Genetic Fusion of Transacting Activator of Transcription Peptide to Cyclized Green Fluorescence Protein Improves Stability, Intracellular Delivery, and Tumor Retention

Construction of a Cyclic Regular-Triangle Trimer of Cytochrome c555 with a Central Hole Using Sortase A

Protein-based supramolecules require precise arrangement of building blocks. A regular-triangle trimer (cp-c555)3 has been constructed using an α-helix-inserted-circular permutant (cp-c555) of Aquifex aeolicus cytochrome (cyt) c555, where the trimers may dissociate to monomers. In this study, we stabilized the regular-triangle structure by constructing a cyclic regular-triangle of three α-helix-linked cyt c555 molecules using sortase-mediated ligation (SML). Comparing SML using sortase A for six cp-c555 variant trimers, the variant with GGG at the N-terminus and LPETG at the C-terminus reacted most efficiently. OP-(c555)3 was designed, in which two cyt c555 molecules were fused using an α-helix, generating a dimer. The cyt c555 C-terminal region was attached to the N-terminus of the dimer, and the cyt c555 N-terminal region was attached to the C-terminus of the dimer using the same α-helix. OP-(c555)3 was expressed in Escherichia coli cells, and the termini were connected by SML, forming a cyclic regular-triangle, CL-(c555)3. CL-(c555)3 showed higher thermostability than (cp-c555)3 and OP-(c555)3. CL-(c555)3 structural stability was confirmed using high-speed atomic force microscopy. The X-ray crystal structure of CL-(c555)3 showed a cyclic structure and a nanoporous supramolecular assembly. These results demonstrate that a nanoporous supramolecular assembly can be constructed by designing a cyclic molecule with a central hole using SML.

Applications of cell penetrating peptide-based drug delivery system in immunotherapy

Cell penetrating peptides (CPPs) are usually positive charged peptides and have good cell membrane permeability. Meanwhile, CPPs are facile to synthesize, and can be functionalized to satisfy different demands, such as cyclization, incorporating unnatural amino acids, and lipid conjugation. These properties have made them as efficient drug-delivery tools to deliver therapeutic molecules to cells and tissues in a nontoxic manner, including small molecules, DNA, siRNA, therapeutic proteins and other various nanoparticles. However, the poor serum stability and low tumor targeting ability also hindered their broad application. Besides, inappropriate chemical modification can lead to membrane disruption and nonspecific toxicity. In this paper, we first reviewed recent advances in the CPP applications for cancer therapy via covalent or non-covalent manners. We carefully analyzed the advantages and disadvantages of each CPP modifications for drug delivery. Then, we concluded the recent progress of their clinical trials for different diseases. Finally, we discussed the challenges and opportunities CPPs met to translate into clinical applications. This review presented a new insight into CPPs for drug delivery, which could provide advice on the design of clinically effective systemic delivery systems using CPPs.

Covalent conjugation and non-covalent complexation strategies for intracellular delivery of proteins using cell-penetrating peptides

Therapeutic proteins provided new opportunities for patients and high sales volumes. However, they are formulated for extracellular targets. The lipophilic barrier of the plasma membrane renders the vast array of intracellular targets out of reach. Peptide-based delivery systems, namely cell-penetrating peptides (CPPs), have few safety concerns, and low immunogenicity, with control over administered doses. This study investigates CPP-based protein delivery systems by classifying them into CPP-protein "covalent conjugation" and CPP: protein "non-covalent complexation" categories. Covalent conjugates ensure the proximity of the CPP to the cargo, which can improve cellular uptake and endosomal escape. We will discuss various aspects of covalent conjugates through non-cleavable (stable) or cleavable bonds. Non-cleavable CPP-protein conjugates are produced by recombinant DNA technology to express the complete fusion protein in a host cell or by chemical ligation of CPP and protein, which ensures stability during the delivery process. CPP-protein cleavable bonds are classified into pH-sensitive and redox-sensitive bonds, enzyme-cleavable bonds, and physical stimuli cleavable linkers (light radiation, ultrasonic waves, and thermo-responsive). We have highlighted the key characteristics of non-covalent complexes through electrostatic and hydrophobic interactions to preserve the conformational integrity of the CPP and cargo. CPP-mediated protein delivery by non-covalent complexation, such as zippers, CPP adaptor methods, and avidin-biotin technology, are featured. Conclusively, non-covalent complexation methods are appropriate when a high number of CPP or protein samples are to be screened. In contrast, when the high biological activity of the protein is critical in the intracellular compartment, conjugation protocols are preferred.

Sortase-mediated labeling: Expanding frontiers in site-specific protein functionalization opens new research avenues

New applications for biomolecules demand novel approaches for their synthesis and modification. Traditional methods for modifying proteins and cells using non-specific labeling chemistry are insufficiently precise to rigorously interrogate the mechanistic biological and physiological questions at the forefront of biomedical science. Site-specific catalytic modification of proteins promises to meet these challenges. Here, we describe recent applications of the enzyme sortase A in facilitating precise biomolecule labeling. We focus on describing new chemistries to broaden the scope of sortase-mediated labeling (sortagging), the development of new probes for imaging via enzymatic labeling, and the modulation of biological systems using probes and reactions mediated by sortase.

Cyclized proteins with tags as permeable and stable cargos for delivery into cells and liposomes

Despite the therapeutic potential of recombinant proteins, their cell permeabilities and stabilities remain significant challenges. Here we demonstrate that cyclized recombinant proteins can be used as universal cargos for permeable and stable delivery into cells and polydiacetylene liposomes. Utilizing a split intein-mediated process, cyclized model fluorescent proteins containing short tetraarginine (R4) and hexahistidine (H6) tags were generated without compromising their native protein functions. Strikingly, as compared to linear R4/H6-tagged proteins, the cyclized counterparts have substantially increased permeabilities in both cancer cells and synthetic liposomes, as well as higher resistances to enzymatic degradation in cancer cells. These properties are likely a consequence of structural constraints imposed on the proteins in the presence of short functional peptides. Additionally, photodynamic therapy by cyclized photoprotein-loaded liposomes in cancer cells was significantly improved in comparison to that by their non-cyclized counterparts. These findings suggest that our strategy will be universally applicable to intercellular delivery of proteins and therapeutics.