Confocal microscopy of giant vesicles supports the absence of HIV-1 neutralizing 2F5 antibody reactivity to plasma membrane phospholipids

A two-step mechanism for the binding of the HIV-1 MPER epitope by the 10E8 antibody onto biosensor-supported lipid bilayers

HIV-1 antibodies targeting the carboxy-terminal area of the membrane-proximal external region (ctMPER) are close to exerting viral pan-neutralization. Here, we reconstituted the ctMPER epitope as the N-terminal extremity of the Env glycoprotein transmembrane domain helix and immobilized it onto biosensor-supported lipid bilayers. We assessed the binding mechanism of anti-MPER antibody 10E8 through Surface Plasmon Resonance, and found, through equilibrium and kinetic binding analyses as a function of bilayer thickness, peptide length, and paratope mutations, that 10E8 engages first with the epitope peptide (encounter), limited by ctMPER helix accessibility at the membrane surface, and then inserts into the lipid bilayer assisted by favorable Fab-membrane interactions (docking). This mechanistic information may help in devising new strategies to develop more efficient MPER-targeting vaccines.

African Swine Fever Virus Gene B117L Encodes a Small Protein Endowed with Low-pH-Dependent Membrane Permeabilizing Activity

African swine fever virus (ASFV) is causing a devastating pandemic in domestic and wild swine in Central Europe to East Asia, resulting in economic losses for the swine industry. The virus contains a large double-stranded DNA genome that contains more than 150 genes, most with no experimentally characterized function. In this study, we evaluate the potential function of the product of ASFV gene B117L, a 115-amino-acid integral membrane protein transcribed at late times during the virus replication cycle and showing no homology to any previously published protein. Hydrophobicity distribution along B117L confirmed the presence of a single transmembrane helix, which, in combination with flanking amphipathic sequences, composes a potential membrane-associated C-terminal domain of ca. 50 amino acids. Ectopic transient cell expression of the B117L gene as a green fluorescent protein (GFP) fusion protein revealed the colocalization with markers of the endoplasmic reticulum (ER). Intracellular localization of various B117L constructs also displayed a pattern for the formation of organized smooth ER (OSER) structures compatible with the presence of a single transmembrane helix with a cytoplasmic carboxy terminus. Using partially overlapping peptides, we further demonstrated that the B117L transmembrane helix has the capacity to establish spores and ion channels in membranes at low pH. Furthermore, our evolutionary analysis showed the high conservation of the transmembrane domain during the evolution of the B117L gene, indicating that the integrity of this domain is preserved by the action of the purifying selection. Collectively our data support a viroporin-like assistant role for the B117L gene-encoded product in ASFV entry. IMPORTANCE ASFV is responsible for an extensively distributed pandemic causing important economic losses in the pork industry in Eurasia. The development of countermeasures is partially limited by the insufficient knowledge regarding the function of the majority of the more than 150 genes present on the virus genome. Here, we provide data regarding the functional experimental evaluation of a previously uncharacterized ASFV gene, B117L. Our data suggest that the B117L gene encodes a small membrane protein that assists in the permeabilization of the ER-derived envelope during ASFV infection.

Design and Applications of a Fluorescent Labeling Technique for Lipid and Surfactant Preformed Vesicles

Amphiphilic molecules such as surfactants, lipids, and block copolymers can be assembled into bilayers and form vesicles. Fluorescent membrane labeling methods require the use of dye molecules that can be inserted into the bilayers at different stages of synthesis. To our knowledge, there is no generalized method for labeling preformed vesicles. Herein, we develop a versatile protocol that is suitable to both surfactant and lipid preformed vesicles and requires no separation or purification steps. On the basis of the lipophilic carbocyanine green dye PKH67, the methodology is assessed on zwitterionic phosphatidylcholine vesicles. To demonstrate its versatility, it is applied to dispersions of anionic or cationic vesicles, such as a drug administrated to premature infants with respiratory distress syndrome, or a vesicle formulation used as a fabric softener for home care applications. By means of fluorescence microscopy, we then visualize the interaction mechanisms of nanoparticles crossing live cell membranes and of surfactants adsorbed on a cotton fabric. These results highlight the advantages of a membrane labeling technique that is simple and applicable to a large number of soft matter systems.

Molecular Characterization of the Viroporin Function of Foot-and-Mouth Disease Virus Nonstructural Protein 2B

Nonstructural protein 2B of foot-and-mouth disease (FMD) virus (FMDV) is comprised of a small, hydrophobic, 154-amino-acid protein. Structure-function analyses demonstrated that FMDV 2B is an ion channel-forming protein. Infrared spectroscopy measurements using partially overlapping peptides that spanned regions between amino acids 28 and 147 demonstrated the adoption of helical conformations in two putative transmembrane regions between residues 60 and 78 and between residues 119 and 147 and a third transmembrane region between residues 79 and 106, adopting a mainly extended structure. Using synthetic peptides, ion channel activity measurements in planar lipid bilayers and imaging of single giant unilamellar vesicles (GUVs) revealed the existence of two sequences endowed with membrane-porating activity: one spanning FMDV 2B residues 55 to 82 and the other spanning the C-terminal region of 2B from residues 99 to 147. Mapping the latter sequence identified residues 119 to 147 as being responsible for the activity. Experiments to assess the degree of insertion of the synthetic peptides in bilayers and the inclination angle adopted by each peptide regarding the membrane plane normal confirm that residues 55 to 82 and 119 to 147 of 2B actively insert as transmembrane helices. Using reverse genetics, a panel of 13 FMD recombinant mutant viruses was designed, which harbored nonconservative as well as alanine substitutions in critical amino acid residues in the area between amino acid residues 28 and 147. Alterations to any of these structures interfered with pore channel activity and the capacity of the protein to permeabilize the endoplasmic reticulum (ER) to calcium and were lethal for virus replication. Thus, FMDV 2B emerges as the first member of the viroporin family containing two distinct pore domains.IMPORTANCE FMDV nonstructural protein 2B is able to insert itself into cellular membranes to form a pore. This pore allows the passage of ions and small molecules through the membrane. In this study, we were able to show that both current and small molecules are able to pass though the pore made by 2B. We also discovered for the first time a virus with a pore-forming protein that contains two independent functional pores. By making mutations in our infectious clone of FMDV, we determined that mutations in either pore resulted in nonviable virus. This suggests that both pore-forming functions are independently required during FMDV infection.

Mutation-induced changes of transmembrane pore size revealed by combined ion-channel conductance and single vesicle permeabilization analyses

Permeabilization of the Endoplasmic Reticulum (ER) is instrumental in the progression of host-cell infection by many viral pathogens. We have described that permeabilization of ER model membranes by the pore-forming domain of the Classical Swine Fever Virus (CSFV) p7 protein depends on two sequence determinants: the C-terminal transmembrane helix, and the preceding polar loop that regulates its activity. Here, by combining ion-channel activity measurements in planar lipid bilayers with imaging of single Giant Unilamellar Vesicles (GUVs), we demonstrate that point substitutions directed to conserved residues within these regions affect ER-like membrane permeabilization following distinct mechanisms. Whereas the polar loop appeared to be involved in protein insertion and oligomerization, substitution of residues predicted to face the lumen of the pore inhibited large conducting channels (>1 nS) over smaller ones (120 pS). Quantitative analyses of the ER-GUV distribution as a function of the solute size revealed a selective inhibition for the permeation of solutes with sizes larger than 4 kDa, further demonstrating that the mutation targeting the transmembrane helix prevented formation of the large pores. Collectively, our data support the idea that the pore-forming domain of p7 may assemble into finite pores with approximate diameters of 1 and 5 nm. Moreover, the observation that the mutation interfering with formation of the larger pores can hamper virus production without affecting ER localization or homo-oligomerization, suggests prospective strategies to block/attenuate pestiviruses.

Automated analysis of giant unilamellar vesicles using circular Hough transformation

MOTIVATION

In order to obtain statistically relevant results, the study of membrane effects at the single-vesicle level requires the analysis of several hundreds of giant unilamellar vesicles (GUVs), which becomes a very time-consuming task if carried out manually. Complete and user-friendly software for fast and bias-free automated analysis has not been reported yet.

RESULTS

We developed a framework for the automated detection, tracking and analysis of individual GUVs on digital microscopy images. Our tool is suited to quantify protein binding to membranes as well as several aspects of membrane permeabilization on single vesicles. We demonstrate the applicability of the approach by comparing alternative activation methods for Bax, a pore-forming protein involved in mitochondrial permeabilization during apoptosis.

AVAILABILITY AND IMPLEMENTATION

The complete software is implemented in MATLAB (The MathWorks, Inc., USA) and available as a standalone as well as the full source code at http://www.ifib.uni-tuebingen.de/research/garcia-saez/guv-software.html.

New biophysical methods to study the membrane activity of Bcl-2 proteins

The proteins of Bcl-2 family are key regulators of apoptosis. Many Bcl-2 proteins have the unique ability to switch between two possible conformations, soluble in the cytosol or associated to cellular membranes. Importantly, their membrane-inserted form is the main responsible for their apoptotic function. Unfortunately, there are only a limited number of methods available to study the membrane activity of these proteins. Here, we present a methodology to study protein binding to membranes and membrane permeabilization at the single vesicle level. It is based on purified proteins and giant unilamellar vesicles and involves directly visualization of the process with a confocal microscope. This approach allows for the characterization of the membrane activity of the Bcl-2 proteins (or of any other pore-forming molecule) with unprecedented detail.

Double-tailed lipid modification as a promising candidate for oligonucleotide delivery in mammalian cells

BACKGROUND

The potential use of nucleic acids as therapeutic drugs has triggered the quest for oligonucleotide conjugates with enhanced cellular permeability. To this end, the biophysical aspects of previously reported potential lipid oligodeoxyribonucleotide conjugates were studied including its membrane-binding properties and cellular uptake.

METHODS

These conjugates were fully characterized by MALDI-TOF mass spectrometry and HPLC chromatography. Their ability to insert into lipid model membrane systems was evaluated by Langmuir balance and confocal microscopy followed by the study of the internalization of a lipid oligodeoxyribonucleotide conjugate bearing a double-tail lipid modification (C28) into different cell lines by confocal microscopy and flow cytometry. This compound was also compared with other lipid containing conjugates and with the classical lipoplex formulation using Transfectin as transfection reagent.

RESULTS

This double-tail lipid modification showed better incorporation into both lipid model membranes and cell systems. Indeed, this lipid conjugation was capable of inserting the oligodeoxyribonucleotide into both liquid-disordered and liquid-ordered domains of model lipid bilayer systems and produced an enhancement of oligodeoxyribonucleotide uptake in cells, even better than the effect caused by lipoplexes. In addition, in β2 integrin (CR3) expressing cells this receptor was directly involved in the enhanced internalization of this compound.

CONCLUSIONS

All these features confirm that the dual lipid modification (C28) is an excellent modification for enhancing nucleic acid delivery without altering their binding properties.

GENERAL SIGNIFICANCE

Compared to the commercial lipoplex approach, oligodeoxyribonucleotide conjugation with C28 dual lipid modification seems to be promising to improve oligonucleotide delivery in mammalian cells.

Caspase-8 binding to cardiolipin in giant unilamellar vesicles provides a functional docking platform for bid

Caspase-8 is involved in death receptor-mediated apoptosis in type II cells, the proapoptotic programme of which is triggered by truncated Bid. Indeed, caspase-8 and Bid are the known intermediates of this signalling pathway. Cardiolipin has been shown to provide an anchor and an essential activating platform for caspase-8 at the mitochondrial membrane surface. Destabilisation of this platform alters receptor-mediated apoptosis in diseases such as Barth Syndrome, which is characterised by the presence of immature cardiolipin which does not allow caspase-8 binding. We used a simplified in vitro system that mimics contact sites and/or cardiolipin-enriched microdomains at the outer mitochondrial surface in which the platform consisting of caspase-8, Bid and cardiolipin was reconstituted in giant unilamellar vesicles. We analysed these vesicles by flow cytometry and confirm previous results that demonstrate the requirement for intact mature cardiolipin for caspase-8 activation and Bid binding and cleavage. We also used confocal microscopy to visualise the rupture of the vesicles and their revesiculation at smaller sizes due to alteration of the curvature following caspase-8 and Bid binding. Biophysical approaches, including Laurdan fluorescence and rupture/tension measurements, were used to determine the ability of these three components (cardiolipin, caspase-8 and Bid) to fulfil the minimal requirements for the formation and function of the platform at the mitochondrial membrane. Our results shed light on the active functional role of cardiolipin, bridging the gap between death receptors and mitochondria.

Rational design of membrane proximal external region lipopeptides containing chemical modifications for HIV-1 vaccination

The inability to generate broadly neutralizing antibody (bnAb) responses to the membrane proximal external region (MPER) of HIV-1 gp41 using current vaccine strategies has hampered efforts to prevent the spread of HIV. To address this challenge, we investigated a novel hypothesis to help improve the anti-MPER antibody response. Guided by structural insights and the unique lipid reactivity of anti-MPER bnAbs, we considered whether amino acid side chain modifications that emulate hydrophilic phospholipid head groups could contribute to the generation of 2F5-like or 4E10-like neutralizing anti-MPER antibodies. To test this hypothesis, we generated a series of chemically modified MPER immunogens through derivatization of amino acid side chains with phosphate or nitrate groups. We evaluated the binding affinity of the chemically modified peptides to their cognate monoclonal antibodies, 2F5 and 4E10, using surface plasmon resonance. The modifications had little effect on binding to the antibodies and did not influence epitope secondary structure when presented in liposomes. We selected five of the chemically modified sequences to immunize rabbits and found that an immunogen containing both the 2F5 and 4E10 epitopes and a phosphorylated threonine at T676 elicited the highest anti-peptide IgG titers, although the high antipeptide titers did not confer higher neutralizing activity. These data indicate that side chain modifications adjacent to known neutralizing antibody epitopes are capable of eliciting antibody responses to the MPER but that these chemically modified gp41 epitopes do not induce neutralizing antibodies.

Giant vesicles as cell models

Tremendous progress has been made in recent years in understanding the working of the living cell, including its micro-anatomy, signalling networks, and regulation of genes. However, an understanding of cellular phenomena using fundamental laws starting from first principles is still very far away. Part of the reason is that a cell is an active and exquisitely complex system where every part is linked to the other. Thus, it is difficult or even impossible to design experiments that selectively and exclusively probe a chosen aspect of the cell. Various kinds of idealised systems and cell models have been used to circumvent this problem. An important example is a giant unilamellar vesicle (GUV, also called giant liposome), which provides a cell-sized confined volume to study biochemical reactions as well as self-assembly processes that occur on the membrane. The GUV membrane can be designed suitably to present selected, correctly-oriented cell-membrane proteins, whose mobility is confined to two dimensions. Here, we present recent advances in GUV design and the use of GUVs as cell models that enable quantitative testing leading to insight into the working of real cells. We briefly recapitulate important classical concepts in membrane biophysics emphasising the advantages and limitations of GUVs. We then present results obtained over the last decades using GUVs, choosing the formation of membrane domains and cell adhesion as examples for in-depth treatment. Insight into cell adhesion obtained using micro-interferometry is treated in detail. We conclude by summarising the open questions and possible future directions.

Interaction of anti-HIV type 1 antibody 2F5 with phospholipid bilayers and its relevance for the mechanism of virus neutralization

Broadly neutralizing monoclonal antibody (MAb) 2F5 targets a linear epitope within the highly conserved membrane proximal external region (MPER) of the HIV-1 envelope protein gp41 integral subunit. Prospective vaccine developments warrant efforts currently underway to unveil the mechanistic and structural basis of its mode of action. One open question relates to the putative role that membrane phospholipids might play in the neutralization process. In this work, we establish experimental conditions that allow monitoring 2F5 insertion into lipid bilayers. Then, we compare the abilities of 2F5-based MAb, Fabs, and 2F5-specific antibodies recovered from immunized rabbits to directly penetrate into lipid bilayers and block the lytic activity of MPER-derived peptides. Antibody insertion induced membrane perturbation, which was blocked on interacting with the peptide epitope, thereby suggesting that such phenomenon was primarily mediated by the epitope-binding site. The long, hydrophobic complementarity-determining region (CDR)-H3 loop contributed little to this effect. In contrast, the CDR-H3 loop was required for blocking the lytic activity of MPER-based peptides and viral neutralization. Thus, our results suggest that core epitope binding plus association with lipid bilayers are not in conjunction sufficient to support viral neutralization by 2F5. Moreover, they support a role for the CDR-H3 loop in establishing secondary interactions with lipids and/or gp41 that would block the membrane-perturbing activity of MPER during fusion.