Single cell fusion events induced by influenza hemagglutinin: studies with rapid-flow, quantitative fluorescence microscopy

Drosophila Myoblast Fusion: Invasion and Resistance for the Ultimate Union

Cell-cell fusion is indispensable for creating life and building syncytial tissues and organs. Ever since the discovery of cell-cell fusion, how cells join together to form zygotes and multinucleated syncytia has remained a fundamental question in cell and developmental biology. In the past two decades, Drosophila myoblast fusion has been used as a powerful genetic model to unravel mechanisms underlying cell-cell fusion in vivo. Many evolutionarily conserved fusion-promoting factors have been identified and so has a surprising and conserved cellular mechanism. In this review, we revisit key findings in Drosophila myoblast fusion and highlight the critical roles of cellular invasion and resistance in driving cell membrane fusion.

Influenza virus-mediated membrane fusion: determinants of hemagglutinin fusogenic activity and experimental approaches for assessing virus fusion

Hemagglutinin (HA) is the viral protein that facilitates the entry of influenza viruses into host cells. This protein controls two critical aspects of entry: virus binding and membrane fusion. In order for HA to carry out these functions, it must first undergo a priming step, proteolytic cleavage, which renders it fusion competent. Membrane fusion commences from inside the endosome after a drop in lumenal pH and an ensuing conformational change in HA that leads to the hemifusion of the outer membrane leaflets of the virus and endosome, the formation of a stalk between them, followed by pore formation. Thus, the fusion machinery is an excellent target for antiviral compounds, especially those that target the conserved stem region of the protein. However, traditional ensemble fusion assays provide a somewhat limited ability to directly quantify fusion partly due to the inherent averaging of individual fusion events resulting from experimental constraints. Inspired by the gains achieved by single molecule experiments and analysis of stochastic events, recently-developed individual virion imaging techniques and analysis of single fusion events has provided critical information about individual virion behavior, discriminated intermediate fusion steps within a single virion, and allowed the study of the overall population dynamics without the loss of discrete, individual information. In this article, we first start by reviewing the determinants of HA fusogenic activity and the viral entry process, highlight some open questions, and then describe the experimental approaches for assaying fusion that will be useful in developing the most effective therapies in the future.

An invasive podosome-like structure promotes fusion pore formation during myoblast fusion

Recent studies in Drosophila have implicated actin cytoskeletal remodeling in myoblast fusion, but the cellular mechanisms underlying this process remain poorly understood. Here we show that actin polymerization occurs in an asymmetric and cell type-specific manner between a muscle founder cell and a fusion-competent myoblast (FCM). In the FCM, a dense F-actin-enriched focus forms at the site of fusion, whereas a thin sheath of F-actin is induced along the apposing founder cell membrane. The FCM-specific actin focus invades the apposing founder cell with multiple finger-like protrusions, leading to the formation of a single-channel macro fusion pore between the two muscle cells. Two actin nucleation-promoting factors of the Arp2/3 complex, WASP and Scar, are required for the formation of the F-actin foci, whereas WASP but not Scar promotes efficient foci invasion. Our studies uncover a novel invasive podosome-like structure (PLS) in a developing tissue and reveal a previously unrecognized function of PLSs in facilitating cell membrane juxtaposition and fusion.

Roles of cholesterol in vesicle fusion and motion

Although it is well established that exocytosis of neurotransmitters and hormones is highly regulated by numerous secretory proteins, such as SNARE proteins, there is an increasing appreciation of the importance of the chemophysical properties and organization of membrane lipids to various aspects of the exocytotic program. Based on amperometric recordings by carbon fiber microelectrodes, we show that deprivation of membrane cholesterol by methyl-beta-cyclodextrin not only inhibited the extent of membrane depolarization-induced exocytosis, it also adversely affected the kinetics and quantal size of vesicle fusion in neuroendocrine PC12 cells. In addition, total internal fluorescence microscopy studies revealed that cholesterol depletion impaired vesicle docking and trafficking, which are believed to correlate with the dynamics of exocytosis. Furthermore, we found that free cholesterol is able to directly trigger vesicle fusion, albeit with less potency and slower kinetics as compared to membrane depolarization stimulation. These results underscore the versatile roles of cholesterol in facilitating exocytosis.

Genetic control of fusion pore expansion in the epidermis of Caenorhabditis elegans

Developmental cell fusion is found in germlines, muscles, bones, placentae, and stem cells. In Caenorhabditis elegans 300 somatic cells fuse during development. Although there is extensive information on the early intermediates of viral-induced and intracellular membrane fusion, little is known about late stages in membrane fusion. To dissect the pathway of cell fusion in C. elegans embryos, we use genetic and kinetic analyses using live-confocal and electron microscopy. We simultaneously monitor the rates of multiple cell fusions in developing embryos and find kinetically distinct stages of initiation and completion of membrane fusion in the epidermis. The stages of cell fusion are differentially blocked or retarded in eff-1 and idf-1 mutants. We generate kinetic cell fusion maps for embryos grown at different temperatures. Different sides of the same cell differ in their fusogenicity: the left and right membrane domains are fusion-incompetent, whereas the anterior and posterior membrane domains fuse with autonomous kinetics in embryos. All but one cell pair can initiate the formation of the largest syncytium. The first cell fusion does not trigger a wave of orderly fusions in either direction. Ultrastructural studies show that epidermal syncytiogenesis require eff-1 activities to initiate and expand membrane merger.

Stoichiometry of envelope glycoprotein trimers in the entry of human immunodeficiency virus type 1

The human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins (Envs) function as a trimer, mediating virus entry by promoting the fusion of the viral and target cell membranes. HIV-1 Env trimers induce membrane fusion through a pH-independent pathway driven by the interaction between an Env trimer and its cellular receptors, CD4 and CCR5/CXCR4. We studied viruses with mixed heterotrimers of wild-type and dominant-negative Envs to determine the number (T) of Env trimers required for HIV-1 entry. To our surprise, we found that a single Env trimer is capable of supporting HIV-1 entry; i.e., T = 1. A similar approach was applied to investigate the entry stoichiometry of envelope glycoproteins from amphotropic murine leukemia virus (A-MLV), avian sarcoma/leukosis virus type A (ASLV-A), and influenza A virus. When pseudotyped on HIV-1 virions, the A-MLV and ASLV-A Envs also exhibit a T = 1 entry stoichiometry. In contrast, eight to nine influenza A virus hemagglutinin trimers function cooperatively to achieve membrane fusion and virus entry, using a pH-dependent pathway. The different entry requirements for cooperativity among Env trimers for retroviruses and influenza A virus may influence viral strategies for replication and evasion of the immune system.

Kinetics of influenza hemagglutinin-mediated membrane fusion as a function of technique

Reliable techniques are required to evaluate the plausibility of proposed membrane fusion mechanisms. Here we have studied the kinetics of establishing the lipidic connection between hemagglutinin-expressing cells (HA-cells) and red blood cells (RBC) labeled with octadecylrhodamine, R18, using three different experimental approaches: (1) the most common approach of monitoring the rate of the R18 dequenching in a cuvette with a suspension of RBC/HA-cell complexes; (2) video fluorescence microscopy (VFM) to detect the waiting times before the onset of R18 redistribution, not dequenching, for each RBC attached to an adherent HA-cell; and (3) a new approach based on blockage of RBC fusion to an adherent HA-cell at different time points by lysophosphatidylcholine (LPC), so that only the cell pairs which, at the time of LPC application, had fused or were irreversibly committed to fusion contributed to the final extent of lipid mixing. The LPC blockage and VFM gave very similar estimates for the fusion kinetics, with LPC monitoring also those sites committed to the lipid mixing process. In contrast, R18 dequenching in the cuvette was much slower, i.e., it monitors a much later stage of dye redistribution.

The influenza haemagglutinin-induced fusion cascade: effects of target membrane permeability changes

To define the stages in influenza haemagglutinin (HA)-mediated fusion the kinetics of fusion between cell pairs consisting of single influenza HA-expressing cells and single erythrocytes (RBC) which had been labelled with both a fluorescent lipid (Dil) in the membrane and a fluorescent solute (calcein) in the aqueous space have been monitored. It is shown that release of solute from the target cell occurs, following the formation of the hemi-fusion diaphragm. These results are discussed in terms of a model in which fusion peptide insertion into the target membrane induces lipid stalks, which results in the formation of a hemifusion diaphragm and a fusion pore. Bilayer expansion due to overproduction of these stalks can give rise to collateral damage of target membranes.

Membrane fusion mediated by baculovirus gp64 involves assembly of stable gp64 trimers into multiprotein aggregates

The baculovirus fusogenic activity depends on the low pH conformation of virally-encoded trimeric glycoprotein, gp64. We used two experimental approaches to investigate whether monomers, trimers, and/or higher order oligomers are functionally involved in gp64 fusion machine. First, dithiothreitol (DTT)- based reduction of intersubunit disulfides was found to reversibly inhibit fusion, as assayed by fluorescent probe redistribution between gp64-expressing and target cells (i.e., erythrocytes or Sf9 cells). This inhibition correlates with disappearance of gp64 trimers and appearance of dimers and monomers in SDS-PAGE. Thus, stable (i.e., with intact intersubunit disulfides) gp64 trimers, rather than independent monomers, drive fusion. Second, we established that merger of membranes is preceded by formation of large (greater than 2 MDa), short-lived gp64 complexes. These complexes were stabilized by cell-surface cross-linking and characterized by glycerol density gradient ultracentrifugation. The basic structural unit of the complexes is stable gp64 trimer. Although DTT-destabilized trimers were still capable of assuming the low pH conformation, they failed to form multimeric complexes. The fact that formation of these complexes correlated with fusion in timing, and was dependent on (a) low pH application, (b) stable gp64 trimers, and (c) cell-cell contacts, suggests that such multimeric complexes represent a fusion machine.

Submaximal responses in calcium-triggered exocytosis are explained by differences in the calcium sensitivity of individual secretory vesicles

A graded response to calcium is the defining feature of calcium-regulated exocytosis. That is, there exist calcium concentrations that elicit submaximal exocytotic responses in which only a fraction of the available population of secretory vesicles fuse. The role of calcium-dependent inactivation in defining the calcium sensitivity of sea urchin egg secretory vesicle exocytosis in vitro was examined. The cessation of fusion in the continued presence of calcium was not due to calcium-dependent inactivation. Rather, the calcium sensitivity of individual vesicles within a population of exocytotic vesicles is heterogeneous. Any specific calcium concentration above threshold triggered subpopulations of vesicles to fuse and the size of the subpopulations was dependent upon the magnitude of the calcium stimulus. The existence of multiple, stable subpopulations of vesicles is consistent with a fusion process that requires the action of an even greater number of calcium ions than the numbers suggested by models based on the assumption of a homogeneous vesicle population.

Lysis of Large Unilamellar Vesicles Induced by Analogs of the Fusion Peptide of Influenza Virus Hemagglutinin

We have developed a micropipette aspiration assay to observe the lysis of large (20-30 &mgr;m diameter) vesicles aspirated using micropipettes. Single membrane lysis events can be seen with the light microscope and are followed using fluorescence assays and video microscopy. In this study, we have examined the ability of two analogs of the fusion peptide from influenza virus hemagglutinin to induce the lysis of large unilamellar egg phosphatidylcholine vesicles, as a function of peptide concentration and pH. X31 is a wild-type peptide from one strain of Influenza A, and E5 is an analogue which has several residues replaced by glutamate residues. Both peptides were found to induce lysis of large vesicles in a pH-dependent manner. Both peptides exhibited maximal activity at pH 5, measured in terms of both rate and extent of lysis. E5 was active at much lower concentrations than X31. Our results with both peptides are compared to results published from other laboratories.

Voltage-dependent translocation of R18 and DiI across lipid bilayers leads to fluorescence changes

We show that the lipophilic, cationic fluorescent dyes R18 and Dil translocate from one monolayer of a phospholipid bilayer membrane to the other in a concentration and voltage-dependent manner. When the probes were incorporated into voltage-clamped planar membranes and potentials were applied, displacement currents resulted. The charged probes sensed a large fraction of the applied field. When these probes were added to only one monolayer, displacement currents were symmetrical around 0 mV, indicating that the probes distributed equally between the two monolayers. Charge translocation required that the bilayer be fluid. When membranes were in a condensed gel phase, displacement currents were not observed; raising the temperature to above the gel-liquid crystalline transition restored the currents. Translocation of R18 was also shown by fluorescence measurements. When R18 was in the bilayer at high, self-quenching concentrations, voltage pulses led to voltage-dependent fluorescence changes. The kinetics of the fluorescence changes and charge translocations correlated. Adding the quencher I- to one aqueous phase caused fluorescence to decrease or increase when voltage moved R18 toward or away from the quencher at low, nonquenching concentrations of R18. In contrast to R18, Dil incorporated into bilayers was a carrier fo I-, and hence I- altered Dil currents. Voltage-driven translocations allow R18 and Dil to be used to probe membrane potential changes.

Dilation of the influenza hemagglutinin fusion pore revealed by the kinetics of individual cell-cell fusion events

We have monitored kinetics of fusion between cell pairs consisting of a single influenza hemaglutinin (HA)-expressing cell and a single erythrocyte (RBC) that had been labeled with both a fluorescent lipid (Dil) in the membrane and a fluorescent solute (calcein) in the aqueous space. Initial fusion pore opening between the RBC and HA-expressing cell produced a change in RBC membrane potential (delta psi) that was monitored by a decrease in Dil fluorescence. This event was followed by two distinct stages of fusion pore dilation: the flux of fluorescent lipid (phi L) and the flux of a large aqueous fluorescent dye (phi s). We have analyzed the kinetics of events that occur as a result of transitions between a fusion pore (FP) and a solute permissive fusion pore (FPs). Our data are consistent with a fusion pore comprising six HA trimers.

Poisson-distributed active fusion complexes underlie the control of the rate and extent of exocytosis by calcium

We have investigated the consequences of having multiple fusion complexes on exocytotic granules, and have identified a new principle for interpreting the calcium dependence of calcium-triggered exocytosis. Strikingly different physiological responses to calcium are expected when active fusion complexes are distributed between granules in a deterministic or probabilistic manner. We have modeled these differences, and compared them with the calcium dependence of sea urchin egg cortical granule exocytosis. From the calcium dependence of cortical granule exocytosis, and from the exposure time and concentration dependence of N-ethylmaleimide inhibition, we determined that cortical granules do have spare active fusion complexes that are randomly distributed as a Poisson process among the population of granules. At high calcium concentrations, docking sites have on average nine active fusion complexes.

Transient domains induced by influenza haemagglutinin during membrane fusion

During low pH-induced fusion of influenza virus with erythrocytes we have observed differential dispersion of viral lipid and haemagglutinin (HA) into the erythrocyte membrane, and viral RNA into the erythrocyte using fluorescence video microscopy. The movement of both viral lipid and HA from virus to cell was restricted during the initial stages of fusion relative to free diffusion. This indicates the existence of relatively long-lived barriers to diffusion subsequent to fusion pore formation. Fluorescence anisotropy of phospholipid analogues incorporated into the viral membrane decreased when the pH was lowered to levels required for optimum fusion. This indicates that the restricted motion of viral membrane components was not due to rigidification of membrane lipids. The movement of HA from the fusion site was also assessed by photosensitized labelling by means of a fluorescent substrate (NBD-taurine) passing through the band 3 sialoglycoprotein (the erythrocyte anion transporter). We also examined the flow of lipid and aqueous markers during fusion of HA-expressing cells with labelled erythrocytes. During this cell-cell fusion, movement of lipid between fusing membranes begins before the fusion pore is wide enough to allow diffusion of aqueous molecules (M(r) > 500). The data indicate that HA is capable of creating domains in the membrane and controlling continuity of aqueous compartments which are bounded by such domains.

Micropipette manipulation technique for the monitoring of pH-dependent membrane lysis as induced by the fusion peptide of influenza virus

We have assembled a micropipette aspiration assay to measure membrane destabilization events in which large (20-30 microns diameter) unilamellar vesicles are manipulated and exposed to membrane destabilizing agents. Single events can be seen with a light microscope and are recorded using both a video camera and a photomultiplier tube. We have performed experiments with a wild-type fusion peptide from influenza virus (X31) and found that it induces pH-dependent, stochastic lysis of large unilamellar vesicles. The rate and extent of lysis are both maximum at pH 5; the maximum rate of lysis is 0.018 s-1 at pH 5. An analysis of our data indicates that the lysis is not correlated either to the size of the vesicles or to the tension created in the vesicle membranes by aspiration.

Restricted movement of lipid and aqueous dyes through pores formed by influenza hemagglutinin during cell fusion

The fusion of cells by influenza hemagglutinin (HA) is the best characterized example of protein-mediated membrane fusion. In simultaneous measurements of pairs of assays for fusion, we determined the order of detectable events during fusion. Fusion pore formation in HA-triggered cell-cell fusion was first detected by changes in cell membrane capacitance, next by a flux of fluorescent lipid, and finally by flux of aqueous fluorescent dye. Fusion pore conductance increased by small steps. A retardation of lipid and aqueous dyes occurred during fusion pore fluctuations. The flux of aqueous dye depended on the size of the molecule. The lack of movement of aqueous dyes while total fusion pore conductance increased suggests that initial HA-triggered fusion events are characterized by the opening of multiple small pores: the formation of a "sieve".

Fluorescence dequenching kinetics of single cell-cell fusion complexes

In an earlier paper which models the cell-cell (or virus-cell) fusion complex as two partial spherical vesicles joined at a narrow neck (Rubin, R. J., and Yi-der Chen. 1990. Biophys. J. 58:1157-1167), the redistribution by diffusion of lipid-like molecules through the neck between the two fused cell surfaces was studied. In this paper, we extend the study to the calculation of the kinetics of fluorescence increase in a single fusion complex when the lipid-like molecules are fluorescent and self-quenching. The formalism developed in this paper is useful in deducing fusion activation mechanisms from cuvette fluorescence measurements in cell-cell fusion systems. Two different procedures are presented: 1) an exact one which is based on the exact local density functions obtained from diffusion equations in our earlier study; and 2) an approximate one which is based on treating the kinetics of transfer of probes between the two fused cells as a two-state chemical reaction. For typical cell-cell fusion complexes, the fluorescence dequencing curves calculated from the exact and approximate procedures are very similar. Due to its simplicity, the approximate method should be very useful in future applications. The formalism is applied to a typical cell-cell fusion complex to study the sensitivity of dequenching curves to changes in various fusion parameters, such as the radii of the cells, the radius of the pore at the fusion junction, and the number of probes initially loaded to the complex.

Dextran sulfate inhibits fusion of influenza virus and cells expressing influenza hemagglutinin with red blood cells

The influence of dextran sulfate with molecular weights of 500,000 and 8000 on binding and fusion of influenza virus (X31 strain) and of cells expressing influenza hemagglutinin (GP4F) with red blood cells (RBC) was investigated by spectrofluorimetry using virus and RBC labeled with the fluorescent dye octadecyl rhodamine B (R18). There was no significant inhibition of binding of virus and GP4F cells to red blood cells by dextran sulfate, but the polymer strongly inhibited the low pH induced fusion. Virus-RBC fusion was completely blocked by the high molecular weight dextran sulfate at concentrations as low as 0.5 mg/ml. Inhibition of RBC-GP4F cell fusion by dextran sulfate in the same concentration range was not as pronounced but the effect was potentiated by Ca2+. The polymer was only inhibitory when added at early steps of the fusion reaction, but the pH-induced conformational change of the hemagglutinin was not affected by dextran sulfate as measured by its susceptibility to proteolytic digestion. Removal of dextran sulfate after low pH-requiring steps allowed the system to fuse at neutral pH indicating that the inhibitory effect requires the continuous presence of dextran sulfate during the fusion reaction.

Acidic pH induces fusion of cells infected with baculovirus to form syncytia

The enveloped baculovirus insect cell system has been used extensively for expression of recombinant proteins, including viral fusion proteins. We tested wild-type baculovirus for endogenous fusion protein activity. Syncytia formation, dye transfer, and capacitance changes were observed after incubating infected Spodoptera frugiperda cells in acidic media, consistent with fusion protein activity. Only a short acidic pulse of 10 s is needed to trigger syncytia formation. Identical results were obtained with recombinant baculovirus. This new system is convenient for studying pH activated cell-cell fusion. However, using this enveloped virus to study the mechanism of recombinant fusion proteins requires caution.