A new peptide vector for efficient delivery of oligonucleotides into mammalian cells

Cell-Penetrating Peptides: Promising Therapeutics and Drug-Delivery Systems for Neurodegenerative Diseases

Currently, one of the most significant and rapidly growing unmet medical challenges is the treatment of neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). This challenge encompasses the imperative development of efficacious therapeutic agents and overcoming the intricacies of the blood-brain barrier for successful drug delivery. Here we focus on the delivery aspect with particular emphasis on cell-penetrating peptides (CPPs), widely used in basic and translational research as they enhance drug delivery to challenging targets such as tissue and cellular compartments and thus increase therapeutic efficacy. The combination of CPPs with nanomaterials such as nanoparticles (NPs) improves the performance, accuracy, and stability of drug delivery and enables higher drug loads. Our review presents and discusses research that utilizes CPPs, either alone or in conjugation with NPs, to mitigate the pathogenic effects of neurodegenerative diseases with particular reference to AD and PD.

Hydrophobicity: The door to drug delivery

The engineering of intracellular delivery systems with the goal of achieving personalized medicine has been encouraged by advances in nanomaterial science as well as a greater understanding of diseases and of the biochemical pathways implicated in many disorders. The development of vectors able to transport the drug to a target location and release it only on demand is undoubtedly the primary issue. From a molecular perspective, the topography of drug carrier surfaces is directly related to the design of an effective drug carrier because it provides a physical hint to modifying its interactions with biological systems. For instance, the initial ratio of hydrophilic to hydrophobic surfaces and the changes brought about by external factors enable the release or encapsulation of a therapeutic molecule and the ability of the nanosystem to cross biological barriers and reach its target without causing systemic toxicity. The first step in creating new materials with enhanced functionality is to comprehend and characterize the interplay between hydrophilic and hydrophobic molecules at the molecular level. Therefore, the focus of this review is on the function of hydrophobicity, which is essential for matching the complexity of biological environments with the intended functionality.

Cellular uptake and trafficking of peptide-based drug delivery systems for miRNA

Today, macromolecular compounds such as microRNAs (miRNAs) are becoming more and more widespread as leading therapeutics. However, their application is limited mostly due to their poor stability, limited cellular uptake, and poor target specificity. Cell-penetrating peptides (CPPs), a group of positively charged peptides, represent a breakthrough as delivery systems for macromolecules. In the present study, we used two types of nanoparticles which differ in the type of CPP used for their manufacturing. The first type is composed of protamine, an arginine rich CPP, which is highly positively charged. The arginine residues are able to form electrostatic interactions with miRNAs, stabilize them, and deliver them to cells. The second type is composed of the N-Ter peptide (also known as MPG), an amphipathic peptide rich in lysine. The positively charged parts of the N-Ter peptide electrostatically stabilize miRNAs, whereas its amphipathic character allows it to successfully traverse cell membranes. We used miRNA-27a, a negative regulator of adipogenesis, to form nanoparticles with the peptides and traced their uptake in 3T3-L1 preadipocytes. Motivated by the lengthy discourse regarding the uptake mechanism of CPPs, the focus of our study was to analyse and understand the internalization of proticles (protamine nanoparticles) and N-Ter complexes. The nanoparticles were characterized regarding size, size distribution, and zeta potential, and their cytotoxicity was tested in 3T3-L1 cells. The uptake studies were performed by varying the experimental conditions such as time, concentration, and temperature, as well as by applying different inhibitors of endocytosis. Furthermore, we assessed the biological effect of miRNA-27a on the pro-adipogenic machinery. The obtained data have shown that protamine and the N-Ter peptide form positively charged nanoparticles through non-covalent complexation. The uptake of proticles and N-Ter complexes was found to be dependent on time, concentration, and temperature, and different uptake pathways were discovered to be involved in the internalization of the different nanoparticles. Furthermore, both types of nanoparticles induced the anti-adipogenic effect of miRNA-27a, demonstrating that this approach can be used as a novel miRNA replacement therapy in the treatment of obesity and obesity-related disorders.

Peptide nanostructures-based delivery of DNA nanomaterial therapeutics for regulating gene expression

Self-assembled branched DNA (bDNA) nanomaterials have exhibited their functionality in various biomedical and diagnostic applications. However, the anionic cellular membrane has restricted the movement of bDNA nanostructures. Recently, amphiphilic peptides have been investigated as cationic delivery agents for nucleic acids. Herein, we demonstrate a strategy for delivering functional bDNA nanomaterials into mammalian cells using self-assembled linear peptides. In this study, antisense oligonucleotides of vascular endothelial growth factor (VEGF) were inserted in the overhangs of bDNAs. Novel linear peptides have been synthesized and the peptide-bound bDNA complex formation was examined using various biophysical experiments. Interestingly, the W4R4-bound bDNAs were found to be exceptionally stable against DNase I compared to other complexes. The delivery of fluorescent-labeled bDNAs into the mammalian cells confirmed the potential of peptide transporters. Furthermore, the functional efficacy of the peptide-bound bDNAs has been examined through RT-PCR and western blot analysis. The observed results revealed that W4R4 peptides exhibited excellent internalization of antisense bDNAs and significantly suppressed (3- to 4-fold) the transcripts and translated product of VEGF compared to the control. In summary, the results highlight the potential use of peptide-based nanocarrier for delivering bDNA nanostructures to regulate the gene expression in cell lines.

Cell penetrating peptides: Highlighting points in cancer therapy

Cell-penetrating peptides (CPPs), first identified in HIV a few decades ago, deserved great attention in the last two decades; especially to support the penetration of anticancer drug means. In the drug delivery discipline, they have been involved in various approaches from mixing with hydrophobic drugs to the use of genetically conjugated proteins. The early classification as cationic and amphipathic CPPs has been extended to a few more classes such as hydrophobic and cyclic CPPs so far. Developing potential sequences utilized almost all methods of modern science: choosing high-efficiency peptides from natural protein sequences, sequence-based comparison, amino acid substitution, obtaining chemical and/or genetic conjugations, in silico approaches, in vitro analysis, animal experiments, etc. The bottleneck effect in this discipline reveals the complications that modern science faces in drug delivery research. Most CPP-based drug delivery systems (DDSs) efficiently inhibited tumor volume and weight in mice, but only in rare cases reduced their levels and continued further processes. The integration of chemical synthesis into the development of CPPs made a significant contribution and even reached the clinical stage as a diagnostic tool. But constrained efforts still face serious problems in overcoming biobarriers to reach further achievements. In this work, we reviewed the roles of CPPs in anticancer drug delivery, focusing on their amino acid composition and sequences. As the most suitable point, we relied on significant changes in tumor volume in mice resulting from CPPs. We provide a review of individual CPPs and/or their derivatives in a separate subsection.

Cell-Penetrating Peptide-Based Delivery of Macromolecular Drugs: Development, Strategies, and Progress

Cell-penetrating peptides (CPPs), developed for more than 30 years, are still being extensively studied due to their excellent delivery performance. Compared with other delivery vehicles, CPPs hold promise for delivering different types of drugs. Here, we review the development process of CPPs and summarize the composition and classification of the CPP-based delivery systems, cellular uptake mechanisms, influencing factors, and biological barriers. We also summarize the optimization routes of CPP-based macromolecular drug delivery from stability and targeting perspectives. Strategies for enhanced endosomal escape, which prolong its half-life in blood, improved targeting efficiency and stimuli-responsive design are comprehensively summarized for CPP-based macromolecule delivery. Finally, after concluding the clinical trials of CPP-based drug delivery systems, we extracted the necessary conditions for a successful CPP-based delivery system. This review provides the latest framework for the CPP-based delivery of macromolecular drugs and summarizes the optimized strategies to improve delivery efficiency.

Lipid and Peptide-Oligonucleotide Conjugates for Therapeutic Purposes: From Simple Hybrids to Complex Multifunctional Assemblies

Antisense and small interfering RNA (siRNA) oligonucleotides have been recognized as powerful therapeutic compounds for targeting mRNAs and inducing their degradation. However, a major obstacle is that unmodified oligonucleotides are not readily taken up into tissues and are susceptible to degradation by nucleases. For these reasons, the design and preparation of modified DNA/RNA derivatives with better stability and an ability to be produced at large scale with enhanced uptake properties is of vital importance to improve current limitations. In the present study, we review the conjugation of oligonucleotides with lipids and peptides in order to produce oligonucleotide conjugates for therapeutics aiming to develop novel compounds with favorable pharmacokinetics.

Cell penetrating peptide: A potent delivery system in vaccine development

One of the main obstacles to most medication administrations (such as the vaccine constructs) is the cellular membrane's inadequate permeability, which reduces their efficiency. Cell-penetrating peptides (CPPs) or protein transduction domains (PTDs) are well-known as potent biological nanocarriers to overcome this natural barrier, and to deliver membrane-impermeable substances into cells. The physicochemical properties of CPPs, the attached cargo, concentration, and cell type substantially influence the internalization mechanism. Although the exact mechanism of cellular uptake and the following processing of CPPs are still uncertain; but however, they can facilitate intracellular transfer through both endocytic and non-endocytic pathways. Improved endosomal escape efficiency, selective cell targeting, and improved uptake, processing, and presentation of antigen by antigen-presenting cells (APCs) have been reported by CPPs. Different in vitro and in vivo investigations using CPP conjugates show their potential as therapeutic agents in various medical areas such as infectious and non-infectious disorders. Effective treatments for a variety of diseases may be provided by vaccines that can cooperatively stimulate T cell-mediated immunity (T helper cell activity or cytotoxic T cell function), and immunologic memory. Delivery of antigen epitopes to APCs, and generation of a potent immune response is essential for an efficacious vaccine that can be facilitated by CPPs. The current review describes the delivery of numerous vaccine components by various CPPs and their immunostimulatory properties.

Cell-penetrating peptide: A powerful delivery tool for DNA-free crop genome editing

Genome editing system based on the CRISPR/Cas (clustered regularly interspaced short palindromic repeats) technology is a milestone for biology. However, public concerns regarding genetically modified organisms (GMOs) and recalcitrance in the crop of choice for regeneration have limited its application. Cell-penetrating peptides (CPPs) are derived from protein transduction domains (PTDs) that can take on various cargoes across the plant wall, and membrane of target cells. Selected CPPs show mild cytotoxicity and are a suitable delivery tool for DNA-free genome editing. Moreover, CPPs may also be applied for the transient delivery of morphogenic transcription factors, also known as developmental regulators (DRs), to overcome the bottleneck of the crop of choice regeneration. In this review, we introduce a brief history of cell-penetrating peptides and discuss the practice of CPP-mediated DNA-free transfection and the prospects of this potential delivery tool for improving crop genome editing.

Changing Fate: Reprogramming Cells via Engineered Nanoscale Delivery Materials

The incorporation of nanotechnology in regenerative medicine is at the nexus of fundamental innovations and early-stage breakthroughs, enabling exciting biomedical advances. One of the most exciting recent developments is the use of nanoscale constructs to influence the fate of cells, which are the basic building blocks of healthy function. Appropriate cell types can be effectively manipulated by direct cell reprogramming; a robust technique to manipulate cellular function and fate, underpinning burgeoning advances in drug delivery systems, regenerative medicine, and disease remodeling. Individual transcription factors, or combinations thereof, can be introduced into cells using both viral and nonviral delivery systems. Existing approaches have inherent limitations. Viral-based tools include issues of viral integration into the genome of the cells, the propensity for uncontrollable silencing, reduced copy potential and cell specificity, and neutralization via the immune response. Current nonviral cell reprogramming tools generally suffer from inferior expression efficiency. Nanomaterials are increasingly being explored to address these challenges and improve the efficacy of both viral and nonviral delivery because of their unique properties such as small size and high surface area. This review presents the state-of-the-art research in cell reprogramming, focused on recent breakthroughs in the deployment of nanomaterials as cell reprogramming delivery tools.

Peptide therapeutics in the management of metastatic cancers

Cancer remains a leading health concern threatening lives of millions of patients worldwide. Peptide-based drugs provide a valuable alternative to chemotherapeutics as they are highly specific, cheap, less toxic and easier to synthesize compared to other drugs. In this review, we have discussed various modes in which peptides are being used to curb cancer. Our review highlights specially the various anti-metastatic peptide-based agents developed by targeting a plethora of cellular factors. Herein we have given a special focus on integrins as targets for peptide drugs, as these molecules play key roles in metastatic progression. The review also discusses use of peptides as anti-cancer vaccines and their efficiency as drug-delivery tools. We hope this work will give the reader a clear idea of the mechanisms of peptide-based anti-cancer therapeutics and encourage the development of superior drugs in the future.

Peptide-Mediated Targeted Delivery of Aloe-Emodin as Anticancer Drug

Breast cancer is one of the most diffuse cancers in the world and despite the availability of the different drugs employed against it, the need for new and particularly more specific molecules is ever growing. In this framework, natural products are increasingly assuming an important role as new anticancer drugs. Aloe-emodin (AE) is one of the best characterized molecules in this field. The functionalization of bioactive natural products with selected peptide sequences to enhance their bioavailability and specificity of action is a powerful and promising strategy. In this study, we analyzed the cell specificity, cell viability effects, intracellular distribution, and immune cell response of a new peptide conjugate of Aloe-emodin in SKBR3 and A549 cell lines by means of viability tests, flow cytometry, and confocal microscopy. The conjugate proved to be more effective at reducing cell viability than AE in both cell lines. Furthermore, the results showed that it was mainly internalized within the SKBR3 cells, showing a nuclear localization, while A459 cells displayed mainly a cytoplasmic distribution. A preserving effect of the conjugate on NKs’ cell function was also observed. The designed conjugate showed a promising specific activity towards HER2-expressing cells coupled with an enhanced water solubility and a higher cytotoxicity; thus, the resulting proof-of-concept molecule can be further improved as an anticancer compound.

Efficient transdermal delivery of functional protein cargoes by a hydrophobic peptide MTD 1067

The skin has a protective barrier against the external environment, making the transdermal delivery of active macromolecules very difficult. Cell-penetrating peptides (CPPs) have been accepted as useful delivery tools owing to their high transduction efficiency and low cytotoxicity. In this study, we evaluated the hydrophobic peptide, macromolecule transduction domain 1067 (MTD 1067) as a CPP for the transdermal delivery of protein cargoes of various sizes, including growth hormone-releasing hexapeptide-6 (GHRP-6), a truncated form of insulin-like growth factor-I (des(1-3)IGF-I), and platelet-derived growth factor BB (PDGF-BB). The MTD 1067-conjugated GHRP-6 (MTD-GHRP-6) was chemically synthesized, whereas the MTD 1067-conjugated des(1-3)IGF-I and PDGF-BB proteins (MTD-des(1-3)IGF-I and MTD-PDGF-BB) were generated as recombinant proteins. All the MTD 1067-conjugated cargoes exhibited biological activities identical or improved when compared to those of the original cargoes. The analysis of confocal microscopy images showed that MTD-GHRP-6, MTD-des(1-3)IGF-I, and MTD-PDGF-BB were detected at 4.4-, 18.8-, and 32.9-times higher levels in the dermis, respectively, compared to the control group without MTD. Furthermore, the MTD 1067-conjugated cargoes did not show cytotoxicity. Altogether, our data demonstrate the potential of MTD 1067 conjugation in developing functional macromolecules for cosmetics and drugs with enhanced transdermal permeability.

Nonviral Delivery Systems of mRNA Vaccines for Cancer Gene Therapy

In recent years, the use of messenger RNA (mRNA) in the fields of gene therapy, immunotherapy, and stem cell biomedicine has received extensive attention. With the development of scientific technology, mRNA applications for tumor treatment have matured. Since the SARS-CoV-2 infection outbreak in 2019, the development of engineered mRNA and mRNA vaccines has accelerated rapidly. mRNA is easy to produce, scalable, modifiable, and not integrated into the host genome, showing tremendous potential for cancer gene therapy and immunotherapy when used in combination with traditional strategies. The core mechanism of mRNA therapy is vehicle-based delivery of in vitro transcribed mRNA (IVT mRNA), which is large, negatively charged, and easily degradable, into the cytoplasm and subsequent expression of the corresponding proteins. However, effectively delivering mRNA into cells and successfully activating the immune response are the keys to the clinical transformation of mRNA therapy. In this review, we focus on nonviral nanodelivery systems of mRNA vaccines used for cancer gene therapy and immunotherapy.

Cell-Penetrating Peptides

In this introductory chapter, we first define cell-penetrating peptides (CPPs), give short overview of CPP history and discuss several aspects of CPP classification. Next section is devoted to the mechanism of CPP penetration into the cells, where direct and endocytic internalization of CPP is explained. Kinetics of internalization is discussed more extensively, since this topic is not discussed in other chapters of this book. At the end of this section some features of the thermodynamics of CPP interaction with the membrane is also presented. Finally, we present different cargoes that can be transferred into the cells by CPPs and briefly discuss the effect of cargo on the rate and efficiency of penetration into the cells.

Methods for Small-Angle Scattering Measurements on Peptiplexes of DNA with Cell-Penetrating Peptides

Peptiplexes are soft biomaterials formed through the noncovalent association between cell-penetrating peptides and nucleic acids. Although internalization often involves electrostatic anchoring followed by endocytosis, the mode of action of these transporters remains elusive in many cases, and proper understanding of mechanisms behind their penetrating capabilities necessarily entails structural data at the nanoscopic scale. In this chapter, we examine the structural landscape of peptiplexes, emphasizing the complex behavior of these polyelectrolyte self-assemblies and how supramolecular order impacts their translocation efficiency. We discuss experimental tools commonly used to investigate the structure of peptiplexes and pay special attention to small-angle X-ray scattering (SAXS) as a suitable method for unveiling their nanoscale organization. A roadmap for standard SAXS measurements in CPP/DNA samples is presented alongside a selection of observations from our own experience dealing with SAXS applied to the investigation of CPPs.

Mitochondrial Targeting Probes, Drug Conjugates, and Gene Therapeutics

Mitochondria represent an important drug target for many phatology, including neurodegeneration, metabolic disease, heart failure, ischemia-reperfusion injury, and cancer. Mitochondrial dysfunctions are caused by mutation in mitochondrial DNA or in nuclear genes encoding mitochondrial proteins. Cell-penetrating peptides (CPPs) have been employed to overcome biological barriers, target this organelle, and therapeuticaly restore mitochondrial functions. Here, we describe recent methods used to deliver oligonucleotides targeting mitochondrial protein by using mitochondrial penetrating peptides. In particular, we highlight recent advances of formulated peptides/oligonucleotides nanocomplexes as a proof-of-principle for pharmaceutical form of peptide-based therapeutics.

TAT for Enzyme/Protein Delivery to Restore or Destroy Cell Activity in Human Diseases

Much effort has been dedicated in the recent decades to find novel protein/enzyme-based therapies for human diseases, the major challenge of such therapies being the intracellular delivery and reaching sub-cellular organelles. One promising approach is the use of cell-penetrating peptides (CPPs) for delivering enzymes/proteins into cells. In this review, we describe the potential therapeutic usages of CPPs (mainly trans-activator of transcription protein, TAT) in enabling the uptake of biologically active proteins/enzymes needed in cases of protein/enzyme deficiency, concentrating on mitochondrial diseases and on the import of enzymes or peptides in order to destroy pathogenic cells, focusing on cancer cells.

Chemistry of Peptide-Oligonucleotide Conjugates: A Review

Peptide-oligonucleotide conjugates (POCs) represent one of the increasingly successful albeit costly approaches to increasing the cellular uptake, tissue delivery, bioavailability, and, thus, overall efficiency of therapeutic nucleic acids, such as, antisense oligonucleotides and small interfering RNAs. This review puts the subject of chemical synthesis of POCs into the wider context of therapeutic oligonucleotides and the problem of nucleic acid drug delivery, cell-penetrating peptide structural types, the mechanisms of their intracellular transport, and the ways of application, which include the formation of non-covalent complexes with oligonucleotides (peptide additives) or covalent conjugation. The main strategies for the synthesis of POCs are viewed in detail, which are conceptually divided into (a) the stepwise solid-phase synthesis approach and (b) post-synthetic conjugation either in solution or on the solid phase, especially by means of various click chemistries. The relative advantages and disadvantages of both strategies are discussed and compared.

Cell-Penetrating Peptides and Transportan

In the most recent 25–30 years, multiple novel mechanisms and applications of cell-penetrating peptides (CPP) have been demonstrated, leading to novel drug delivery systems. In this review, I present a brief introduction to the CPP area with selected recent achievements. This is followed by a nostalgic journey into the research in my own laboratories, which lead to multiple CPPs, starting from transportan and paving a way to CPP-based therapeutic developments in the delivery of bio-functional materials, such as peptides, proteins, vaccines, oligonucleotides and small molecules, etc.

Non-Covalent Carrier Hydrophobicity as a Universal Predictor of Intracellular Protein Activity

Over the past decade, extensive optimization of polymeric cell-penetrating peptide (CPP) mimics (CPPMs) by our group has generated a substantial library of broadly effective carriers which circumvent the need for covalent conjugation often required by CPPs. In this study, design rules learned from CPPM development were applied to reverse-engineer the first library of simple amphiphilic block copolypeptides for non-covalent protein delivery, namely, poly(alanine-block-arginine), poly(phenylalanine-block-arginine), and poly(tryptophan-block-arginine). This new CPP library was screened for enhanced green fluorescent protein and Cre recombinase delivery alongside a library of CPPMs featuring equivalent side-chain configurations. Due to the added hydrophobicity imparted by the polymer backbone as compared to the polypeptide backbone, side-chain functionality was not a universal predictor of carrier performance. Rather, overall carrier hydrophobicity predicted the top performers for both internalization and activity of protein cargoes, regardless of backbone identity. Furthermore, comparison of protein uptake and function revealed carriers which facilitated high gene recombination despite remarkably low Cre internalization, leading us to formalize the concept of intracellular availability (IA) of the delivered cargo. IA, a measure of cargo activity per quantity of cargo internalized, provides valuable insight into the physical relationship between cellular internalization and bioavailability, which can be affected by bottlenecks such as endosomal escape and cargo release. Importantly, carriers with maximal IA existed within a narrow hydrophobicity window, more hydrophilic than those exhibiting maximal cargo uptake. Hydrophobicity may be used as a scaffold-independent predictor of protein uptake, function, and IA, enabling identification of new, effective carriers which would be overlooked by uptake-based screening methods.

Peptide-Based Nanoparticles for Therapeutic Nucleic Acid Delivery

Gene therapy offers the possibility to skip, repair, or silence faulty genes or to stimulate the immune system to fight against disease by delivering therapeutic nucleic acids (NAs) to a patient. Compared to other drugs or protein treatments, NA-based therapies have the advantage of being a more universal approach to designing therapies because of the versatility of NA design. NAs (siRNA, pDNA, or mRNA) have great potential for therapeutic applications for an immense number of indications. However, the delivery of these exogenous NAs is still challenging and requires a specific delivery system. In this context, beside other non-viral vectors, cell-penetrating peptides (CPPs) gain more and more interest as delivery systems by forming a variety of nanocomplexes depending on the formulation conditions and the properties of the used CPPs/NAs. In this review, we attempt to cover the most important biophysical and biological aspects of non-viral peptide-based nanoparticles (PBNs) for therapeutic nucleic acid formulations as a delivery system. The most relevant peptides or peptide families forming PBNs in the presence of NAs described since 2015 will be presented. All these PBNs able to deliver NAs in vitro and in vivo have common features, which are characterized by defined formulation conditions in order to obtain PBNs from 60 nm to 150 nm with a homogeneous dispersity (PdI lower than 0.3) and a positive charge between +10 mV and +40 mV.

Cell-penetrating peptides (CPPs): an overview of applications for improving the potential of nanotherapeutics

In the field of nanotherapeutics, gaining cellular entry into the cytoplasm of the target cell continues to be an ultimate challenge. There are many physicochemical factors such as charge, size and molecular weight of the molecules and delivery vehicles, which restrict their cellular entry. Hence, to dodge such situations, a class of short peptides called cell-penetrating peptides (CPPs) was brought into use. CPPs can effectively interact with the cell membrane and can assist in achieving the desired intracellular entry. Such strategy is majorly employed in the field of cancer therapy and diagnosis, but now it is also used for other purposes such as evaluation of atherosclerotic plaques, determination of thrombin levels and HIV therapy. Thus, the current review expounds on each of these mentioned aspects. Further, the review briefly summarizes the basic know-how of CPPs, their utility as therapeutic molecules, their use in cancer therapy, tumor imaging and their assistance to nanocarriers in improving their membrane penetrability. The review also discusses the challenges faced with CPPs pertaining to their stability and also mentions the strategies to overcome them. Thus, in a nutshell, this review will assist in understanding how CPPs can present novel possibilities for resolving the conventional issues faced with the present-day nanotherapeutics.

Evolutionary Timeline of Genetic Delivery and Gene Therapy

There are more than 3,500 genes that are being linked to hereditary diseases or correlated with an elevated risk of certain illnesses. As an alternative to conventional treatments with small molecule drugs, gene therapy has arisen as an effective treatment with the potential to not just alleviate disease conditions but also cure them completely. In order for these treatment regimens to work, genes or editing tools intended to correct diseased genetic material must be efficiently delivered to target sites. There have been many techniques developed to achieve such a goal. In this article, we systematically review a variety of gene delivery and therapy methods that include physical methods, chemical and biochemical methods, viral methods, and genome editing. We discuss their historical discovery, mechanisms, advantages, limitations, safety, and perspectives.

Gene Delivery Technologies with Applications in Microalgal Genetic Engineering

Microalgae and cyanobacteria are photosynthetic microbes that can be grown with the simple inputs of water, carbon dioxide, (sun)light, and trace elements. Their engineering holds the promise of tailored bio-molecule production using sustainable, environmentally friendly waste carbon inputs. Although algal engineering examples are beginning to show maturity, severe limitations remain in the transformation of multigene expression cassettes into model species and DNA delivery into non-model hosts. This review highlights common and emerging DNA delivery methods used for other organisms that may find future applications in algal engineering.

Endolysin-Based Autolytic E. coli System for Facile Recovery of Recombinant Proteins

Recovery of recombinant proteins from the Escherichia coli cytoplasm depends on cell disruption by mechanical, chemical, and/or enzymatic methods, which usually cause incomplete cell breakage or protein denaturation. Controllable autolytic E. coli strains have been designed to facilitate the purification of recombinant proteins; however, these strains suffer from low recovery yield, slow cell lysis, or extensive strain engineering. Herein, we report an improved, highly efficient programmable autolytic E. coli platform, in which cell lysis is initiated upon the induced expression of T4 lysozyme with N-terminal fusion of a cell-penetrating peptide. Through the engineering of the peptide sequence and copy number, and by incorporating the fusion lytic gene into the E. coli genome, more than 99.97% of cells could be lysed within 30 min of induction regardless of cell age. We further tested the expression and release of a recombinant enzyme lysostaphin (Lst) and demonstrated that 4 h induction of the lytic gene after 3 h of Lst expression resulted in 98.97% cell lysis. Lst obtained from this system had the same yield, yet 1.63-fold higher activity, compared with that obtained from cells lysed by freeze-thawing and sonication. This autolytic platform shows potential for use in large-scale microbial production of proteins and other biopolymers.

In vivo delivery of a multiepitope peptide and Nef protein using novel cell-penetrating peptides for development of HIV-1 vaccine candidate

OBJECTIVES

A potent HIV vaccine should overcome some limitations such as polymorphism of human HLA, the diversity of HIV-1 virus, and the lack of an effective delivery system. In this study, a DNA construct encoding Nef_60-84, Nef_126-144, Vpr_34-47, Vpr_60-75, Gp160_30-53, Gp160_308-323, and P24_8-151 epitopes was designed using bioinformatics tools. The pcDNA3.1-nef-vpr-gp160-p24 and pcDNA3.1-nef constructs were prepared in large scale as endotoxin-free form. Moreover, the recombinant Nef-Vpr-Gp160-p24 polypeptide and Nef protein were generated inE. coli. These constructs were delivered using cell penetrating peptides (CPPs) in vivo, and immune responses were assessed for different modalities in BALB/c mice.

RESULTS

The recombinant DNA constructs were confirmed as the ~ 867 bp and ~ 648 bp bands related tonef-vpr-gp160-p24 andnef genes on agarose gel. Moreover, the purified Nef-Vpr-Gp160-p24 polypeptide and Nef protein showed the ~ 32 kDa and ~ 30 kDa bands on SDS-PAGE, respectively. The results of immune responses indicated that the heterologous prime/boost regimens using both Nef-Vpr-Gp160-P24 and Nef antigens induced significantly the secretion of IgG2a, IgG2b, IFN-γ and Granzyme B compared to other groups. The levels of Granzyme B in mice immunized with Nef antigen were higher than those immunized with Nef-Vpr-Gp160-P24 antigen. The CPPs showed the same potency with Montanide adjuvant for eliciting immune responses.

CONCLUSIONS

The heterologous prime/boost regimens for both antigens could significantly direct immune responses toward Th1 and CTL activity compared to other regimens. Comparing the efficiency of Nef-Vpr-Gp160-P24 and Nef constructs, the Nef-Vpr-Gp160-P24 constructs delivered by CPPs showed promising results as an HIV vaccine candidate.

Approaches for the discovery of new cell-penetrating peptides

Introduction: The capability of cell-penetrating peptides (CPP), also known as protein transduction domains (PTD), to enter into cells possibly with an attached cargo, makes their application as delivery vectors or as direct therapeutics compelling. They are generally biocompatible, nontoxic, and easy to synthesize and modify. Three decades after the discovery of the first CPPs, ~2,000 CPP sequences have been identified, and many more predicted. Nevertheless, the field has a strong commitment to authenticate new, more efficient, and specific CPPs.Areas covered: Although a scattering of CPPs have been found by chance, various systematic approaches have been developed and refined over the years to directly aid the identification and depiction of new peptide-based delivery vectors or therapeutics. Here, the authors give an overview of CPPs, and review various approaches of discovering new ones. An emphasis is placed on in silico methods, as these have advanced rapidly in recent years.Expert opinion: Although there are many known CPPs, there is a need to find more efficient and specific CPPs. Several approaches are used to identify such sequences. The success of these approaches depends on the advancement of others and the successful prediction of CPP sequences relies on experimental data.

Cell-Penetrating Peptides Escape the Endosome by Inducing Vesicle Budding and Collapse

Cell-penetrating peptides (CPPs) are capable of delivering membrane-impermeable cargoes (including small molecules, peptides, proteins, nucleic acids, and nanoparticles) into the cytosol of mammalian cells and have the potential to revolutionize biomedical research and drug discovery. However, the mechanism of action of CPPs has remained poorly understood, especially how they escape from the endosome into the cytosol following endocytic uptake. We show herein that CPPs exit the endosome by inducing budding and collapse of CPP-enriched vesicles from the endosomal membrane. This mechanism provides a theoretical basis for designing CPPs and other delivery vehicles of improved efficiencies.

Caspase-Activated Oligonucleotide Probe

Light-activated ("caged") oligonucleotides provide a strategy for modulating the activity of antisense oligos, siRNA, miRNA, aptamers, DNAzymes, and mRNA-capturing probes with high spatiotemporal resolution. However, the near-UV and visible wavelengths that promote these bond-breaking reactions poorly penetrate living tissue, which limits some biological applications. To address this issue, we describe the first example of a protease-activated oligonucleotide probe, capable of reporting on caspase-3 during cellular apoptosis. The 2'-F RNA-peptide substrate-peptide nucleic acid (PNA) hairpin structure was generated in 30% yield in a single bioconjugation step.

The world of cell penetrating: the future of medical applications

Cell-penetrating peptides present huge biomedical applications in a variety of pathologies, thanks to their ability to penetrate membranes and carry a variety of cargoes inside cells. Progress in peptide synthesis has produced a greater availability of virtually any synthetic peptide, increasing their attractiveness. Most molecules when associated to a cell-penetrating peptides can be delivered into a cell, however, understanding of the critical factors influencing the uptake mechanism is of paramount importance to construct nanoplatforms for effective delivery in vitro and in vivo in medical applications. Focus is now on the state-of-art of the mechanisms enabling therapeutics/diagnostics to reach the site target of their activities, and in support of scientists developing platforms for drug delivery and personalized therapies.

Polyamide Backbone Modified Cell Targeting and Penetrating Peptides in Cancer Detection and Treatment

Cell penetrating and targeting peptides (CPPs and CTPs) encompass an important class of biochemically active peptides owning the capabilities of targeting and translocating within selected cell types. As such, they have been widely used in the delivery of imaging and therapeutic agents for the diagnosis and treatment of various diseases, especially in cancer. Despite their potential utility, first generation CTPs and CPPs based on the native peptide sequences are limited by poor biological and pharmacological properties, thereby restricting their efficacy. Therefore, medicinal chemistry approaches have been designed and developed to construct related peptidomimetics. Of specific interest herein, are the design applications which modify the polyamide backbone of lead CTPs and CPPs. These modifications aim to improve the biochemical characteristics of the native peptide sequence in order to enhance its diagnostic and therapeutic capabilities. This review will focus on a selected set of cell penetrating and targeting peptides and their related peptidomimetics whose polyamide backbone has been modified in order to improve their applications in cancer detection and treatment.

TAT-dextran-mediated mitochondrial transfer enhances recovery from models of reperfusion injury in cultured cardiomyocytes

Acute myocardial infarction is a leading cause of death among single organ diseases. Despite successful reperfusion therapy, ischaemia reperfusion injury (IRI) can induce oxidative stress (OS), cardiomyocyte apoptosis, autophagy and release of inflammatory cytokines, resulting in increased infarct size. In IRI, mitochondrial dysfunction is a key factor, which involves the production of reactive oxygen species, activation of inflammatory signalling cascades or innate immune responses, and apoptosis. Therefore, intercellular mitochondrial transfer could be considered as a promising treatment strategy for ischaemic heart disease. However, low transfer efficiency is a challenge in clinical settings. We previously reported uptake of isolated exogenous mitochondria into cultured cells through co‐incubation, mediated by macropinocytosis. Here, we report the use of transactivator of transcription dextran complexes (TAT‐dextran) to enhance cellular uptake of exogenous mitochondria and improve the protective effect of mitochondrial replenishment in neonatal rat cardiomyocytes (NRCMs) against OS. TAT‐dextran–modified mitochondria (TAT‐Mito) showed a significantly higher level of cellular uptake. Mitochondrial transfer into NRCMs resulted in anti‐apoptotic capability and prevented the suppression of oxidative phosphorylation in mitochondria after OS. Furthermore, TAT‐Mito significantly reduced the apoptotic rates of cardiomyocytes after OS, compared to simple mitochondrial transfer. These results indicate the potential of mitochondrial replenishment therapy in OS‐induced myocardial IRI.

Selective anticancer activity of synthetic peptides derived from the host defence peptide tritrpticin

Antimicrobial peptides (AMPs) constitute a diverse family of peptides with the ability to protect their host against microbial infections. In addition to their ability to kill microorganisms, several AMPs also exhibit selective cytotoxicity towards cancer cells and are collectively referred to as anticancer peptides (ACPs). Here a large library of AMPs, mainly derived from the porcine cathelicidin peptide, tritrpticin (VRRFPWWWPFLRR), were assessed for their anticancer activity against the Jurkat T cell leukemia line. These anticancer potencies were compared to the cytotoxicity of the peptides towards normal cells isolated from healthy donors, namely peripheral blood mononuclear cells (PBMCs) and red blood cells (RBCs; where hemolytic activity was assessed). Among the active tritrpticin derivatives, substitution of Arg by Lys enhanced the selectivity of the peptides towards Jurkat cells when compared to PBMCs. Additionally, the side chain length of the Lys residues was also optimized to further enhance the tritrpticin ACP selectivity at low concentrations. The mechanism of action of the peptides with high selectivity involved the permeabilization of the cytoplasmic membrane of Jurkat cells, without formation of apoptotic bodies. The incorporation of non-natural Lys-based cationic amino acids could provide a new strategy to improve the selectivity of other synthetic ACPs to enhance their potential for therapeutic use against leukemia cells.

Stepwise Development of Biomimetic Chimeric Peptides for Gene Delivery

Gene-based therapy largely relies on the vector type that allows a selective and efficient transfection into the target cells with maximum efficacy and minimal toxicity. Although, genes delivered utilizing modified viruses transfect efficiently and precisely, these vectors can cause severe immunological responses and are potentially carcinogenic. A promising method of overcoming this limitation is the use of non-viral vectors, including cationic lipids, polymers, dendrimers, and peptides, which offer potential routes for compacting DNA for targeted delivery. Although non-viral vectors exhibit reduced transfection efficiency compared to their viral counterpart, their superior biocompatibility, non-immunogenicity and potential for large-scale production make them increasingly attractive for modern therapy. There has been a great deal of interest in the development of biomimetic chimeric peptides. Biomimetic chimeric peptides contain different motifs for gene translocation into the nucleus of the desired cells. They have motifs for gene targeting into the desired cell, condense DNA into nanosize particles, translocate the gene into the nucleus and enhance the release of the particle into the cytoplasm. These carriers were developed in recent years. This review highlights the stepwise development of the biomimetic chimeric peptides currently being used in gene delivery.

Peptide carriers to the rescue: overcoming the barriers to siRNA delivery for cancer treatment

Cancer is a significant health concern worldwide and its clinical treatment presents many challenges. Consequently, much research effort has focused on the development of new anticancer drugs to combat this disease. One area of exploration, in particular, has been in the therapeutic application of RNA interference (RNAi). Although RNAi appears to be an attractive therapeutic tool for the treatment of cancer, one of the primary obstacles towards its pervasive use in the clinic has been cell/tissue type-specific cytosolic delivery of therapeutic small interfering RNA (siRNA) molecules. Consequently, varied drug delivery platforms have been developed and widely explored for siRNA delivery. Among these candidate drug delivery systems, peptides have shown great promise as siRNA carriers due to their varied physiochemical properties and functions, simple formulations, and flexibility in design. In this review, we will focus on distinguishing between the different classes of peptide carriers based on their functions, as well as summarize and discuss the various design strategies and advancements that have been made in circumventing the barriers to siRNA delivery for cancer treatment. Resolution of these challenges by peptide carriers will accelerate the translation of RNAi-based therapies to the clinic.

A C-terminal peptide of TFPI-1 facilitates cytosolic delivery of nucleic acid cargo into mammalian cells

Efficient intracellular nucleic acid delivery into mammalian cells remains a long-standing challenge owing to poor cell permeability and uptake of naked nucleic acids across the cell membrane and limited cargo stability. Conventional delivery methods have several drawbacks, such as cytotoxicity, limited cell-type applicability, low efficiency, hindrances that limit the potential of oligonucleotide delivery in functional genomics, therapeutics and diverse research applications. Thus, new approaches that are robust, safe, effective and valid across multiple cell types are much needed. Here, we demonstrate that GGL27, a TFPI-1-derived novel cationic host defence peptide, facilitates the delivery of nucleic acid cargo into the cytosol of a range of mammalian cells. The GGL27 peptide is non-cytotoxic and is internalized in a broad range of mammalian cell-types, including transformed cell lines and primary cells. GGL27 spontaneously forms complexes with nucleic acids of variable sizes, protects them from nuclease degradation, and delivers cargo effectively. Together, our observations demonstrate the versatile cell-penetrating property of GGL27, providing an excellent template for developing a simple, non-toxic peptide-based cytosolic delivery tool for wide use in biomedical research.

Conversion of human fibroblasts into multipotent cells by cell-penetrating peptides

The potential application of human induced pluripotent stem cells (hiPSCs) brings great expectations to regenerative medicine. However, several safety concerns, such as oncogenic transformation, remain. A number of methods have been developed to produce hiPSCs with potentially reduced risks. Cell-penetrating peptides (CPPs) are expected to improve the efficiency of nonviral reprogramming by delivering biologically active molecules into cells. Here, we show that the transfection of CPPs alone into normal adult human fibroblasts generated embryonic body (EB)-like cell clusters in the absence of reprogramming factors. The CPP-generated cell clusters were positive for a set of multipotency markers and differentiated into endodermal, ectodermal, and mesodermal cells in vitro. These results suggest that CPPs converted normal human adult somatic cells into multipotent cells. Moreover, we show that CPPs dissociated histone deacetylase 1 and lysine-specific demethylase 1 from the promoter/enhancer regions of reprogramming factors to reactivate their expression. This is the first report of an easy and quick method for somatic cell reprogramming by CPPs and a novel mechanism of reprogramming. The potential application of CPP-generated multipotent cells resolves several concerns, especially safety issues, in regenerative medicine.

Nucleic Acids Delivery Into the Cells Using Pro-Apoptotic Protein Lactaptin

Cell penetrating peptides (CPP) are promising agents for transporting diverse cargo into the cells. The amino acid sequence and the mechanism of lactaptin entry into the cells allow it to be included into CPP group. Lactaptin, the fragment of human milk kappa-casein, and recombinant lactaptin (RL2) were initially discovered as molecules that induced apoptosis of cultured cancer cells and did not affect non-malignant cells. Here, we analyzed the recombinant lactaptin potency to form complexes with nucleic acids and to act as a gene delivery system. To study RL2-dependent delivery, three type of nucleic acid were used as a models: plasmid DNA (pDNA), siRNA, and non-coding RNA which allow to detect intracellular localization through their functional activity. We have demonstrated that RL2 formed positively charged noncovalent 110-nm-sized complexes with enhanced green fluorescent protein (EGFP)-expressing plasmid DNA. Ca²⁺ ions stabilized these complexes, whereas polyanion heparin displaced DNA from the complexes. The functional activity of delivered nucleic acids were assessed by fluorescent microscopy using A549 lung adenocarcinoma cells and A431 epidermoid carcinoma cells. We observed that RL2:pDNA complexes provided EGFP expression in the treated cells and that strongly confirmed the entering pDNA into the cells. The efficiency of cell transformation by these complexes increased when RL2:pDNA ratio increased. Pre-treatment of the cells with anti-RL2 antibodies partly inhibited the entry of pDNA into the cells. RL2-mediated delivery of siRNA against EGFP was analyzed when A549 cells were co-transfected with EGFP-pDNA and RL2:siRNA complexes. siRNA against EGFP efficiently inhibited the expression of EGFP being delivered as RL2:siRNA complexes. We have previously demonstrated that non-coding U25 small nucleolar RNA (snoRNA) can decrease cell viability. Cancer cell transfection with RL2-snoRNA U25 complexes lead to a substantial decrease of cell viability, confirming the efficiency of snoRNA U25 delivery. Collectively, these findings indicate that recombinant lactaptin is able to deliver noncovalently associated nucleic acids into cancer cells in vitro.

Design and in vitro delivery of HIV-1 multi-epitope DNA and peptide constructs using novel cell-penetrating peptides

OBJECTIVES

Developing an effective HIV vaccine that stimulates the humoral and cellular immune responses is still challenging because of the diversity of HIV-1 virus, polymorphism of human HLA and lack of a suitable delivery system.

RESULTS

Using bioinformatics tools, we designed a DNA construct encoding multiple epitopes. These epitopes were highly conserved within prevalent HIV-1 subtypes and interacted with prevalent class I and II HLAs in Iran and the world. The designed DNA construct included Nef_60-84, Nef_126-144, Vpr_34-47, Vpr_60-75, Gp160_30-53, Gp160_308-323 and P24_8-151 epitopes (i.e., nef-vpr-gp160-p24 DNA) which was cloned into pET-24a(+) and pEGFP-N1 vectors. The recombinant polyepitope peptide (rNef-Vpr-Gp160-P24; ~ 32 kDa) was successfully generated in E. coli expression system. The pEGFP-nef-vpr-gp160-p24 and rNef-Vpr-Gp160-P24 polyepitope peptide were delivered into HEK-293 T cells using cell-penetrating peptides (CPPs). The MPG and HR9 CPPs, as well as the novel LDP-NLS and CyLoP-1 CPPs, were utilized for DNA and peptide delivery into the cells, respectively. SEM results confirmed the formation of stable MPG/pEGFP-N1-nef-vpr-gp160-p24, HR9/pEGFP-N1-nef-vpr-gp160-p24, LDP-NLS/rNef-Vpr-Gp160-P24 and CyLoP-1/rNef-Vpr-Gp160-P24 nanoparticles with a diameter of < 200 nm through non-covalent bonds. MTT assay results indicated that these nanoparticles did not have any major toxicity in vitro. Fluorescence microscopy, flow cytometry and western blot data demonstrated that these CPPs could significantly deliver the DNA and peptide constructs into HEK-293 T cells.

CONCLUSION

The use of these CPPs can be considered as an approach in HIV vaccine development for in vitro and in vivo delivery of DNA and peptide constructs into mammalian cells.

Recent Advances in Cell Penetrating Peptide-Based Anticancer Therapies

Cell-penetrating-peptides (CPPs) are small amino-acid sequences characterized by their ability to cross cellular membranes. They can transport various bioactive cargos inside cells including nucleic acids, large proteins, and other chemical compounds. Since 1988, natural and synthetic CPPs have been developed for applications ranging from fundamental to applied biology (cell imaging, gene editing, therapeutics delivery). In recent years, a great number of studies reported the potential of CPPs as carriers for the treatment of various diseases. Apart from a good efficacy due to a rapid and potent delivery, a crucial advantage of CPP-based therapies is the peptides low toxicity compared to most drug carriers. On the other hand, they are quite unstable and lack specificity. Higher specificity can be obtained using a cell-specific CPP to transport the therapeutic agent or using a non-specific CPP to transport a cargo with a targeted activity. CPP-cargo complexes can also be conjugated to another moiety that brings cell- or tissue-specificity. Studies based on all these approaches are showing promising results. Here, we focus on recent advances in the potential usage of CPPs in the context of cancer therapy, with a particular interest in CPP-mediated delivery of anti-tumoral proteins.

A cell-penetrating peptide-assisted nanovaccine promotes antigen cross-presentation and anti-tumor immune response

Exogenous antigens processed in the cytosol and subsequently cross-presented on major histocompatibility complex class I (MHC-I) molecules activate cytotoxic CD8⁺ lymphocytes (CTL), which are crucial in cancer immunotherapy.

Cell-penetrating Peptides: Efficient Vectors for Vaccine Delivery

Subunit vaccines are composed of pathogen fragments that, on their own, are generally poorly immunogenic. Therefore, the incorporation of an immunostimulating agent, e.g. adjuvant, into vaccine formulation is required. However, there are only a limited number of licenced adjuvants and their immunostimulating ability is often limited, while their toxicity can be substantial. To overcome these problems, a variety of vaccine delivery systems have been proposed. Most of them are designed to improve the stability of antigen in vivo and its delivery into immune cells. Cell-penetrating peptides (CPPs) are especially attractive component of antigen delivery systems as they have been widely used to enhance drug transport into the cells. Fusing or co-delivery of antigen with CPPs can enhance antigen uptake, processing and presentation by antigen presenting cells (APCs), which are the fundamental steps in initiating an immune response. This review describes the different mechanisms of CPP intercellular uptake and various CPP-based vaccine delivery strategies.

Cell penetrating peptides in preclinical and clinical cancer diagnosis and therapy

Delivery of imaging reagents and drugs to tumors is essential for cancer diagnosis and therapy. In addition to therapeutic and diagnostic functionalities, peptides have potential benefits such as biocompatibility, ease to synthesize, smaller size, by-passing off-target side effects, and achieving the beneficial effects with lower-administered dosages. A particular type of peptide known as cell penetrating peptides (CPP) have been predominantly studied during last twenty years as they are not only capable to translocate themselves across membranes but also allow carrier drugs to translocate across plasma membrane, by different mechanisms depending on the CPP. This is of great potential importance in drug delivery systems, as the ability to pass across membranes is crucial to many drug delivery systems. In spite of significant progress in design and application of CPP, more investigations are required to further improve their delivery to tumors, with reduced side-effect and enhanced therapeutic efficacy. In this review, we emphasis on current advancements in preclinical and clinical trials based on using CPP for more efficient delivery of anti-cancer drugs and imaging reagents to cancer tissues and individual cells associated with them. We discuss the evolution of the CPPs-based strategies for targeted delivery, their current status and strengths, along with summarizing the role of CPPs in targeted drug delivery. We also discuss some recently reported diagnostic applications of engineered protease-responsive substrates and activable imaging complexes. We highlight the recent clinical trial data by providing a road map for better design of the CPPs for future preclinical and clinical applications.

Comparing Agent-Based Delivery of DNA and PNA Forced Intercalation (FIT) Probes for Multicolor mRNA Imaging

Fluorogenic oligonucleotide probes allow mRNA imaging in living cells. A key challenge is the cellular delivery of probes. Most delivery agents, such as cell‐penetrating peptides (CPPs) and pore‐forming proteins, require interactions with the membrane. Charges play an important role. To explore the influence of charge on fluorogenic properties and delivery efficiency, we compared peptide nucleic acid (PNA)‐ with DNA‐based forced intercalation (FIT) probes. Perhaps counterintuitively, fluorescence signaling by charged DNA FIT probes proved tolerant to CPP conjugation, whereas CPP–FIT PNA conjugates were affected. Live‐cell imaging was performed with a genetically engineered HEK293 cell line to allow the inducible expression of a specific mRNA target. Blob‐like features and high background were recurring nuisances of the tested CPP and lipid conjugates. By contrast, delivery by streptolysin‐O provided high enhancements of the fluorescence of the FIT probe upon target induction. Notably, DNA‐based FIT probes were brighter and more responsive than PNA‐based FIT probes. Optimized conditions enabled live‐cell multicolor imaging of three different mRNA target sequences.