Diffusion and redistribution of lipid-like molecules between membranes in virus-cell and cell-cell fusion systems

Mathematical Modeling of Virus-Mediated Syncytia Formation: Past Successes and Future Directions

Many viruses have the ability to cause cells to fuse into large multi-nucleated cells, known as syncytia. While the existence of syncytia has long been known and its importance in helping spread viral infection within a host has been understood, few mathematical models have incorporated syncytia formation or examined its role in viral dynamics. This review examines mathematical models that have incorporated virus-mediated cell fusion and the insights they have provided on how syncytia can change the time course of an infection. While the modeling efforts are limited, they show promise in helping us understand the consequences of syncytia formation if future modeling efforts can be coupled with appropriate experimental efforts to help validate the models.

Visualization of expanding fusion pores in secretory cells

Abbineni et al. examine recent imaging work on fusion pores and discuss the dynamics of PI-4,5-P2 accumulation on granule membranes.

IFITM3 restricts influenza A virus entry by blocking the formation of fusion pores following virus-endosome hemifusion

Interferon-induced transmembrane proteins (IFITMs) inhibit infection of diverse enveloped viruses, including the influenza A virus (IAV) which is thought to enter from late endosomes. Recent evidence suggests that IFITMs block virus hemifusion (lipid mixing in the absence of viral content release) by altering the properties of cell membranes. Consistent with this mechanism, excess cholesterol in late endosomes of IFITM-expressing cells has been reported to inhibit IAV entry. Here, we examined IAV restriction by IFITM3 protein using direct virus-cell fusion assay and single virus imaging in live cells. IFITM3 over-expression did not inhibit lipid mixing, but abrogated the release of viral content into the cytoplasm. Although late endosomes of IFITM3-expressing cells accumulated cholesterol, other interventions leading to aberrantly high levels of this lipid did not inhibit virus fusion. These results imply that excess cholesterol in late endosomes is not the mechanism by which IFITM3 inhibits the transition from hemifusion to full fusion. The IFITM3's ability to block fusion pore formation at a post-hemifusion stage shows that this protein stabilizes the cytoplasmic leaflet of endosomal membranes without adversely affecting the lumenal leaflet. We propose that IFITM3 interferes with pore formation either directly, through partitioning into the cytoplasmic leaflet of a hemifusion intermediate, or indirectly, by modulating the lipid/protein composition of this leaflet. Alternatively, IFITM3 may redirect IAV fusion to a non-productive pathway, perhaps by promoting fusion with intralumenal vesicles within multivesicular bodies/late endosomes.

Interferon-induced transmembrane proteins (IFITMs) block infection of many enveloped viruses, including the influenza A virus (IAV) that enters from late endosomes. IFITMs are thought to prevent virus hemifusion (merger of contacting leaflets without formation of a fusion pore) by altering the properties of cell membranes. Here we performed single IAV imaging and found that IFITM3 did not interfere with hemifusion, but prevented complete fusion. Also, contrary to a current view that excess cholesterol in late endosomes of IFITM3-expressing cells inhibits IAV entry, we show that cholesterol-laden endosomes are permissive for virus fusion. The ability of IFITM3 to block the formation of fusion pores implies that this protein stabilizes the cytoplasmic leaflet of endosomal membranes, either directly or indirectly, through altering its physical properties. IFITM3 may also redirect IAV to a non-productive pathway by promoting fusion with intralumenal vesicles of late endosomes instead of their limiting membrane.

Hemifusion and fusion of giant vesicles induced by reduction of inter-membrane distance

Proteins involved in membrane fusion, such as SNARE or influenza virus hemagglutinin, share the common function of pulling together opposing membranes in closer contact. The reduction of inter-membrane distance can be sufficient to induce a lipid transition phase and thus fusion. We have used functionalized lipids bearing DNA bases as head groups incorporated into giant unilamellar vesicles in order to reproduce the reduction of distance between membranes and to trigger fusion in a model system. In our experiments, two vesicles were isolated and brought into adhesion by the mean of micromanipulation; their evolution was monitored by fluorescence microscopy. Actual fusion only occurred in about 5% of the experiments. In most cases, a state of "hemifusion" is observed and quantified. In this state, the outer leaflets of both vesicles' bilayers merged whereas the inner leaflets and the aqueous inner contents remained independent. The kinetics of the lipid probes redistribution is in good agreement with a diffusion model in which lipids freely diffuse at the circumference of the contact zone between the two vesicles. The minimal density of bridging structures, such as stalks, necessary to explain this redistribution kinetics can be estimated.

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.

Lipid flow through fusion pores connecting membranes of different tensions

When two membranes fuse, their components mix; this is usually described as a purely diffusional process. However, if the membranes are under different tensions, the material will spread predominantly by convection. We use standard fluid mechanics to rigorously calculate the steady-state convective flux of lipids. A fusion pore is modeled as a toroid shape, connecting two planar membranes. Each of the membrane monolayers is considered separately as incompressible viscous media with the same shear viscosity, etas. The two monolayers interact by sliding past each other, described by an intermonolayer viscosity, etar. Combining a continuity equation with an equation that balances the work provided by the tension difference, Deltasigma, against the energy dissipated by flow in the viscous membrane, yields expressions for lipid velocity, upsilon, and area of lipid flux, Phi. These expressions for upsilon and Phi depend on Deltasigma, etas, etar, and geometrical aspects of a toroidal pore, but the general features of the theory hold for any fusion pore that has a roughly hourglass shape. These expressions are readily applicable to data from any experiments that monitor movement of lipid dye between fused membranes under different tensions. Lipid velocity increases nonlinearly from a small value for small pore radii, rp, to a saturating value at large rp. As a result of velocity saturation, the flux increases linearly with pore radius for large pores. The calculated lipid flux is in agreement with available experimental data for both large and transient fusion pores.

Meta-stability of the hemifusion intermediate induced by glycosylphosphatidylinositol-anchored influenza hemagglutinin

Fusion between influenza virus and target membranes is mediated by the viral glycoprotein hemagglutinin (HA). Replacement of the transmembrane domain of HA with a glycosylphosphatidylinositol (GPI) membrane anchor allows lipid mixing but not the establishment of cytoplasmic continuity. This observation led to the proposal that the fusion mechanism passes through an intermediate stage corresponding to hemifusion between outer monolayers. We have used confocal fluorescence microscopy to study the movement of probes for specific bilayer leaflets of erythrocytes fusing with HA-expressing cells. N-Rh-PE and NBD-PC were used for specific labeling of the outer and inner membrane leaflet, respectively. In the case of GPI-HA-induced fusion, different behaviors of lipid transfer were observed, which include 1) exclusive movement of N-Rh-PE (hemifusion), 2) preferential movement of N-Rh-PE relative to NBD-PC, and 3) equal movement of both lipid analogs. The relative population of these intermediate states was dependent on the time after application of a low pH trigger for fusion. At early time points, hemifusion was more common and full redistribution of both bilayers was rare, whereas later full redistribution of both probes was frequently observed. In contrast to wild-type HA, the latter was not accompanied by mixing of the cytoplasmic marker Lucifer Yellow. We conclude that 1) the GPI-HA-mediated hemifusion intermediate is meta-stable and 2) expansion of an aqueous fusion pore requires the transmembrane and/or cytoplasmic domain of HA.

Analysis of delay times of hemagglutinin-mediated fusion between influenza virus and cell membranes

We have studied the kinetics of low pH-induced fusion between influenza virus A/PR 8/34 and human erythrocyte membranes in suspension by using an assay based on fluorescence dequenching (FDQ) of the lipophilic dye octadecylrhodamine B chloride (R 18). As shown previously (Clague et al. 1991) the onset of FDQ is preceded by a characteristic lag time (tlag) following pH reduction. Whereas tlag represents only a subpopulation of fusing viruses with the shortest delay time we suggest here that a representative mean lag time mu lag of virus-cell fusion can be deduced from the R 18-assay. Kinetics of FDQ reflects the cumulative distribution function of lag times tau lag of single fusion events with the mean value mu lag. We show that tau lag obtained from the onset of FDQ does not always reflect the fusion behaviour of the whole population of fusing viruses. While both lag times, taulag and mu lag, exhibit a similar temperature dependence we found a significantly different dependence of both delay times on virus inactivation by low pH-pretreatment. We conclude that the mean lag time mu lag appears to be a more appropriate parameter describing the kinetics of virus-cell fusion. The analysis of delay times offers a new approach to test the validity of different kinetic models of HA-mediated fusion and to gain valuable information about HA-mediated fusion. The analysis confirms that the inactivation process proceeds via steps of the formation of the fusion pore. Although the increase of lag times can be explained by a depletion of fusion competent HA's, our data suggest that intermediate structures of HA along the inactivation pathway can still transform into a fusion site.

A triggered mechanism retrieves membrane in seconds after Ca(2+)-stimulated exocytosis in single pituitary cells

In neuroendocrine cells, cytosolic Ca2+ triggers exocytosis in tens of milliseconds, yet known pathways of endocytic membrane retrieval take minutes. To test for faster retrieval mechanisms, we have triggered short bursts of exocytosis by flash photolysis of caged Ca2+, and have tracked subsequent retrieval by measuring the plasma membrane capacitance. We find that a limited amount of membrane can be retrieved with a time constant of 4 s at 21-26 degrees C, and that this occurs partially via structures larger than coated vesicles. This novel mechanism may be arrested at a late step. Incomplete retrieval structures then remain on the cell surface for minutes until the consequences of a renewed increase in cytosolic [Ca2+] disconnect them from the cell surface in < 1 s. Our results provide evidence for a rapid, triggered membrane retrieval pathway in excitable cells.

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.

Membrane flux through the pore formed by a fusogenic viral envelope protein during cell fusion

We have investigated the mechanism of cell fusion mediated by HA, the fusogenic hemagglutinin of the Influenza viral envelope. Single erythrocytes (RBCs) were attached to fibroblasts expressing the HA on their cell surface, and fusion of the paired cells was triggered by rapid acidification. The RBC membrane was stained with fluorescent lipid, and the fusion-induced escape of lipid into the fibroblast was observed by quantitative image analysis. At the same time, the formation of an aqueous connection (i.e., the fusion pore) between the two cells was monitored electrically. Within minutes after acidification, an electrical conductance between the two cells appeared abruptly as the fusion pore opened, and then increased gradually as the pore dilated. Later, fluorescent lipid diffused into the fibroblast, approaching equilibrium over the next 5-20 min. No lipid flux was seen while the pore conductance remained 0.5 nS or less. Evidently lipid flux requires a threshold pore size. Our finding suggests that the smallest and earliest fusion pores are surrounded by a ring of protein. A fusion pore expands by breaking this ring and recruiting lipid into its circumference.

Computer detection of the rapid diffusion of fluorescent membrane fusion markers in images observed with video microscopy

We have developed an algorithm for automated detection of the dynamic pattern characterizing flashes of fluorescence in video images of membrane fusion. The algorithm detects the spatially localized, transient increases and decreases in brightness that result from the dequenching of fluorescent dye in phospholipid vesicles or lipid-enveloped virions fusing with a planar membrane. The flash is identified in video images by its nonzero time derivative and the symmetry of its spatial profile. Differentiation is implemented by forward and backward subtractions of video frames. The algorithm groups spatially connected pixels brighter than a user-specified threshold into distinct objects in forward- and backward-differentiated images. Objects are classified as either flashes or noise particles by comparing the symmetries of matched forward and backward difference profiles and then by tracking each profile in successive difference images. The number of flashes identified depends on the brightness threshold, the size of the convolution kernel used to filter the image, and the time difference between the subtracted video frames. When these parameters are changed so that the algorithm identifies an increasing percentage of the flashes recognized by eye, an increasing number of noise objects are mistakenly identified as flashes. These mistaken flashes can be eliminated by a human observer. The algorithm considerably shortens the time needed to analyze video data. Tested extensively with phospholipid vesicle and virion fusion with planar membranes, our implementation of the algorithm accurately determined the rate of fusion of influenza virions labeled with the lipophilic dye octadecylrhodamine (R18).

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

Fusion of individual human erythrocytes to fibroblasts expressing the influenza virus hemagglutinin Cells were attached to coverslips fitted in a specially designed flow chamber mounted on a microscope stage, and fusion was triggered by rapid acidification to pH less than 5.2. Fusion between single cell pairs was monitored by a fluorescence increase due to redistribution of fluorescent dyes between either membrane or cytoplasmic compartments of fusing cells. The single cell fusion events were broadly heterogenous in lag times, rise times, and overall shape of the curves. Lag times obtained with a water-soluble dye were within the range obtained with a water-soluble dye were within the range obtained with the membrane-bound fluorophores, (10-160 s). Fusion was both all-or-nothing and irreversible, in that once dye redistribution in any cell commenced, it completed, regardless of pH. Short pulses of pH 4.9 for 6-10 s led to about half of the cell pairs fusing, but pulses greater than 14 s were as effective as constant low pH. Pulses that were too short to trigger fusion did not partially activate nor deactivate the fusion process, as shown by the ability of a second acidification to cause fusion of the same cells, with similar lag times. These results indicate that the overall hemagglutinin-mediated fusion process is composed of at least two stages, one required for commitment of the hemagglutinin to a fusogenic state that is pH-dependent and a maturation stage that is pH-independent.