decomplexation, in particular when combined with markers of subcellular compartments that enable a quantitative analysis of colocalization and thereby mapping of trafficking routes. In this chapter, we describe protocols for a highly defined formation of polyplexes. We then point out the use of fluorescent fusion proteins to identify subcellular trafficking compartments and image analysis protocols to obtain quantitative information on trafficking routes and endosomal release.

Protein Expression Correlates Linearly with mRNA Dose over Up to Five Orders of Magnitude In Vitro and In Vivo

Messenger RNA is rapidly gaining significance as a therapeutic modality. Here, we address the dependence of dose-response functions on the type of delivery vehicle, cell line, and incubation time. Knowledge of these characteristics is crucial for the application of mRNA. As delivery vehicles, a lipid-based formulation and the cell-penetrating peptide Pepfect14 (PF14) were employed. As cell lines, we included a glomerular endothelial cell line (mGEnC) as a model for differentiated cells, HeLa cells, and SKOV-3 ovarian carcinoma cells. Uptake and expression were detected by flow cytometry, using a Cy5-labelled mRNA coding for enhanced green fluorescent protein (EGFP). There was a linear correlation of dose, uptake, and expression, and this correlation was maintained for over up to 72 h. Through application of a multistep kinetic model, we show that differences in expression levels can already be explained by the number of mRNAs packaged per delivery vehicle. Using luciferase as a reporter protein, linearity of expression was observed over 5 orders of magnitude in vitro and 3 orders of magnitude in vivo. Overall, the results demonstrate that mRNA provides excellent quantitative control over protein expression, also over extended periods of time.

Super-resolution Imaging of Structure, Molecular Composition, and Stability of Single Oligonucleotide Polyplexes

The successful application of gene therapy relies on the development of safe and efficient delivery vectors. Cationic polymers such as cell-penetrating peptides (CPPs) can condense genetic material into nanoscale particles, called polyplexes, and induce cellular uptake. With respect to this point, several aspects of the nanoscale structure of polyplexes have remained elusive because of the difficulty in visualizing the molecular arrangement of the two components with nanometer resolution. This limitation has hampered the rational design of polyplexes based on direct structural information. Here, we used super-resolution imaging to study the structure and molecular composition of individual CPP-mRNA polyplexes with nanometer accuracy. We use two-color direct stochastic optical reconstruction microscopy (dSTORM) to unveil the impact of peptide stoichiometry on polyplex structure and composition and to assess their destabilization in blood serum. Our method provides information about the size and composition of individual polyplexes, allowing the study of such properties on a single polyplex basis. Furthermore, the differences in stoichiometry readily explain the differences in cellular uptake behavior. Thus, quantitative dSTORM of polyplexes is complementary to the currently used characterization techniques for understanding the determinants of polyplex activity in vitro and inside cells.

Effects of a human recombinant alkaline phosphatase during impaired mitochondrial function in human renal proximal tubule epithelial cells

Sepsis-associated acute kidney injury is a multifactorial syndrome in which inflammation and renal microcirculatory dysfunction play a profound role. Subsequently, renal tubule mitochondria reprioritize cellular functions to prevent further damage. Here, we investigated the putative protective effects of human recombinant alkaline phosphatase (recAP) during inhibition of mitochondrial respiration in conditionally immortalized human proximal tubule epithelial cells (ciPTEC). Full inhibition of mitochondrial oxygen consumption was obtained after 24h antimycin A treatment, which did not affect cell viability. While recAP did not affect the antimycin A-induced decreased oxygen consumption and increased hypoxia-inducible factor-1α or adrenomedullin gene expression levels, the antimycin A-induced increase of pro-inflammatory cytokines IL-6 and IL-8 was attenuated. Antimycin A tended to induce the release of detrimental purines ATP and ADP, which reached statistical significance when antimycin A was co-incubated with lipopolysaccharide, and were completely converted into cytoprotective adenosine by recAP. As the adenosine A_2A receptor was up-regulated after antimycin A exposure, an adenosine A_2A receptor knockout ciPTEC cell line was generated in which recAP still provided protection. Together, recAP did not affect oxygen consumption but attenuated the inflammatory response during impaired mitochondrial function, an effect suggested to be mediated by dephosphorylating ATP and ADP into adenosine.

Molecular parameters of siRNA--cell penetrating peptide nanocomplexes for efficient cellular delivery

Cell-penetrating peptides (CPPs) are versatile tools for the intracellular delivery of various biomolecules, including siRNA. Recently, CPPs were introduced that showed greatly enhanced delivery efficiency. However, the molecular basis of this increased activity is poorly understood. Here, we performed a detailed analysis of the molecular and physicochemical properties of seven different siRNA-CPP nanoparticles. In addition, we determined which complexes are internalized most efficiently into the leukemia cell-line SKNO-1, and subsequently inhibited the expression of a luciferase reporter gene. We demonstrated effective complexation of siRNA for all tested CPPs, and optimal encapsulation of the siRNA was achieved at very similar molar ratios independent of peptide charge. However, CPPs with an extreme high or low overall charge proved to be exceptions, suggesting an optimal range of charge for CPP-siRNA nanoparticle formation based on opposite charge. The most active CPP (PepFect6) displayed high serum resistance but also high sensitivity to decomplexation by polyanionic macromolecules, indicating the necessity for partial decomplexation for efficient uptake. Surprisingly, CPP-siRNA complexes acquired a negative ζ-potential in the presence of serum. These novel insights shed light on the observation that cell association is necessary but not sufficient for activity and motivate new research into the nature of the nanoparticle-cell interaction. Overall, our results provide a comprehensive molecular basis for the further development of peptide-based oligonucleotide transfection agents.