中央研究院 原子與分子科學研究所

Investigation of intracellular transport at the molecular scale requires measurements at high spatial and temporal resolutions. We demonstrate label-free, direct imaging and tracking of native cell vesicles in live cells at ultrahigh spatiotemporal resolution. Using coherent brightfield (COBRI) microscopy, we monitor individual cell vesicles traveling inside the cell with nanometer spatial precision in 3D at 30,000 frames per second. Stepwise directional motion of the vesicle on the cytoskeletal track is clearly resolved. We also observe repeated switching of transport direction of the vesicle in a continuous trajectory. Our high-resolution measurement unveils transient pausing and subtle bidirectional motion of the vesicle, taking place over tens of nanometers in tens of milliseconds. By tracking multiple particles simultaneously, we found strong correlations between the motions of two neighboring vesicles. Our label-free ultrahigh-speed optical imaging provides the opportunity to visualize intracellular cargo transport at the nanoscale in the microsecond timescale with minimal perturbation.

Viral infection starts with a virus particle landing on a cell surface followed by penetration of the plasma membrane. Due to the difficulty of measuring the rapid motion of small-sized virus particles on the membrane, little is known about how a virus particle reaches an endocytic site after landing at a random location. Here, we use coherent brightfield (COBRI) microscopy to investigate early-stage viral infection with ultrahigh spatiotemporal resolution. By detecting intrinsic scattered light via imaging-based interferometry, COBRI microscopy allows us to track the motion of a single vaccinia virus particle with nanometer spatial precision (< 3 nm) in 3D and microsecond temporal resolution (up to 100,000 frames per second). We explore the possibility of differentiating the virus signal from cell background based on their distinct spatial and temporal behaviors via digital image processing. Through image post-processing, relatively stationary background scattering of cellular structures is effectively removed, generating a background-free image of the diffusive virus particle for precise localization. Using our method, we unveil single virus particles exploring cell plasma membranes after attachment. We found that immediately after attaching to the membrane (within a second), the virus particle is locally confined within hundreds of nanometers. Surprisingly, within this confinement, the virus particle diffuses laterally with a very high diffusion coefficient (~1 μm²/s) at microsecond timescales. During this fast local exploration of the membrane, the virus particle is transiently associated with nanoscopic zones for sub-milliseconds. The ultrahigh-speed scattering-based optical imaging provides opportunities for resolving rapid virus-receptor interactions with nanometer clarity.

Lipid rafts are membrane nanodomains that facilitate important cell functions. Despite recent advances in identifying the biological significance of rafts, nature and regulation mechanism of rafts are largely unknown due to the difficulty of resolving dynamic molecular interaction of rafts at the nanoscale. Here, we investigate organization and single-molecule dynamics of rafts by monitoring lateral diffusion of single molecules in raft-containing reconstituted membranes. Using high-speed interferometric scattering (iSCAT) optical microscopy and small gold nanoparticles as labels, motion of single lipids is recorded via single-particle tracking (SPT) with nanometer spatial precision and microsecond temporal resolution. Processes of single molecules partitioning into and escaping from the raft-mimetic liquid-ordered (L_o) domains are directly visualized in a continuous manner with unprecedented clarity. Importantly, we observe subdiffusion of saturated lipids in the L_o domain in microsecond timescale, indicating the nanoscopic heterogeneous molecular arrangement of the L_o domain. Further analysis of the diffusion trajectory shows the presence of nano-subdomains of the L_o phase, as small as 10 nm, which transiently trap the lipids. Our results provide the first experimental evidence of non-uniform molecular organization of the L_o phase, giving a new view of how rafts recruit and confine molecules in cell membranes.