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Capturing the start point of the virus–cell interaction with high-speed 3D single-virus tracking

Abstract

The early stages of the virus–cell interaction have long evaded observation by existing microscopy methods due to the rapid diffusion of virions in the extracellular space and the large three-dimensional cellular structures involved. Here we present an active-feedback single-particle tracking method with simultaneous volumetric imaging of the live cell environment called 3D-TrIm to address this knowledge gap. 3D-TrIm captures the extracellular phase of the infectious cycle in what we believe is unprecedented detail. We report what are, to our knowledge, previously unobserved phenomena in the early stages of the virus–cell interaction, including skimming contact events at the millisecond timescale, orders of magnitude change in diffusion coefficient upon binding and cylindrical and linear diffusion modes along cellular protrusions. Finally, we demonstrate how this method can move single-particle tracking from simple monolayer culture toward more tissue-like conditions by tracking single virions in tightly packed epithelial cells. This multiresolution method presents opportunities for capturing fast, three-dimensional processes in biological systems.

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Fig. 1: 3D tracking and imaging (3D-TrIm).
Fig. 2: Minute-long high-resolution 3D tracking and volumetric live cell imaging reveals transient VSV-G VLP contacts with the cell surface.
Fig. 3: VSV-G VLP binding.
Fig. 4: Virus interaction with cellular protrusions.
Fig. 5: VSV-G VLP diffusing through multi-layered epithelial cells.
Fig. 6: Internalized viral trafficking.

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Data availability

Raw data files for 3D-TrIm trajectories in all main text figures can be found at the Duke University Research Data Repository at https://doi.org/10.7924/r4bp07h15.

Code availability

Code to analyze single virus trajectories (MATLAB) and render them (Amira) is available at https://github.com/welsherlab/3dtrim.

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Acknowledgements

We acknowledge financial support from the National Institute of General Medical Sciences of the National Institutes of Health under award number R35GM124868 (K.D.W.). We also acknowledge the Duke Viral Vector Core for assistance with virus-like particle generation and the Duke Cell Culture Facility for access to cell lines used in this study. We further thank LSM Tech for help customizing the laser scanning microscope for integration into the 3D-TrIm setup. We are also grateful to Duke Office of Information Technology (OIT) and Duke Research Computing for facilitating access to Amira 3D 2021.1 for data rendering.

Author information

Authors and Affiliations

Authors

Contributions

C.J., J.E. and K.D.W. conceptualized the study. C.J., J.E. and K.D.W. were responsible for the methodology. C.J., J.E. and Y.L. performed investigations. C.J., J.E., Y.L., J.A. and K.D.W. curated the data. C.J., J.E., Y.L., J.A. and K.D.W. performed visualizations. K.D.W. was responsible for funding acquisition. C.J., J.E. and K.D.W. wrote the original draft and reviewed and edited the manuscript.

Corresponding author

Correspondence to Kevin D. Welsher.

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The authors declare no competing interests.

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Peer review information

Nature Methods thanks Hari Shroff and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Madhura Mukhopadhyay, in collaboration with the Nature Methods team. Peer reviewer reports are available.

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Extended data

Extended Data Fig. 1 Instrument Diagram.

3D-TrIm Instrument Diagram. The 3D-TrIm microscope consists of two excitation sources: one for single-particle tracking and one for imaging. The tracking laser position is modulated using two electro-optic deflectors (EOD) and tunable acoustic gradient (TAG) lens. The imaging laser focus is modulated by an electrically tunable lens (ETL) and the beam is sampled by beam-splitters (BS) and evaluated using a system of photodiodes (PD). This focus is relayed using two lenses (IL). The tracking laser enters the underside of the LSM410 through a dichroic mirror (DCM2) where it ultimately couples with the imaging laser through DCM1 toward the shared objective lens and piezoelectric stage. After exciting the sample, the fluorescence emission pathway is shared between tracking and imaging until separated by DCM3.

Extended Data Fig. 2 Comparison of image-based tracking and imaging (spinning disk confocal) with active-feedback tracking and complementary 3D imaging (3D-TrIm).

Fluorescently-labeled virus-like particles (eGFP-Vpr VSVG) were introduced to cultures of live HeLa cells stained with SiR650-actin (100 nm). a-e, Data collected on Andor Dragon fly spinning disk confocal microscope. The same area was sampled continuously. Each volume has a depth of 8 μm split into 16 z planes. Virus particles and cells were imaged simultaneously with a camera exposure time of 40 msec. f-j, Data collected on 3D-TrIm microscope. A single virus particle was tracked continuously (1 msec sampling shown) and the surrounding area was imaged 5 μm below the particle and 3 μm above, to give an approximate 8 μm volume. The resulting trajectories over the entire acquisition period are shown and color-coded by time. Gray box in XZ projections shows the size of the spinning disk imaging volume for comparison. Experiments were performed in live-cell image solution with 2% FBS and maintained at 37 °C.

Extended Data Fig. 3 High resolution tracking and 3D imaging data, related to Fig. 2.

a, Local Volume MIPs. b, Tracking intensity trace. Both a and b are associated with VSV-G/HeLa tracking and imaging render, Fig. 2c–g. c, Local Volume MIPs, local volumes from 138-162 sec have been omitted as trajectory diffuses a greater distance above the cells than the axial range of the ETL. Note in volume 2 from 17-34s there is a noted absence of intensity due to transit in Z of the particle away from the cells. This can appear as striping due to unsampled voxels and is a feature of local volumes, especially when displayed as MIPs. d, Tracking intensity trace. Both c and d are associated VSV-G/GM701 tracking and imaging render, Fig. 2h-j.

Extended Data Fig. 4 Virus-to-cell distance map calculation.

a and b, The full trajectory is shown in white. Using the top-hat-generated isosurface (gray, see Supplementary Information 3.4) from 3D-FASTR imaging data the distance from each trajectory point to each surface vertex is calculated as describe in Supplementary Information Section 3.5. The minimum distance is then assigned to each data point (purple lines, only every 50th data point is shown for clarity and every 10th data point in the insert). The vectors from the vertex to the data point corresponding to the minimum distance are also shown as spheres on the cellular contour. a is related to trajectory shown in Fig. 2c-g, b is related to trajectory shown in Fig. 2h-j.

Extended Data Fig. 5 Additional example 1 of VSV-G VLP freely diffusing in the extracellular space.

a, Top-down view of virus trajectory pincering live HeLa cell. b, Virus-to-cell distance representation of trajectory in a. a and b share the same axes and scale bar. c,d, Different view of the trajectory shown in a, b. Circular insets shows close-up views of virus in close contact with the cell surface. c and d share the same axes and scale bar. e, Cross-section with slice along x and y axis, overlaid with trajectory color-coded by diffusion coefficient. a, c, and e are color-coded by image intensity, while b and d display virus-to-cell distance map on surface of cell. f, Top, correlation between diffusivity and distance from the cell surface of VSV-G VLP, close-approaches (below 0.5 µm) are color patched purple. Dashed purple line, 0.5 µm from cell surface. Below left, enlarged view of data between 14.5-25.5 sec, associated with trajectory segment displayed in circular inset of d. Below right, enlarged view of data between 43.25-56 sec, associated with trajectory segment displayed in circular inset of d. g, Visual representation of virus-to-cell distance calculation. Isosurface volume render of cell image (gray) overlayed with trajectory sampled at 3 msec (white). Distance vectors are calculated from each trajectory timepoint to the isosurface (generated by top-hat transform) and displayed as color-coded spheres on the cell surface.

Extended Data Fig. 6 Additional example of VSV-G VLP Binding Event.

a, 3D-TrIm render of BJ Fibroblast cells stained with SiR-650 (f-Actin). b, Distance-diffusivity trace of viral trajectory. This trajectory begins with rapid extracellular diffusion before binding to the surface. A small ~1 second detachment occurs before re-binding for several seconds and detaching again where skimming is observed with inhibited diffusivity compared to the initial free diffusion, followed by free diffusion that has greater diffusivity than observed while skimming, but lower than the initial free diffusion.

Extended Data Fig. 7 Additional example 1 of VLP-protrusion interaction.

a, Top-down view of VSV-G VLP landing on protrusion from the surface of a live cell. 3D volume rendering of live HeLa cells (stained with SYTO61) co-registered with high resolution virus trajectory from 4D tracking and imaging data. b, c Alternate views of the trajectory shown in a. d, Magnified views showing cylindrical nature of trajectory and proximity to cell surface. e, Top-down view, with trajectory colored by diffusion coefficient, free portion with D > 1.5 × 10−2 μm2/s colored gray. f, Color-coded trajectory, showing free diffusion (D > 1.5 × 10−2 μm2/s, gray) and bound diffusion (green-yellow). g, Cylindrical fitting of the bound portion of the trajectory in a-f (28-120 sec), fitted cylinder (magenta) has a radius of 122 ± 8.4 nm after taking into consideration the tracked particle diameter. h, View of trajectory along the cylinder axis. i, Intensity trace of tracked VLP.

Extended Data Fig. 8 High resolution tracking and simultaneous 3D imaging localizes VSV-G VLP diffusion along fibroblast protrusion.

a, Left, top-down view of VSV-G VLP ‘surfing’ on actin-stained protrusion of BJ fibroblast. Trajectory colored by time. The lower, brighter, z-planes have been omitted to help visualize the protrusion the VLP is localized to. Right, same view but trajectory color-coded by diffusivity. b, Snapshots of 3D reconstruction. The sustained linear motion correlates with the lateral surface of the protrusion, large changes in diffusivity where detected. c, (left) XY MIP, (middle) magnified, and (right) YZ MIP of viral diffusion on actin-rich protrusion.

Extended Data Fig. 9 Additional example of VSV-G VLP circumnavigating HT29-MTX cells.

a, 3D reconstruction from a 4D data set covering 3 local volumes, at 16 FPV of suspended HT29-MTX cells grown on inverted matrix stained with SiR650-actin with high resolution trajectory. Below, axial view. b, Magnified view. c, Cell boundary reconstruction highlighting sample density. d, XY MIP. e, 3D reconstruction of cellular imaging data in a with trajectory split into unique segments of diffusion behavior via change-point analysis. f, VLP intensity trace.

Extended Data Fig. 10 Multi-trajectory acquisition.

a, Left: 10 VSV-G VLP trajectories collected in the same area of live HeLa cells stained with SYTO61 (see also: Supplementary Fig. 29 and Supplementary Video 8). Right: top-down (xy) MIP with multiple co-registered VLP trajectories. Colorbar represents trajectory number, with each trajectory uniquely colored. b, The dwell time of 27 VSV-G VLP trajectories (starting from free diffusion) collected in the same area as a were binned into voxels and represented as spheres, color-mapped by diffusivity. This correlated analysis reveals commonly visited regions and highlights the slower diffusion experienced by the VLP population nearer the cell surface. Inset: two different VLPs land in close proximity on the same cell. c, Exploded views showing multiple VLPs contact the same cell.

Supplementary information

Supplementary Information

Supplementary methods and Figs. 1–30, and description of Supplementary Videos.

Reporting Summary

Peer Review File

Supplementary Video 1

3D-TrIm method animation, related to Fig. 1. Demonstration of 3D-TrIm operating principle. Animation sequence begins with overview of experimental setup in which a heated sample containing virus-like particles (VLP) and live cells are mounted on a piezoelectric stage with an objective lens shared by both tracking and imaging microscope sources. This overview is followed by an animation of 3D-SMART real-time tracking, demonstrating how a pair of Electro-Optic Deflectors (EOD) create a lateral Knight’s Tour grid pattern, followed by the use of a Tunable Acoustic Gradient (TAG lens) to scan a focal range above and below the center of the focal volume. A final animation demonstrates the principle of 3D-FASTR point-scan imaging.

Supplementary Video 2

VSV-G exploring the extracellular matrix, related to Fig. 2a,b. 3D reconstruction of real-time VSV-G VLP trajectory in extracellular matrix of live GM701 cells (stained with F-actin label SiR650-actin), from a 4D dataset covering 10 local volumes, at 16 FPV. Trajectory (~162 s) is segmented into 25 segments per second (25 frames per second when playback rate is 1×) and color mapped by time. The progress bar shows how the trajectory is further categorized: (1) Free diffusion period (playback rate: 2×): 0–14 s, 18–38 s, 44–62 s, 70–108 s. (2) Skimming period (playback rate: 1×): 14–18 s, 38–44 s, 62–70 s, 108–122 s. (3) Detachment (playback rate: 2×): 122–162 s. Sphere represents the VLP position in the current frame (refreshing rate is consistent with the trajectory, that is, 25 FPS at 1× playback rate). Image volumes formed from maximum intensity projection over time from local volumes acquired over 16 frame-times. In a, cells are color-coded by imaging intensity, while in b, cells are color-coded depending on distance of the virus from the cell surface. Panels a and b share the same trajectory color scale, camera angle and camera path; however, a is magnified compared with b.

Supplementary Video 3

VSV-G VLP skimming and binding, related to Fig. 3a–c. 3D reconstruction of live HeLa cells (stained with nucleic acid label SYTO61) corendered with virus trajectory. Trajectory (~123 s) is segmented into 25 segments per second (25 frames per second when playback rate is 1×) and color mapped by time. The progress bar shows how the trajectory is further categorized: (1) Free diffusion period (playback rate: 2×): 14–30 s, 41–70 s. (2) Skimming period (playback rate: 1×): 0–14 s, 30–41 s. (3) Immobilized period (playback rate: 4×): 70–123 s. The end points of trajectory segments are labeled with spheres (refreshing rate is consistent with the trajectory, that is, 25 FPS at 1× playback rate), representing the position of the viral particle. The diffusional status of viral particle is further illustrated by the color change of the sphere (gray, free motion; cyan, skimming behavior; purple, immobilization). Image volumes formed from maximum intensity projection over time from local volumes acquired over 16 frame-times. Cells are color-coded by image intensity.

Supplementary Video 4

VSV-G VLP transient binding, related to Fig. 3e–g. 3D reconstruction of live GM701 cells (stained with F-actin label SiR650-actin) corendered with virus trajectory. Trajectory (~54 s) is segmented into 25 segments per second and color mapped by diffusivity. The playback rate is constant 1×. Image volumes formed from maximum intensity projection over time from local volumes acquired over 16 frame-times. Cells are color-coded by image intensity. We observed the VLP repeatedly landing and detaching.

Supplementary Video 5

VSV-G VLP interaction with cell surface protrusion, related to Fig. 4. 3D reconstruction of live HeLa cells (stained with nucleic acid label SYTO61) corendered with virus trajectory. Trajectory (~133 s) is segmented into 25 segments per second (25 frames per second when playback rate is 1×) and color mapped by time. Image volumes formed from maximum intensity projection over time from local volumes acquired over 16 frame-times. Cells are color-coded by image intensity. Shown in the progress bar, the trajectory is further categorized into two segments: (1) Free diffusion (playback rate: 1×): 0–16 s. (2) Membrane-bound diffusion on protrusion: 16–133 s.

Supplementary Video 6

VSV-G VLP diffusing through multi-layered epithelial cells, related to Fig. 5. 3D reconstruction of suspended HT29-MTX cells grown on inverted matrix support (stained with F-actin label SiR650-actin) corendered with virus trajectory. Trajectory (~166 s) is segmented into 25 segments per second (25 frames per second) and color mapped by time. The playback rate is constant 4×. This video shows how the global volume intensity is accumulated from local volumes acquired over four frame-times. Cells are color-coded by image intensity. Axes grid represents approximate position of matrix support on which the cells are suspended.

Supplementary Video 7

Internalized viral trafficking, related to Fig. 6. 3D reconstruction of live HeLa cells (stained with F-actin label SiR650) corendered with virus trajectory. Trajectory (~325 s) color mapped by time. Global image volume formed from maximum intensity projection over time from local volumes acquired over 16 frame-times. Cells are color-coded by image intensity. Initially, the cells are opaque and the trajectory is not visible; a cut-through reveals the complete internalized trajectory. Viewing this embedded trajectory from above reveals high-intensity, actin-rich regions which the trajectory appears to navigate. Finally, the camera is rotated to a lateral view to show the position of the trajectory within the 3D cell.

Supplementary Video 8

Multi-trajectory overlay, related to Extended Data Fig. 10. 3D reconstruction of a single assembled area containing live HeLa cells (stained with nucleic acid label SYTO61) corendered with ten virus trajectories. The total imaging data were acquired during the ten trajectories and coregistered to form a single common area. Image volumes formed from maximum intensity projection over time from the assembled volumes acquired over 16 frame-times. Cells are color-coded by image intensity. Ten trajectories combined (~490 s in total) are segmented into 25 segments per second (25 frames per second when playback rate is 1×). The trajectories are color mapped with respect to trajectory number (the color legend in lower left indicates the initial color of each trajectory). Each trajectory is further color mapped by time, as indicated in the progress bar. Trajectory no. 5 is related to Supplementary Video 3.

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Johnson, C., Exell, J., Lin, Y. et al. Capturing the start point of the virus–cell interaction with high-speed 3D single-virus tracking. Nat Methods 19, 1642–1652 (2022). https://doi.org/10.1038/s41592-022-01672-3

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