Heat shock protein 70 kDa (HSP70) is a major protein family in the cell protections against stress-induced denaturation and aggregation and in the folding of nascent proteins. It is a highly conserved protein that can be found in most organisms and is strongly connected to several intracellular pathways such as protein folding and refolding, protein degradation and regulation, and protection against intense stress. Cellular delivery of HSP70 would be of high impact for clarification of its role in these cellular processes.

PepFect14 is a cell-penetrating peptide known to be able to mediate the transfection of various oligonucleotides to multiple cell lines with a higher efficacy than most commercially available transfection agents and without inducing significant toxic effects.

In this study we demonstrated that PepFect14 was able to form a complex with HSP70 and to deliver it inside cells in the same fashion with oligonucleotide delivery. The delivered HSP70 showed an effect in the cell regulation indicating that the protein was biologically available in the cytoplasm and the interactions with PepFect14 did not impeach its active sites once the plasma barrier crossed.

This study reports the first successful delivery of HSP70 to our knowledge and the first protein transfection mediated by PepFect14. It opens new fields of research for both PepFect14 as a delivery agent and HSP70 as a therapeutic agent; with potential in peptide aggregation caused diseases such as Parkinson’s and Alzheimer’s diseases.

Cell-penetrating peptides are a promising therapeutic strategy for a wide variety of degenerative diseases, ageing, and cancer. Among the multitude of cell-penetrating peptides, PepFect14 has been preferentially used in our laboratory for oligonucleotide delivery into cells and in vivo mouse models. However, this activity has mainly been reported towards cytoplasm and nuclei, while the mentioned disorders have been linked to mitochondrial defects. Here, we report a library generated from a combinatorial covalent fusion of a mitochondrial-penetrating peptide, mtCPP1, and PepFect14 in order to deliver therapeutic biomolecules to influence mitochondrial protein expression. The non-covalent complexation of these peptides with oligonucleotides resulted in nano-complexes affecting biological functions in the cytoplasm and on mitochondria. This delivery system proved to efficiently target mitochondrial genes, providing a framework for the development of mitochondrial peptide-based oligonucleotide technologies with the potential to be used as a treatment for patients with mitochondrial disorders.

Mitochondria have simply been known as the cell’s powerhouse for a long time, with its vital function of producing ATP. However, substantially more attention was directed towards these organelles once they were recognized to perform several essential functions having an impact in cell biology, pharmaceutics and medicine. Dysfunctions of these organelles have been linked to several diseases such as diabetes, cancer, neurodegenerative diseases and cardiovascular disorders. Mitochondrial medicine emerged once the relationship of reactive oxygen species and mutations of the mitochondrial DNA linked to diseases was shown, referred to as mitochondrial dysfunction. This has led to the need to deliver therapeutic molecules in their active form not only to the target cells but more importantly into the targeted organelles.

In this thesis, cell-penetrating peptides (CPPs) used as mitochondrial drug delivery system and the pathways involved in the uptake mechanisms of a CPP are described. In particular, Paper I describes a novel cell-penetrating peptide targeting mitochondria with intrinsic antioxidant properties. Paper II expands upon this first finding and show that the same peptide can carry a glutathione analogue peptide with improved radical scavenging ability into cytoplasm and mitochondria. Paper III introduces mitochondrial targeting peptides for delivery of therapeutic biomolecules to modify mitochondrial gene expression. In Paper IV, the uptake mechanisms of the CPP delivery strategy has been investigated to gain a better understanding of the used transfection system.

Overall, this thesis summarizes our current effort regarding cell-penetrating peptides delivery system to target mitochondria and the progress made towards a potential gene therapy. It contributes to the field of CPPs and drug delivery with a set of peptides with radical scavenging ability, a strategy to deliver oligonucleotides to mitochondria as proof-of-concept for mitochondrial gene therapy, and to help understanding the pathways involved in CPPs uptake.

Mitochondria are key organelles with essential functions and fundamental roles in cell death and survival signaling. Consequently, they are involved in a wide range of diseases with a great diversity of clinical appearance, which makes them attractive as target for drugs to treat metabolic and degenerative diseases and cancer. Efficient methods for specific intracellular delivery of exogenous compounds, including biochemically active small molecules, imaging agents, peptides, peptide nucleic acids, proteins, RNA, DNA, and nanoparticles, would be beneficial for research and patients. A sustained effort in the last 20 years has been done to exploit cell-penetrating peptides (CPPs) for the delivery of such useful cargoes in vitro  and in vivo  because of their biocompatibility, ease of synthesis, and controllable physical chemistry. Here, we discuss the mechanisms by which CPPs can function, the use of this alternative as well as strategies used to target mitochondria and the implications for drug delivery.

INTRODUCTION: One of the major limiting steps in order to have an effective drug is the passage through one or more cell membranes to reach its site of action. To reach the action-site, the specific macromolecules are required to be delivered specifically to the cell compartment/organelle in their (pre)active form.

AREAS COVERED: In this review, we will discuss cell-penetrating peptides (CPPs) developed in the last decade to transport small RNA/DNA, plasmids, antibodies, and nanoparticles into specific sites of the cell. The article describes CPPs in complex with cargo molecules that target specific intracellular organelles and their potential for pharmacological or clinical use.

EXPERT OPINION: Organelle targeting is the ultimate goal to ensure selective delivery to the site of action in the cells. CPP technologies represent an important strategy to address drug delivery to specific intracellular compartments by covalent conjugation to targeting sequences, potentially enabling strategies to combat genomic diseases as well as infections, cancer, neurodegenerative and hereditary diseases. They have proven to be successful in delivering various therapeutic agents into cells however, further in vivo experiments and clinical trials are required to demonstrate the efficacy of this technology.

Previously, we designed and synthesized a library of mitochondrial antioxidative cell-penetrating peptides (mtCPPs) superior to the parent peptide, SS31, to protect mitochondria from oxidative damage. A library of antioxidative glutathione analogs called glutathione peptides (UPFs), exceptional in hydroxyl radical elimination compared with glutathione, were also designed and synthesized. Here, a follow-up study is described, investigating the effects of the most promising members from both libraries on reactive oxidative species scavenging ability. None of the peptides influenced cell viability at the concentrations used. Fluorescence microscopy studies showed that the fluorescein-mtCPP1-UPF25 (mtgCPP) internalized into cells, and spectrofluorometric analysis determined the presence and extent of peptide into different cell compartments. mtgCPP has superior antioxidative activity compared with mtCPP1 and UPF25 against H2O2 insult, preventing ROS formation by 2- and 3-fold, respectively. Moreover, we neither observed effects on mitochondrial membrane potential nor production of ATP. These data indicate that mtgCPP is targeting mitochondria, protecting them from oxidative damage, while also being present in the cytosol. Our hypothesis is based on a synergistic effect resulting from the fused peptide. The mitochondrial peptide segment is targeting mitochondria, whereas the glutathione analog peptide segment is active in the cytosol, resulting in increased scavenging ability.

Cell-penetrating peptides (CPPs) uptake mechanism is still in need of more clarification to have a better understanding of their action in the mediation of oligonucleotide transfection. In this study, the effect on early events (1 h treatment) in transfection by PepFect14 (PF14), with or without oligonucleotide cargo on gene expression, in HeLa cells, have been investigated. The RNA expression profile was characterized by RNA sequencing and confirmed by qPCR analysis. The gene regulations were then related to the biological processes by the study of signaling pathways that showed the induction of autophagy-related genes in early transfection. A ligand library interfering with the detected intracellular pathways showed concentration-dependent effects on the transfection efficiency of splice correction oligonucleotide complexed with PepFect14, proving that the autophagy process is induced upon the uptake of complexes. Finally, the autophagy induction and colocalization with autophagosomes have been confirmed by confocal microscopy and transmission electron microscopy. We conclude that autophagy, an inherent cellular response process, is triggered by the cellular uptake of CPP-based transfection system. This finding opens novel possibilities to use autophagy modifiers in future gene therapy.

More than twenty years after the discovery of the first cell-penetrating peptide (CPP), a large number of both naturally occurring as well as engineered CPPs have been discovered. Generally, CPPs are short polycationic sequences of less than 30 amino acids that are able to translocate different cargoes into cells. They are amphipathic and net positively charged at physiological pH. The cargo can be covalently attached to the CPP, which can be achieved by expression as a fusion construct or by chemical coupling; or the cargo and carrier could bind each other non-covalently mainly through ionic interactions.

A series of CPPs targeting mitochondria (mtCPPs) were studied in an effort to optimize their applications for the reduction of reactive oxygen species targeting this therapeutically important organelle. Mitochondria have evolved to play a vital role in both life and death of eukaryotic cells, through involvement in numerous cellular functions, such as the proficient production of energy from ATP biosynthesis and the regulation of programmed cell death. As a result, dysfunction in the biochemical processes housed within this organelle is implicated in diverse diseases, including cancer, diabetes, and neurodegenerative disorders. Advancing mitochondrial medicine by probing the subcellular biochemistry or targeting therapeutics into this organelle has motivated the development of effective mitochondrial delivery vectors. A fluorescent probe was covalently attached at the N-terminus of the analog peptides to determine the cellular internalization and the possibility to be transported to mitochondria by mtCPPs. The results report the development of a novel cationic peptides (mtCPP-1), which is readily cell permeable and preferentially localize into the mitochondria of living mammalian cells. By substitutions with both natural and synthetic amino acids, and monitoring the intracellular localization by fluorescence microscopy, the mitochondrial accumulation with a cationic peptide was achieved. The biological and chemical characterization of mtCPP-1 revealed the importance of balancing the opposing characteristics of positive charge and lipophilicity to attain preferential sequestration into mitochondria, as well as provide evidence that this antioxidant peptide will be suitable as mitochondrial delivery vector.

Cell-penetrating peptides (CPPs) are short, nontoxic peptides with cationic and/or amphipathic properties able to cross the cellular membrane. CPPs are used for the delivery of a wide variety of cargoes, such as proteins, oligonucleotides, and therapeutic molecules. The aim of the present study was to synthesize unusually small novel CPPs targeting mitochondria based on the Szeto-Schiller peptide (SS-31) to influence intramitochondrial processes and to improve the biologic effects. All the peptides used were synthesized manually using 9-fluorenylmethyloxycarbonyl chemistry. In the first part of the study, HeLa 705, U87, and bEnd.3 cells were used as in vitro delivery model. Cells were incubated for 24 h at 37°C and 5% CO2 with different concentrations of our peptides. Cell proliferation assay was performed to evaluate cell viability. Biologic effects such as mitochondrial membrane potential and antioxidant activity were evaluated. H2O2 was used as positive control. Uptake studies were performed using peptides conjugated with 5(6)-carboxyfluorescein (FAM). Fluorescent microscopy was used to determine presence and localization of peptides into the cells. Isolated mitochondria from pretreated cells and mitochondria treated after isolation were used to confirm the targeting ability of the peptide. Uptake of FAM alone was used as negative control. Microscopy studies confirmed the ability of peptides to penetrate cell. Localization analysis showed increase in uptake by 35% compared with SS-31. Mitochondrial CPP 1 (mtCPP-1) had no effect on mitochondrial membrane potential and prevented reactive oxygen species formation in bEnd.3 cells by 2-fold compared with SS-31. No cytotoxicity was observed even at high concentration (100 µM). These data suggest that mtCPP-1 is a mitochondrial CPP and protect mitochondria from oxidative damage due to its own antioxidant activities.-Cerrato, C. P., Pirisinu M., Vlachos E. N., Langel, Ü. Novel cell-penetrating peptide targeting mitochondria.