The Effect of Phosphatidylserine on a pH-Responsive Peptide Is Defined by Its Noninserting End

Tumor-targeted liposomes with platycodin D2 promote apoptosis in colorectal cancer

Conventional chemotherapy for colorectal cancer (CRC), though efficacious, is discouraging due to its limited targeting capability, lack of selectivity, and chemotherapy-associated side effects. With the advent of nanomedicines, a liposomal delivery system making use of a combination of anticancer phytochemicals is fast gaining popularity as one of the most promising nanoplatforms for CRC treatment. Rising evidence supports phytochemicals such as platycosides for their anticancer potency. To this end, a combination therapy including tumor-targeted liposomes along with phytochemicals might have a greater therapeutic potential against cancer. In this study, we developed acidity-triggered rational membrane (ATRAM) along with conjugated platycodin D2 (PCD2) and liposomes (PCD2-Lipo-ATRAM) as a tumor-targeting therapy. The PCD2-Lipo-ATRAM treatment demonstrated a successful tumor-targeting ability in the CRC xenografts, in which PCD2 not only exerted a potent antitumor effect by inducing apoptotic cell death and but also functioned as a liposome membrane stabilizer. Moreover, PCD2-Lipo-ATRAM suppressed antiapoptotic BCL-2 family proteins, resulting in enhanced cytotoxicity toward CRC cells by inducing intrinsic caspase-9/-3 mediated apoptosis. Thus, our data has shown that tumor-targeting PCD2-based liposomal systems represent a promising strategy for CRC therapy, since they directly target the tumors, unlike other therapies that can miss the target.

The Candida albicans virulence factor candidalysin polymerizes in solution to form membrane pores and damage epithelial cells

Candida albicans causes severe invasive candidiasis. C. albicans infection requires the virulence factor candidalysin (CL) which damages target cell membranes. However, the mechanism that CL uses to permeabilize membranes is unclear. We reveal that CL forms membrane pores using a unique mechanism. Unexpectedly, CL readily assembled into polymers in solution. We propose that the basic structural unit in polymer formation is a CL oligomer, which is sequentially added into a string configuration that can close into a loop. CL loops appear to spontaneously insert into the membrane to become pores. A CL mutation (G4W) inhibited the formation of polymers in solution and prevented pore formation in synthetic lipid systems. Epithelial cell studies showed that G4W CL failed to activate the danger response pathway, a hallmark of the pathogenic effect of CL. These results indicate that CL polymerization in solution is a necessary step for the damage of cellular membranes. Analysis of CL pores by atomic force microscopy revealed co-existence of simple depressions and more complex pores, which are likely formed by CL assembled in an alternate oligomer orientation. We propose that this structural rearrangement represents a maturation mechanism that stabilizes pore formation to achieve more robust cellular damage. To summarize, CL uses a previously unknown mechanism to damage membranes, whereby pre-assembly of CL loops in solution leads to formation of membrane pores. Our investigation not only unravels a new paradigm for the formation of membrane pores, but additionally identifies CL polymerization as a novel therapeutic target to treat candidiasis.

Spiers Memorial Lecture: Analysis and de novo design of membrane-interactive peptides

Membrane-peptide interactions play critical roles in many cellular and organismic functions, including protection from infection, remodeling of membranes, signaling, and ion transport. Peptides interact with membranes in a variety of ways: some associate with membrane surfaces in either intrinsically disordered conformations or well-defined secondary structures. Peptides with sufficient hydrophobicity can also insert vertically as transmembrane monomers, and many associate further into membrane-spanning helical bundles. Indeed, some peptides progress through each of these stages in the process of forming oligomeric bundles. In each case, the structure of the peptide and the membrane represent a delicate balance between peptide-membrane and peptide-peptide interactions. We will review this literature from the perspective of several biologically important systems, including antimicrobial peptides and their mimics, α-synuclein, receptor tyrosine kinases, and ion channels. We also discuss the use of de novo design to construct models to test our understanding of the underlying principles and to provide useful leads for pharmaceutical intervention of diseases.

Expanding MPEx Hydropathy Analysis to Account for Electrostatic Contributions to Protein Interactions with Anionic Membranes

Hydropathy plots are a crucial tool to guide experimental design, as they generate predictions of protein-membrane interactions and their bilayer topology. The predictions are based on experimentally determined hydrophobicity scales, which provide an estimate for the propensity and stability of these interactions. A significant improvement to the accuracy of hydropathy analyses was provided by the development of the popular Wimley-White interfacial and octanol hydrophobicity scales. These scales have been previously incorporated into the freely available MPEx (Membrane Protein Explorer) online application. Here, we introduce a substantial update to MPEx that allows for the consideration of electrostatic contributions to the bilayer partitioning free energy. This component originates from the Coulombic attraction or repulsion of charges between proteins and membranes. Its inclusion in hydropathy calculations increases the accuracy of hydropathy plot predictions and extends their use to more complex systems (i.e., anionic membranes). We illustrate the application of this analysis to studies on the membrane selectivity of antimicrobial peptides, the membrane partitioning of ion-channel gating modifiers, and the amyloid proteins α-synuclein and Tau, as well as pH-dependent bilayer interactions of diphtheria toxin and apoptotic inhibitor Bcl-xL.

Improved growth of Escherichia coli in aminoglycoside antibiotics by the zor-orz toxin-antitoxin system

Type I toxin-antitoxin systems consist of a small protein (under 60 amino acids) whose overproduction can result in cell growth stasis or death, and a small RNA that represses translation of the toxin mRNA. Despite their potential toxicity, type I toxin proteins are increasingly linked to improved survival of bacteria in stressful environments and antibiotic persistence. While the interaction of toxin mRNAs with their cognate antitoxin sRNAs in some systems are well characterized, additional translational control of many toxins and their biological roles are not well understood. Using an ectopic overexpression system, we show that the efficient translation of a chromosomally encoded type I toxin, ZorO, requires mRNA processing of its long 5' untranslated region (UTR; Δ28 UTR). The severity of ZorO induced toxicity on growth inhibition, membrane depolarization, and ATP depletion were significantly increased if expressed from the Δ28 UTR versus the full-length UTR. ZorO did not form large pores as evident via a liposomal leakage assay, in vivo morphological analyses, and measurement of ATP loss. Further, increasing the copy number of the entire zor-orz locus significantly improved growth of bacterial cells in the presence of kanamycin and increased the minimum inhibitory concentration against kanamycin and gentamycin; however, no such benefit was observed against other antibiotics. This supports a role for the zor-orz locus as a protective measure against specific stress agents and is likely not part of a general stress response mechanism. Combined, these data shed more insights into the possible native functions for type I toxin proteins. IMPORTANCE Bacterial species can harbor gene pairs known as type I toxin-antitoxin systems where one gene encodes a small protein that is toxic to the bacteria producing it and a second gene that encodes a small RNA antitoxin to prevent toxicity. While artificial overproduction of type I toxin proteins can lead to cell growth inhibition and cell lysis, the endogenous translation of type I toxins appears to be tightly regulated. Here, we show translational regulation controls production of the ZorO type I toxin and prevents subsequent negative effects on the cell. Further, we demonstrate a role for zorO and its cognate antitoxin in improved growth of E. coli in the presence of aminoglycoside antibiotics.

PIP2 promotes conformation-specific dimerization of the EphA2 membrane region

The impact of the EphA2 receptor on cancer malignancy hinges on the two different ways it can be activated. EphA2 induces antioncogenic signaling after ligand binding, but ligand-independent activation of EphA2 is pro-oncogenic. It is believed that the transmembrane (TM) domain of EphA2 adopts two alternate conformations in the ligand-dependent and the ligand-independent states. However, it is poorly understood how the difference in TM helical crossing angles found in the two conformations impacts the activity and regulation of EphA2. We devised a method that uses hydrophobic matching to stabilize two conformations of a peptide comprising the EphA2 TM domain and a portion of the intracellular juxtamembrane (JM) segment. The two conformations exhibit different TM crossing angles, resembling the ligand-dependent and ligand-independent states. We developed a single-molecule technique using styrene maleic acid lipid particles to measure dimerization in membranes. We observed that the signaling lipid PIP2 promotes TM dimerization, but only in the small crossing angle state, which we propose corresponds to the ligand-independent conformation. In this state the two TMs are almost parallel, and the positively charged JM segments are expected to be close to each other, causing electrostatic repulsion. The mechanism PIP2 uses to promote dimerization might involve alleviating this repulsion due to its high density of negative charges. Our data reveal a conformational coupling between the TM and JM regions and suggest that PIP2 might directly exert a regulatory effect on EphA2 activation in cells that is specific to the ligand-independent conformation of the receptor.

pH-responsive high stability polymeric nanoparticles for targeted delivery of anticancer therapeutics

The practical application of nanoparticles (NPs) as chemotherapeutic drug delivery systems is often hampered by issues such as poor circulation stability and targeting inefficiency. Here, we have utilized a simple approach to prepare biocompatible and biodegradable pH-responsive hybrid NPs that overcome these issues. The NPs consist of a drug-loaded polylactic-co-glycolic acid (PLGA) core covalently 'wrapped' with a crosslinked bovine serum albumin (BSA) shell designed to minimize interactions with serum proteins and macrophages that inhibit target recognition. The shell is functionalized with the acidity-triggered rational membrane (ATRAM) peptide to facilitate internalization specifically into cancer cells within the acidic tumor microenvironment. Following uptake, the unique intracellular conditions of cancer cells degrade the NPs, thereby releasing the chemotherapeutic cargo. The drug-loaded NPs showed potent anticancer activity in vitro and in vivo while exhibiting no toxicity to healthy tissue. Our results demonstrate that the ATRAM-BSA-PLGA NPs are a promising targeted cancer drug delivery platform.