Determinants of membrane activity from mutational analysis of the HIV fusion peptide

On the role of surrounding regions in the fusion peptide in dengue virus infection

Dengue virus infection depends on its fusion with the host membrane, where the binding occurs through interaction between proteins on the virus cell surface and specific viral receptors on target membranes. This process is mediated by the fusion peptide located between residues 98 and 112 (DRGWGNGCGLFGKGG) that forms a loop in domain II of dengue E glycoprotein. In this study, we evaluated the role of fusion peptide surrounding regions (88-97 and 113-123) of the Dengue 2 subtype on its interaction with the membrane and fusion activity. These sequences are important to stabilize the fusion peptide loop and increase fusion activity. Three peptides, besides the fusion peptide, were synthesized by SPPS using the Fmoc chemical approach. The first contains the fusion peptide and the C-terminal region of the loop (sequence 98-123); another contains the N-terminal region (88-112) and the larger peptide contains both regions (88-123). The peptides were able to interact with a model membrane. Differences in morphology of the monolayer promoted by the peptides were assessed by Brewster Angle Microscopy (BAM). Our data indicated that the C-terminal region of fusion peptide loop is more efficient in promoting fusion and interacting with the membrane than the N-terminal sequence, which is responsible for the electrostatic initial interaction. We propose a 2-step mechanism for the interaction of the dengue virus fusion peptide with the host membrane, where the N-terminal sequence docks electrostatically on the headgroups and then the C-terminal interacts via hydrophobic forces in the acyl chains.

Conversion of human fibroblasts into multipotent cells by cell-penetrating peptides

The potential application of human induced pluripotent stem cells (hiPSCs) brings great expectations to regenerative medicine. However, several safety concerns, such as oncogenic transformation, remain. A number of methods have been developed to produce hiPSCs with potentially reduced risks. Cell-penetrating peptides (CPPs) are expected to improve the efficiency of nonviral reprogramming by delivering biologically active molecules into cells. Here, we show that the transfection of CPPs alone into normal adult human fibroblasts generated embryonic body (EB)-like cell clusters in the absence of reprogramming factors. The CPP-generated cell clusters were positive for a set of multipotency markers and differentiated into endodermal, ectodermal, and mesodermal cells in vitro. These results suggest that CPPs converted normal human adult somatic cells into multipotent cells. Moreover, we show that CPPs dissociated histone deacetylase 1 and lysine-specific demethylase 1 from the promoter/enhancer regions of reprogramming factors to reactivate their expression. This is the first report of an easy and quick method for somatic cell reprogramming by CPPs and a novel mechanism of reprogramming. The potential application of CPP-generated multipotent cells resolves several concerns, especially safety issues, in regenerative medicine.

Efficient Fusion at Neutral pH by Human Immunodeficiency Virus gp41 Trimers Containing the Fusion Peptide and Transmembrane Domains

Human immunodeficiency virus (HIV) is membrane-enveloped, and an initial infection step is joining/fusion of viral and cell membranes. This step is catalyzed by gp41, which is a single-pass integral viral membrane protein. The protein contains an ∼170-residue ectodomain located outside the virus that is important for fusion and includes the fusion peptide (FP), N-helix, loop, C-helix, and viral membrane-proximal external region (MPER). The virion initially has noncovalent complexes between three gp41 ectodomains and three gp120 proteins. A gp120 contains ∼500 residues and functions to identify target T-cells and macrophages via binding to specific protein receptors of the target cell membrane. gp120 moves away from the gp41 ectodomain, and the ectodomain is thought to bind to the target cell membrane and mediate membrane fusion. The secondary and tertiary structures of the ectodomain are different in the initial complex with gp120 and the final state without gp120. There is not yet imaging of gp41 during fusion, so the temporal relationship between the gp41 and membrane structures is not known. This study describes biophysical and functional characterization of large gp41 constructs that include the ectodomain and transmembrane domain (TM). Significant fusion is observed of both neutral and anionic vesicles at neutral pH, which reflects the expected conditions of HIV/cell fusion. Fusion is enhanced by the FP, which in HIV/cell fusion likely contacts the host membrane, and the MPER and TM, which respectively interfacially contact and traverse the HIV membrane. Initial contact with vesicles is made by protein trimers that are in a native oligomeric state that reflects the initial complex with gp120 and also is commonly observed for the ectodomain without gp120. Circular dichroism data support helical structure for the N-helix, C-helix, and MPER and nonhelical structure for the FP and loop. Distributions of monomer, trimer, and hexamer states are observed by size-exclusion chromatography (SEC), with dependences on solubilizing detergent and construct. These SEC and other data are integrated into a refined working model of HIV/cell fusion that includes dissociation of the ectodomain into gp41 monomers followed by folding into hairpins that appose the two membranes, and subsequent fusion catalysis by trimers and hexamers of hairpins. The monomer and oligomer gp41 states may therefore satisfy dual requirements for HIV entry of membrane apposition and fusion.

Assessment of RNA carrier function in peptide amphiphiles derived from the HIV fusion peptide

A small library of amphiphilic peptides has been evaluated for duplex RNA carrier function into A549 cells. We studied peptides in which a C-terminal 7-residue cationic domain is attached to a neutral/hydrophobic 23-residue domain that is based on the viral fusion peptide of HIV. We also examined peptides in which the cationic charge was evenly distributed throughout the peptide. Strikingly, subtle sequence variations in the hydrophobic domain that do not alter net hydrophobicity result in wide variation in RNA uptake. Additionally, cyclic cystine variants are much less active as RNA carriers than their open-chain cysteine analogs. With regard to electrostatic effects, we find that lysine is less effective than arginine in facilitating uptake, and that even distribution of cationic residues throughout the peptide sequence results in especially effective RNA carrier function. Overall, minor changes in peptide hydrophobicity, flexibility and charge distribution can significantly alter carrier function. We hypothesize this is due to altered properties of the peptide-RNA assembly rather than peptide secondary structure.

Interaction of peptides with cell membranes: insights from molecular modeling

The investigation of the interaction of peptides with cell membranes is the focus of active research. It can enhance the understanding of basic membrane functions such as membrane transport, fusion, and signaling processes, and it may shed light on potential applications of peptides in biomedicine. In this review, we will present current advances in computational studies on the interaction of different types of peptides with the cell membrane. Depending on the properties of the peptide, membrane, and external environment, the peptide-membrane interaction shows a variety of different forms. Here, on the basis of recent computational progress, we will discuss how different peptides could initiate membrane pores, translocate across the membrane, induce membrane endocytosis, produce membrane curvature, form fibrils on the membrane surface, as well as interact with functional membrane proteins. Finally, we will present a conclusion summarizing recent progress and providing some specific insights into future developments in this field.

Controlled fusion of synthetic lipid membrane vesicles

Lipid membrane fusion is a fundamental noncovalent transformation as well as a central process in biology. The complex and highly controlled biological machinery of fusion has been the subject of intense investigation. In contrast, fewer synthetic approaches that demonstrate selective membrane fusion have been developed. Artificial recapitulation of membrane fusion is an informative pursuit in that fundamental biophysical concepts of biomembrane merger may be generally tested in a controlled reductionist system. A key concept that has emerged from extensive studies on lipid biophysics and biological membrane fusion is that selective membrane fusion derives from the coupling of surface recognition with local membrane disruption, or strain. These observations from native systems have guided the development of de novo-designed biomimetic membrane fusion systems that have unequivocally established the generality of these concepts in noncovalent chemistry. In this Account, we discuss the function and limitations of the artificial membrane fusion systems that have been constructed to date and the insights gained from their study by our group and others. Overall, the synthetic systems are highly reductionist and chemically selective, though there remain aspects of membrane fusion that are not sufficiently understood to permit designed function. In particular, membrane fusion with efficient retention of vesicular contents within the membrane-bound compartments remains a challenge. We discuss examples in which lipid mixing and some degree of vesicle-contents mixing is achieved, but the determinants of aqueous-compartment mixing remain unclear and therefore are difficult to generally implement. The ability to fully design membrane fusogenic function requires a deeper understanding of the biophysical underpinnings of membrane fusion, which has not yet been achieved. Thus, it is critical that biological and synthetic studies continue to further elucidate this biologically important process. Examination of lipid membrane fusion from a synthetic perspective can also reveal the governing noncovalent principles that drive chemically determined release and controlled mixing within nanometer-scale compartments. These are processes that figure prominently in numerous biotechnological and chemical applications. A rough guide to the construction of a functional membrane fusion system may already be assembled from the existing studies: surface-directed membrane apposition may generally be elaborated into selective fusion by coupling to a membrane-disruptive element, as observed over a range of systems that include small-molecule, DNA, or peptide fusogens. Membrane disruption may take different forms, and we briefly describe our investigation of the sequence determinants of fusion and lysis in membrane-active viral fusion peptide variants. These findings set the stage for further investigation of the critical elements that enable efficient, fully functional fusion of both membrane and aqueous compartments and the application of these principles to unite synthetic and biological membranes in a directed fashion. Controlled fusion of artificial and living membranes remains a chemical challenge that is biomimetic of native chemical transport and has a direct impact on drug delivery approaches.

Liposomes with double-stranded DNA anchoring the bilayer to a hydrogel core

Liposomes are important biomolecular nanostructures for handling membrane-associated molecules in the lab and delivering drugs in the clinic. In addition to their biomedical applications, they have been widely used as model cell membranes in biophysical studies. Here we present a liposome-based model membrane that mimics the attachment of membrane-resident molecules to the cytoskeleton. To facilitate this attachment, we have developed a lipid-based hybrid nanostructure in which the liposome bilayer membrane is covalently anchored to a biocompatible poly(ethylene) glycol (PEG) hydrogel core using short double-stranded DNA (dsDNA) linkers. The dsDNA linkers connect cholesterol groups that reside in the bilayer to vinyl groups that are incorporated in the cross-linked hydrogel backbone. Size exclusion chromatography (SEC) of intact and surfactant-treated nanoparticles confirms the formation of anchored hydrogel structures. Transmission electron microscopy (TEM) shows ~100 nm nanoparticles even after removal of unanchored phospholipids. The location of dsDNA groups at the hydrogel-bilayer interface is confirmed with a fluorescence assay. Using DNA as a linker between the bilayer and a hydrogel core allows for temperature-dependent release of the anchoring interaction, produces polymer nanogels with addressible hybridization sites on their surface, and provides a prototype structure for potential future oligonucleotide drug delivery applications.

Phosphatase-triggered fusogenic liposomes for cytoplasmic delivery of cell-impermeable compounds

License to fuse! A phosphorylated fusion peptide can mediate membrane fusion when the phosphates (green triangles, see scheme) are removed by phosphatases (blue spheres), delivering the contents of the liposome into the cytosol. This phosphatase-triggered approach may be useful to create target-specific lipid nanocarriers.

Line-tension controlled mechanism for influenza fusion

Our molecular simulations reveal that wild-type influenza fusion peptides are able to stabilize a highly fusogenic pre-fusion structure, i.e. a peptide bundle formed by four or more trans-membrane arranged fusion peptides. We rationalize that the lipid rim around such bundle has a non-vanishing rim energy (line-tension), which is essential to (i) stabilize the initial contact point between the fusing bilayers, i.e. the stalk, and (ii) drive its subsequent evolution. Such line-tension controlled fusion event does not proceed along the hypothesized standard stalk-hemifusion pathway. In modeled influenza fusion, single point mutations in the influenza fusion peptide either completely inhibit fusion (mutants G1V and W14A) or, intriguingly, specifically arrest fusion at a hemifusion state (mutant G1S). Our simulations demonstrate that, within a line-tension controlled fusion mechanism, these known point mutations either completely inhibit fusion by impairing the peptide's ability to stabilize the required peptide bundle (G1V and W14A) or stabilize a persistent bundle that leads to a kinetically trapped hemifusion state (G1S). In addition, our results further suggest that the recently discovered leaky fusion mutant G13A, which is known to facilitate a pronounced leakage of the target membrane prior to lipid mixing, reduces the membrane integrity by forming a 'super' bundle. Our simulations offer a new interpretation for a number of experimentally observed features of the fusion reaction mediated by the prototypical fusion protein, influenza hemagglutinin, and might bring new insights into mechanisms of other viral fusion reactions.

Cell-based analysis of Chikungunya virus E1 protein in membrane fusion

BACKGROUND

Chikungunya fever is a pandemic disease caused by the mosquito-borne Chikungunya virus (CHIKV). E1 glycoprotein mediation of viral membrane fusion during CHIKV infection is a crucial step in the release of viral genome into the host cytoplasm for replication. How the E1 structure determines membrane fusion and whether other CHIKV structural proteins participate in E1 fusion activity remain largely unexplored.

METHODS

A bicistronic baculovirus expression system to produce recombinant baculoviruses for cell-based assay was used. Sf21 insect cells infected by recombinant baculoviruses bearing wild type or single-amino-acid substitution of CHIKV E1 and EGFP (enhanced green fluorescence protein) were employed to investigate the roles of four E1 amino acid residues (G91, V178, A226, and H230) in membrane fusion activity.

RESULTS

Western blot analysis revealed that the E1 expression level and surface features in wild type and mutant substituted cells were similar. However, cell fusion assay found that those cells infected by CHIKV E1-H230A mutant baculovirus showed little fusion activity, and those bearing CHIKV E1-G91D mutant completely lost the ability to induce cell-cell fusion. Cells infected by recombinant baculoviruses of CHIKV E1-A226V and E1-V178A mutants exhibited the same membrane fusion capability as wild type. Although the E1 expression level of cells bearing monomeric-E1-based constructs (expressing E1 only) was greater than that of cells bearing 26S-based constructs (expressing all structural proteins), the sizes of syncytial cells induced by infection of baculoviruses containing 26S-based constructs were larger than those from infections having monomeric-E1 constructs, suggesting that other viral structure proteins participate or regulate E1 fusion activity. Furthermore, membrane fusion in cells infected by baculovirus bearing the A226V mutation constructs exhibited increased cholesterol-dependences and lower pH thresholds. Cells bearing the V178A mutation exhibited a slight decrease in cholesterol-dependence and a higher-pH threshold for fusion.

CONCLUSIONS

Cells expressing amino acid substitutions of conserved protein E1 residues of E1-G91 and E1-H230 lost most of the CHIKV E1-mediated membrane fusion activity. Cells expressing mutations of less-conserved amino acids, E1-V178A and E1-A226V, retained membrane fusion activity to levels similar to those expressing wild type E1, but their fusion properties of pH threshold and cholesterol dependence were slightly altered.