Haifan Wu

Improving the Stability of Aptamers by Chemical Modification

Ever since the invention of SELEX (systematic evolution of ligands by exponential enrichment), there has been rapid development for aptamers over the last two decades, making them a promising approach in therapeutic applications as either drug candidates or diagnostic tools. For therapeutic purposes, a durable performance of aptamers in biofluids is required, which is, however, hampered by the lack of stability of most aptamers. Not only are the nucleic acid aptamers susceptible to nucleases, the peptide aptamers are also subjective to degradation by proteases. With the advancement of chemical biology, numerous attempts have been made to overcome this obstacle, many resulting in significant improvements in stability. In this review, chemical modifications to increase the stability of three main types of aptamers, DNA, RNA and peptide are comprehensively summarized. For nucleic acid aptamers, development of modified SELEX coupled with mutated polymerase is discussed, which is adaptive to a number of modifications in aptamers and in a large extent facilitates the research of aptamer-modifications. For peptide aptamers, approaches in molecular biology with introduction of stabilizing protein as well as the switch of scaffold protein are included, which may represent a future direction of chemical conjugations to aptamers.

Inserting CO2 into Aryl C−H Bonds of Metal–Organic Frameworks: CO2 Utilization for Direct Heterogeneous C−H Activation

Described for the first time is that carbon dioxide (CO2 ) can be successfully inserted into aryl C-H bonds of the backbone of a metal-organic framework (MOF) to generate free carboxylate groups, which serve as Brønsted acid sites for efficiently catalyzing the methanolysis of epoxides. The work delineates the very first example of utilizing CO2 for heterogeneous C-H activation and carboxylation reactions on MOFs, and opens a new avenue for CO2 chemical transformations under mild reaction conditions.

Lipo-γ-AApeptides as a New Class of Potent and Broad-Spectrum Antimicrobial Agents

There is increasing demand to develop antimicrobial peptides (AMPs) as next generation antibiotic agents, as they have the potential to circumvent emerging drug resistance against conventional antibiotic treatments. Non-natural antimicrobial peptidomimetics are an ideal example of this, as they have significant potency and in vivo stability. Here we report for the first time the design of lipidated γ-AApeptides as antimicrobial agents. These lipo-γ-AApeptides show potent broad-spectrum activities against fungi and a series of Gram-positive and Gram-negative bacteria, including clinically relevant pathogens that are resistant to most antibiotics. We have analyzed their structure-function relationship and antimicrobial mechanisms using membrane depolarization and fluorescent microscopy assays. Introduction of unsaturated lipid chain significantly decreases hemolytic activity and thereby increases the selectivity. Furthermore, a representative lipo-γ-AApeptide did not induce drug resistance in S. aureus, even after 17 rounds of passaging. These results suggest that the lipo-γ-AApeptides have bactericidal mechanisms analogous to those of AMPs and have strong potential as a new class of novel antibiotic therapeutics.

Identification of γ-AApeptides with potent and broad-spectrum antimicrobial activity

We report the identification of a new class of antimicrobial peptidomimetics-γ-AApeptides with potent and broad-spectrum activity, including clinically-relevant strains that are unresponsive to most antibiotics. They are also not prone to select for drug-resistance.

Development of self-immolative dendrimers for drug delivery and sensing

Traditional dendrimers possess unique cascade-branched structural properties that allow for multivalent modifications with drug cargos, targeting/delivery agents and imaging tools. In addition to multivalency, the dendrimers macromolecular size also brings about the enhanced permeability and retention (EPR) effect, which makes it an attracting agent for drug delivery and biosensing. Similar to other macromolecules, therapeutic application of dendrimers in the human body faces practical challenges such as target specificity and toxicity. The latter represents a substantial issue due to the dendrimers unnatural chemical structure and relatively large size, which prohibit its in vivo degradation and excretion from the body. To date, a class of self-immolative dendrimers has been developed to overcome these obstacles, which takes advantage of its unique structural backbone to allow for cascade decompositions upon a simple triggering event. The specific drug release can be achieved through a careful design of the trigger, and as a result of the fragmentation, the generated small molecules are either biodegradable or easily excreted from the body. Though still at a preliminary stage, the development of this novel approach represents an important direction in nanoparticle-mediated drug delivery and sensor design, thereby opening up an insightful frontier of dendrimer based applications.

Lipidated cyclic γ-AApeptides display both antimicrobial and anti-inflammatory activity.

Antimicrobial peptides (AMPs) are host-defense agents capable of both bacterial membrane disruption and immunomodulation. However, the development of natural AMPs as potential therapeutics is hampered by their moderate activity and susceptibility to protease degradation. Herein we report lipidated cyclic γ-AApeptides that have potent antibacterial activity against clinically relevant Gram-positive and Gram-negative bacteria, many of which are resistant to conventional antibiotics. We show that lipidated cyclic γ-AApeptides mimic the bactericidal mechanism of AMPs by disrupting bacterial membranes. Interestingly, they also harness the immune response and inhibit lipopolysaccharide (LPS) activated Toll-like receptor 4 (TLR4) signaling, suggesting that lipidated cyclic γ-AApeptides have dual roles as novel antimicrobial and anti-inflammatory agents.

γ-AApeptides bind to RNA by mimicking RNA-binding proteins.

The interactions between proteins and RNAs are of vital importance for many cellular processes, including transcription and processing of RNA, translation, and viral infections. Here we report an γ-AApeptide that can mimic HIV-1 Tat protein and bind to TAR RNAs of HIV and BIV with nanomolar affinity, comparable to that of the RNA-binding fragment of Tat (amino acids 49-58). The interaction is resistant to the presence of a large excess of tRNA. With resistance to proteolytic hydrolysis and limitless potential for diversification, γ-AApeptides may emerge as a new class of peptidomimetics to modulate RNA-protein interactions.

Helical Antimicrobial Sulfono-γ-AApeptides

Host-defense peptides (HDPs) such as magainin 2 have emerged as potential therapeutic agents combating antibiotic resistance. Inspired by their structures and mechanism of action, herein we report the first example of antimicrobial helical sulfono-γ-AApeptide foldamers. The lead molecule displays broad-spectrum and potent antimicrobial activity against multi-drug-resistant Gram-positive and Gram-negative bacterial pathogens. Time-kill studies and fluorescence microscopy suggest that sulfono-γ-AApeptides eradicate bacteria by taking a mode of action analogous to that of HDPs. Clear structure-function relationships exist in the studied sequences. Longer sequences, presumably adopting more-defined helical structures, are more potent than shorter ones. Interestingly, the sequence with less helical propensity in solution could be more selective than the stronger helix-forming sequences. Moreover, this class of antimicrobial agents are resistant to proteolytic degradation. These results may lead to the development of a new class of antimicrobial foldamers combating emerging antibiotic-resistant pathogens.

Recent development of small antimicrobial peptidomimetics

Antimicrobial peptides (AMPs) hold promise to circumvent the emergence of drug resistance occurring in the treatment of bacteria using many conventional antibiotics. Antimicrobial peptidomimetics, which mimic bactericidal mechanisms of AMPs, may overcome the disadvantages of AMPs and become the new generation of antibiotic therapeutics. In this review, some recent examples in the development of antimicrobial peptidomimetics are highlighted. The potential of antimicrobial agents has been demonstrated for therapeutic uses. Meanwhile, perspectives on their further development and applications are also presented.

Design and synthesis of unprecedented cyclic γ-AApeptides for antimicrobial development

Antimicrobial drug resistance is one of the greatest threats facing mankind. Antimicrobial peptides (AMPs) can potentially circumvent drug resistance, probably through a bacterial membrane-disruption mechanism. However, they suffer from low in vivo stability, potential immunogenicity, and difficulty in optimization. The development of antimicrobial peptidomimetics is therefore an emerging research area as they avoid the potential disadvantages of AMPs. Cyclic peptidomimetics are of significant interest since constraints induced by cyclization are expected to further improve their antimicrobial activity. Nonetheless, the report of cyclic oligomeric peptidomimetics for antimicrobial development is rare. Herein, for the first time, we report the design and synthesis of cyclic γ-AApeptides via an on-resin cyclization. These cyclic γ-AApeptides are potent and broad-spectrum active against fungus and multi-drug resistant Gram-positive and Gram-negative bacterial pathogens. Our results demonstrate the potential of cyclic γ-AApeptides as a new class of antibiotics to circumvent drug resistance by mimicking the bactericidal mechanism of AMPs. Meanwhile, the facile synthesis of cyclic γ-AApeptides may further expand the applications of γ-AApeptides in biomedical sciences.