Phage display and targeting peptides: surface functionalization of nanocarriers for delivery of small non-coding RNAs

摘要

Introduction Small non-coding RNAs are known as a clinically relevant category of non-coding RNAs (ncRNAs) that have gained growing attention for their therapeutic values. miRNA and siRNA are of the most important and well-documented types of small ncRNAs. These regulatory RNA molecules are short–approximately 22 nucleotides long–with a similar mechanism of generation in which they are excised from longer double-stranded RNA precursor molecules. Both miRNA and siRNA target protein-coding genes via an antisense-based strategy and follow the same processing manner to reach gene-silencing effect (Chen et al., 2015 ). Functionally, small ncRNAs play a determining part in the modulation of cellular gene expression. The impressive role of these molecules as the controlling machinery of cells has proposed promising capacities for their potential translation into the clinic. In this context, tuning the regulatory components (such as ncRNAs) rather than the regulated components (protein-coding gens) has been suggested to be a more convenient and more effective approach to correct many cellular dysfunctions (Taft et al., 2010 ). This emerging concept symbolizes the dramatic potential of small ncRNAs, as a significant part of the regulatory apparatus of cells, to treat human diseases. Small ncRNA delivery: active targeting and the dream of magic bullet Within the recent years, nanotechnology-based approaches have emerged as potentially powerful and efficient systems for cellular delivery of ncRNAs. The exploitation of nanocarriers shows promise to bypass a variety of biological barriers for systematically administered RNA therapeutics (Miele et al., 2012 ; Segovia et al., 2015 ). Nanoparticles provide protection from serum nucleases for RNA-based therapeutic molecules, decrease their sequestration by phagocytes of the reticulo-endothelial system (RES) and are able to carry both hydrophilic and hydrophobic substances (Aagaard and Rossi, 2007 ; Peer et al., 2007 ). The excellent characteristics of nanocarrier platforms aid in enhancing overall bioavailability, extending in vivo stability, and improving the cellular delivery of RNA therapeutics such as small ncRNAs. For the successful delivery of small ncRNA, one of the most important problems that needs to be solved is targeting of these drugs into the desired sites. The delivery of small ncRNAs to the diseased cells, tissues or organs not only raises their silencing potency, but also leads to a considerable reduction of side effects by avoiding normal cells (Daka and Peer, 2012 ). Active targeting plays a highly critical part in achieving efficient in vivo biodistribution and optimized systemic delivery of ncRNA-based therapeutics. Active targeting becomes possible when the surface of nanocarriers that are embedded with small ncRNAs is modified through the covalent attachment of a cell/tissue-specific ligand. These surface-conjugated ligands will specifically interact with receptors that are expressed on the surface of target cells (Bertrand et al., 2014 ). In this manner, the delivery of therapeutic small ncRNA molecules will be poised to materialize the dream of constructing magic bullet; the concept that was theoretically fathered by Paul Ehrlich to describe targeted transportation of therapeutic agents to the desired sites of the body. Peptides are known as one of the most interesting categories of targeting ligands that have recently received huge attention. Due to some unique properties, peptides have become favorable targeting ligands for formulating powerful and sophisticated drug delivery platforms. Phage display libraries: a treasure of cell-targeting peptides Phage display that is defined as the ability to genetically modify bacteriophages for introducing specific amino acids onto their surface is a worthwhile tool in biomedicine. The birth of phage display, as a kind of virus-based molecular chimera, goes back to three decades ago when Smith for the first time reported the expression of a foreign polypeptide on the surface of phage particles (Smith, 1985 ). The power of phage display for gaining momentum as a revolutionary strategy for gene/drug delivery studies comes from two distinctive features: (i) establishing a physical connection between the phenotype (the displayed ligand) and the genotype (the DNA sequence encoding the displayed ligand) within the same viral particle and (ii) producing very large libraries of ligands displayed on the surface of the phage particles. The most widely-used phage display libraries are random peptide libraries (RPL). In simple terms, a random peptide library is a collection of phages carrying on their surface extremely diverse types of peptides. Due to the phenotype-genotype link, phage peptide libraries can be screened against any target of interest for selecting target-specific binders. This screening strategy displays similarity to the approach of natural selection but with distinctive difference of being performed i