Strategies for the eradication of intracellular bacterial pathogens.

Intracellular pathogens affect a significant portion of world population and cause millions of deaths each year. They can invade host cells and survive inside them and are extremely resistant to immune systems and antibiotics. Current treatments have limitations, and therefore, new effective therapies are needed to combat this ongoing health challenge. Active research efforts have been made to develop many new strategies to eradicate these intracellular pathogens. In this review, we focus on the intracellular bacterial pathogens and first introduce several representative intracellular bacteria and the diseases they cause. We then discuss the challenges in eradicating these bacteria and summarize the current therapeutics for intracellular bacteria. Finally, recent advances in intracellular bacteria eradication are highlighted.

Leveraging intracellular ALDH1A1 activity for selective cancer stem-like cell labeling and targeted treatment via in vivo click reaction.

Inhibition of overexpressed enzymes is among the most promising approaches for targeted cancer treatment. However, many cancer-expressed enzymes are "nonlethal," in that the inhibition of the enzymes' activity is insufficient to kill cancer cells. Conventional antibody-based therapeutics can mediate efficient treatment by targeting extracellular nonlethal targets but can hardly target intracellular enzymes. Herein, we report a cancer targeting and treatment strategy to utilize intracellular nonlethal enzymes through a combination of selective cancer stem-like cell (CSC) labeling and Click chemistry-mediated drug delivery. A de novo designed compound, AAMCHO [N-(3,4,6-triacetyl- N-azidoacetylmannosamine)-cis-2-ethyl-3-formylacrylamideglycoside], selectively labeled cancer CSCs in vitro and in vivo through enzymatic oxidation by intracellular aldehyde dehydrogenase 1A1. Notably, azide labeling is more efficient in identifying tumorigenic cell populations than endogenous markers such as CD44. A dibenzocyclooctyne (DBCO)-toxin conjugate, DBCO-MMAE (Monomethylauristatin E), could next target the labeled CSCs in vivo via bioorthogonal Click reaction to achieve excellent anticancer efficacy against a series of tumor models, including orthotopic xenograft, drug-resistant tumor, and lung metastasis with low toxicity. A 5/7 complete remission was observed after single-cycle treatment of an advanced triple-negative breast cancer xenograft (~500 mm3).

Hydrophilic nanoparticles that kill bacteria while sparing mammalian cells reveal the antibiotic role of nanostructures.

To dissect the antibiotic role of nanostructures from chemical moieties belligerent to both bacterial and mammalian cells, here we show the antimicrobial activity and cytotoxicity of nanoparticle-pinched polymer brushes (NPPBs) consisting of chemically inert silica nanospheres of systematically varied diameters covalently grafted with hydrophilic polymer brushes that are non-toxic and non-bactericidal. Assembly of the hydrophilic polymers into nanostructured NPPBs doesn't alter their amicability with mammalian cells, but it incurs a transformation of their antimicrobial potential against bacteria, including clinical multidrug-resistant strains, that depends critically on the nanoparticle sizes. The acquired antimicrobial potency intensifies with small nanoparticles but subsides quickly with large ones. We identify a threshold size (dsilica ~ 50 nm) only beneath which NPPBs remodel bacteria-mimicking membrane into 2D columnar phase, the epitome of membrane pore formation. This study illuminates nanoengineering as a viable approach to develop nanoantibiotics that kill bacteria upon contact yet remain nontoxic when engulfed by mammalian cells.

Open-air synthesis of oligo(ethylene glycol)-functionalized polypeptides from non-purified N-carboxyanhydrides.

With PEG-like properties, such as hydrophilicity and stealth effect against protein absorption, oligo(ethylene glycol) (OEG)-functionalized polypeptides have emerged as a new class of biomaterials alternative to PEG with polypeptide-like properties. Synthesis of this class of materials, however, has been demonstrated very challenging, as the synthesis and purification of OEG-functionalized N-carboxyanhydrides (OEG-NCAs) in high purity, which is critical for the success in polymerization, is tedious and often results in low yield. OEG-functionalized polypeptides are therefore only accessible to a few limited labs with expertise in this specialized NCA chemistry and materials. Here, we report the controlled synthesis of OEG-functionalized polypeptides in high yield directly from the OEG-functionalized amino acids via easy and reproducible polymerization of non-purified OEG-NCAs. The prepared amphiphilic block copolypeptides can self-assemble into narrowly dispersed nanoparticles in water, which show properties suitable for drug delivery applications.

Accelerated polymerization of N-carboxyanhydrides catalyzed by crown ether.

The recent advances in accelerated polymerization of N-carboxyanhydrides (NCAs) enriched the toolbox to prepare well-defined polypeptide materials. Herein we report the use of crown ether (CE) to catalyze the polymerization of NCA initiated by conventional primary amine initiators in solvents with low polarity and low hydrogen-bonding ability. The cyclic structure of the CE played a crucial role in the catalysis, with 18-crown-6 enabling the fastest polymerization kinetics. The fast polymerization kinetics outpaced common side reactions, enabling the preparation of well-defined polypeptides using an α-helical macroinitiator. Experimental results as well as the simulation methods suggested that CE changed the binding geometry between NCA and propagating amino chain-end, which promoted the molecular interactions and lowered the activation energy for ring-opening reactions of NCAs. This work not only provides an efficient strategy to prepare well-defined polypeptides with functionalized C-termini, but also guides the design of catalysts for NCA polymerization.

Recent advances in design of antimicrobial peptides and polypeptides toward clinical translation.

The recent outbreaks of infectious diseases caused by multidrug-resistant pathogens have sounded a piercing alarm for the need of new effective antimicrobial agents to guard public health. Among different types of candidates, antimicrobial peptides (AMPs) and the synthetic mimics of AMPs (SMAMPs) have attracted significant enthusiasm in the past thirty years, due to their unique membrane-active antimicrobial mechanism and broad-spectrum antimicrobial activity. The extensive research has brought many drug candidates into clinical and pre-clinical development. Despite tremendous progresses have been made, several major challenges inherent to current design strategies have slowed down the clinical translational development of AMPs and SMAMPs. However, these challenges also triggered many efforts to redesign and repurpose AMPs. In this review, we will first give an overview on AMPs and their synthetic mimics, and then discuss the current status of their clinical translation. Finally, the recent advances in redesign and repurposing AMPs and SMAMPs are highlighted.

"Metaphilic" Cell-Penetrating Polypeptide-Vancomycin Conjugate Efficiently Eradicates Intracellular Bacteria via a Dual Mechanism.

Infections by intracellular pathogens are difficult to treat because of the poor accessibility of antibiotics to the pathogens encased by host cell membranes. As such, a strategy that can improve the membrane permeability of antibiotics would significantly increase their efficiency against the intracellular pathogens. Here, we report the design of an adaptive, metaphilic cell-penetrating polypeptide (CPP)-antibiotic conjugate (VPP-G) that can effectively eradicate the intracellular bacteria both in vitro and in vivo. VPP-G was synthesized by attaching vancomycin to a highly membrane-penetrative guanidinium-functionalized metaphilic CPP. VPP-G effectively kills not only extracellular but also far more challenging intracellular pathogens, such as S. aureus, methicillin-resistant S. aureus, and vancomycin-resistant Enterococci. VPP-G enters the host cell via a unique metaphilic membrane penetration mechanism and kills intracellular bacteria through disruption of both cell wall biosynthesis and membrane integrity. This dual antimicrobial mechanism of VPP-G prevents bacteria from developing drug resistance and could also potentially kill dormant intracellular bacteria. VPP-G effectively eradicates MRSA in vivo, significantly outperforming vancomycin, which represents one of the most effective intracellular antibacterial agents reported so far. This strategy can be easily adapted to develop other conjugates against different intracellular pathogens by attaching different antibiotics to these highly membrane-penetrative metaphilic CPPs.

Environmentally Benign Nanoantibiotics with a Built-in Deactivation Switch Responsive to Natural Habitats.

The massive use of antibiotics in healthcare and agriculture has led to their artificial accumulation in natural habitats, which risks the structure and function of the microbial communities in ecosystems, threatens food and water security, and accelerates the development of resistome. Ideally, antibiotics should remain fully active in clinical services while becoming deactivated rapidly once released into the environment, but none of the current antibiotics meet this criterion. Here, we show a nanoantibiotic design that epitomizes the concept of carrying a built-in "OFF" switch responsive to natural stimuli. The environmentally benign nanoantibiotics consist of cellulose backbones covalently grafted with hydrophilic polymer brushes that by themselves are antimicrobially inactive. In their nanostructured forms in services, these cellulose-based polymer molecular brushes are potent killers for both Gram-positive and Gram-negative bacteria, including clinical multidrug-resistant strains; after services and being discharged into the environment, they are shredded into antimicrobially inactive pieces by cellulases that do not exist in the human body but are abundant in natural habitats. This study illuminates a new concept of mitigating the environmental footprints of antibiotics with rationally designed nanoantibiotics that can be dismantled and disabled by bioorthogonal chemistry occurring exclusively in natural habitats.

Targeting infected host cells in vivo via responsive azido-sugar mediated metabolic cell labeling followed by click reaction.

The early in vivo diagnosis of infectious disease foci is largely hindered by invasion and concealment of pathogens in host cells, making it difficult for conventional probes to detect and analyze intracellular pathogens. Taking advantage of the excessively produced reactive oxygen species (ROS) within host cells, herein we report the design of thiol-hemiketal blocked N-azidoacetyl galactosamine (Ac3GalNAzSP), an azido unnatural sugar bearing an unprecedent designed ROS-responsive moiety for targeted labelling of infected host cells. Ac3GalNAzSP showed great stability under physiological conditions, specifically released active unnatural sugar in host cells overproducing ROS, metabolically labeled infected host cells with azido groups, and enabled targeting in vivo infection sites by subsequent Click Chemistry reactions, substantiating an unprecedented approach for targeting infected host cells. This technique could be a powerful tool for early in vivo diagnosis and targeted treatment of infectious disease.

Polymer Nanodiscs: Discoidal Amphiphilic Block Copolymer Membranes as a New Platform for Membrane Proteins.

Lipid nanodiscs are playing increasingly important roles in studies of the structure and function of membrane proteins. Development of lipid nanodiscs as a membrane-protein-supporting platform, or a drug targeting and delivery vehicle in general, is undermined by the fluidic and labile nature of lipid bilayers. Here, we report the discovery of polymer nanodiscs, i.e., discoidal amphiphilic block copolymer membrane patches encased within membrane scaffold proteins, as a novel two-dimensional nanomembrane that maintains the advantages of lipid nanodiscs while addressing their weaknesses. Using MsbA, a bacterial ATP-binding cassette transporter as a membrane protein prototype, we show that the protein can be reconstituted into the polymer nanodiscs in an active state. As with lipid nanodiscs, reconstitution of detergent-solubilized MsbA into the polymer nanodiscs significantly enhances its activity. In contrast to lipid nanodiscs that undergo time- and temperature-dependent structural changes, the polymer nanodiscs experience negligible structural evolution under similar environmental stresses, revealing a critically important property for the development of nanodisc-based characterization methodologies or biotechnologies. We expect that the higher mechanical and chemical stability of block copolymer membranes and their chemical versatility for adaptation will open new opportunities for applications built upon diverse membrane protein functions, or involved with drug targeting and delivery.

Polymer-encased nanodiscs with improved buffer compatibility.

Styrene-maleic acid copolymers allow for solubilization and reconstitution of membrane proteins into nanodiscs. These polymer-encased nanodiscs are promising platforms for studies of membrane proteins in a near-physiologic environment without the use of detergents. However, current styrene-maleic acid copolymers display severe limitations in terms of buffer compatibility and ensued flexibility for various applications. Here, we present a new family of styrene-maleic acid copolymers that do not aggregate at low pH or in the presence of polyvalent cations, and can be used to solubilize membrane proteins and produce nanodiscs of controlled sizes.

Hydrophilic Phage-Mimicking Membrane Active Antimicrobials Reveal Nanostructure-Dependent Activity and Selectivity.

The prevalent wisdom on developing membrane active antimicrobials (MAAs) is to seek a delicate, yet unquantified, cationic-hydrophobic balance. Inspired by phages that use nanostructured protein devices to invade bacteria efficiently and selectively, we study here the antibiotic role of nanostructures by designing spherical and rod-like polymer molecular brushes (PMBs) that mimic the two basic structural motifs of bacteriophages. Three model PMBs with different well-defined geometries consisting of multiple, identical copies of densely packed poly(4-vinyl-N-methylpyridine iodide) branches are synthesized by controlled/"living" polymerization, reminiscent of the viral structural motifs comprised of multiple copies of protein subunits. We show that, while the individual linear-chain polymer branch that makes up the PMBs is hydrophilic and a weak antimicrobial, amphiphilicity is not a required antibiotic trait once nanostructures come into play. The nanostructured PMBs induce an unusual topological transition of bacterial but not mammalian membranes to form pores. The sizes and shapes of the nanostructures further help define the antibiotic activity and selectivity of the PMBs against different families of bacteria. This study highlights the importance of nanostructures in the design of MAAs with high activity, low toxicity, and target specificity.

Structure and Dynamics of Extracellular Loops in Human Aquaporin-1 from Solid-State NMR and Molecular Dynamics.

Multiple moderate-resolution crystal structures of human aquaporin-1 have provided a foundation for understanding the molecular mechanism of selective water translocation in human cells. To gain insight into the interfacial structure and dynamics of human aquaporin-1 in a lipid environment, we performed nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics simulations. Using magic angle spinning solid-state NMR, we report a near complete resonance assignment of the human aquaporin-1. Chemical shift analysis of the secondary structure identified pronounced deviations from crystallographic structures in extracellular loops A and C, including the cis Y37-P38 bond in loop A, as well as ordering and immobilization of loop C. Site-specific H/D exchange measurements identify a number of protected nitrogen-bearing side chains and backbone amide groups, involved in stabilizing the loops. A combination of molecular dynamics simulations with NMR-derived restraints and filtering based on solvent accessibility allowed for the determination of a structural model of extracellular loops largely consistent with NMR results. The simulations reveal loop stabilizing interactions that alter the extracellular surface of human AQP1, with possible implications for water transport regulation through the channel. Modulation of water permeation may occur as a result of rearrangement of side chains from loop C in the extracellular vestibule of hAQP1, affecting the aromatic arginine selectivity filter.

A Usual G-Protein-Coupled Receptor in Unusual Membranes.

G-protein-coupled receptors (GPCRs) are the largest family of membrane-bound receptors and constitute about 50% of all known drug targets. They offer great potential for membrane protein nanotechnologies. We report here a charge-interaction-directed reconstitution mechanism that induces spontaneous insertion of bovine rhodopsin, the eukaryotic GPCR, into both lipid- and polymer-based artificial membranes. We reveal a new allosteric mode of rhodopsin activation incurred by the non-biological membranes: the cationic membrane drives a transition from the inactive MI to the activated MII state in the absence of high [H(+)] or negative spontaneous curvature. We attribute this activation to the attractive charge interaction between the membrane surface and the deprotonated Glu134 residue of the rhodopsin-conserved ERY sequence motif that helps break the cytoplasmic "ionic lock". This study unveils a novel design concept of non-biological membranes to reconstitute and harness GPCR functions in synthetic systems.

Small-Anion Selective Transmembrane "Holes" Induced by an Antimicrobial Peptide Too Short to Span Membranes.

Whereas many membrane-destabilization modes have been suggested for membrane-spanning antimicrobial peptides (AMPs), few are available for those too short to span membrane thickness. Here we show that ORB-1, a 15-residue disulfide-bridged AMP that is only ∼20 Å long even when fully stretched like a hairpin, may act by inducing small anion-selective transmembrane "holes" of negative mean curvature. In model membranes of Gram-negative bacteria, ORB-1 induces chloride transmembrane transport and formation of transmembrane channels of negative mean curvature, whereas the inactive analogue, ORB-N, does not, suggesting a correlation between antibacterial activity and ability to induce transmembrane channels. Given that ORB-N is the C-terminus amidated form of ORB-1, our results further suggest that formation of membrane-spanning dimers may be required to initiate the observed channel induction. Moreover, ORB-1 renders model bacterial membranes permeable to anions with effective hydration diameters of <1 nm (e.g., Cl(-) and NO3(-)), but not cations of similar sizes (e.g., H3O(+)), indicative of anion-selective transmembrane channels with an effective inner diameter of ≤1 nm. In addition, negative-intrinsic-curvature (NIC) lipids such as phosphoethanolamine (PE) may facilitate the membrane-destabilization process of ORB-1. Our findings may expand current understandings on how AMPs destabilize membranes and facilitate the pharmaceutical development of ORB-1.

In situ structural studies of Anabaena sensory rhodopsin in the E. coli membrane.

Magic-angle spinning nuclear magnetic resonance is well suited for the study of membrane proteins in the nativelike lipid environment. However, the natural cellular membrane is invariably more complex than the proteoliposomes most often used for solid-state NMR (SSNMR) studies, and differences may affect the structure and dynamics of the proteins under examination. In this work we use SSNMR and other biochemical and biophysical methods to probe the structure of a seven-transmembrane helical photoreceptor, Anabaena sensory rhodopsin (ASR), prepared in the Escherichia coli inner membrane, and compare it to that in a bilayer formed by DMPC/DMPA lipids. We find that ASR is organized into trimers in both environments but forms two-dimensional crystal lattices of different symmetries. It favors hexagonal packing in liposomes, but may form a square lattice in the E. coli membrane. To examine possible changes in structure site-specifically, we perform two- and three-dimensional SSNMR experiments and analyze the differences in chemical shifts and peak intensities. Overall, this analysis reveals that the structure of ASR is largely conserved in the inner membrane of E. coli, with many of the important structural features of rhodopsins previously observed in ASR in proteoliposomes being preserved. Small, site-specific perturbations in protein structure that occur as a result of the membrane changes indicate that the protein can subtly adapt to its environment without large structural rearrangement.

Long hydrophilic-and-cationic polymers: a different pathway toward preferential activity against bacterial over mammalian membranes.

We show that simply converting the hydrophobic moiety of an antimicrobial peptide (AMP) or synthetic mimic of AMPs (SMAMP) into a hydrophilic one could be a different pathway toward membrane-active antimicrobials preferentially acting against bacteria over host cells. Our biostatistical analysis on natural AMPs indicated that shorter AMPs tend to be more hydrophobic, and the hydrophilic-and-cationic mutants of a long AMP experimentally demonstrated certain membrane activity against bacteria. To isolate the effects of antimicrobials' hydrophobicity and systematically examine whether hydrophilic-and-cationic mutants could inherit the membrane activity of their parent AMPs/SMAMPs, we constructed a minimal prototypical system based on methacrylate-based polymer SMAMPs and compared the antibacterial membrane activity and hemolytic toxicity of analogues with and without the hydrophobic moiety. Antibacterial assays showed that the hydrophobic moiety of polymer SMAMPs consistently promoted the antibacterial activity but diminished in effectiveness for long polymers, and the resultant long hydrophilic-and-cationic polymers were also membrane active against bacteria. What distinguished these long mutants from their parent SMAMPs were their drastically reduced hemolytic toxicities and, as a result, strikingly enhanced selectivity. Similar toxicity reduction was observed with the hydrophilic-and-cationic mutants of long AMPs. Taken together, our results suggest that long hydrophilic-and-cationic polymers could offer preferential membrane activity against bacteria over host cells, which may have implications in future antimicrobial development.

"Frozen" block copolymer nanomembranes with light-driven proton pumping performance.

Cellular membranes are natural nanoengineering devices, where matter transport, information processing, and energy conversion across the nanoscale boundaries are mediated by membrane proteins (MPs). Despite the great potential of MPs for nanotechnologies, their broad utility in engineered systems is limited by the fluidic and often labile nature of MP-supporting membranes. Little is known on how to direct spontaneous reconstitution of MPs into robust synthetic nanomembranes or how to tune MP functions through rational design of these membranes. Here we report that proteorhodopsin (PR), a light-driven proton pump, can be spontaneously reconstituted into "frozen" (i.e., glassy state) amphiphilic block copolymer membranes via a charge-interaction-directed reconstitution mechanism. We show that PR is not enslaved by a fluidic or lipid-based membrane environment. Rather, well-defined block copolymer nanomembranes, with their tunable membrane moduli, act as allosteric regulators to support the structural integrity and function of PR. Versatile membrane designs exist to modulate the conformational energetics of reconstituted MPs, therefore optimizing proteomembrane stability and performance in synthetic systems.

Interface for Light-Driven Electron Transfer by Photosynthetic Complexes Across Block Copolymer Membranes.

Incorporation of membrane proteins into nanodevices to mediate recognition and transport in a collective and scalable fashion remains a challenging problem. We demonstrate how nanoscale photovoltaics could be designed using robust synthetic nanomembranes with incorporated photosynthetic reaction centers (RCs). Specifically, RCs from Rhodobacter sphaeroides are reconstituted spontaneously into rationally designed polybutadiene membranes to form hierarchically organized proteopolymer membrane arrays via a charge-interaction-directed reconstitution mechanism. Once incorporated, the RCs are fully active for prolonged periods based upon a variety of spectroscopic measurements, underscoring preservation of their 3D pigment configuration critical for light-driven charge transfer. This result provides a strategy to construct solar conversion devices using structurally versatile proteopolymer membranes with integrated RC functions to harvest broad regions of the solar spectrum.