Microscopy technique visualises 'traffic jams' inside cells

Researchers at South Korea’s Institute for Basic Science (IBS) have unveiled a novel label-free microscopy technique, dubbed cargo-localisation interferometric scattering (CL-iSCAT) microscopy — an optical imaging method that enables real-time tracking of intracellular cargo movement within living cells without the need for traditional fluorescent labelling. Their breakthrough has been published in the journal Nature Communications.

Understanding how intracellular cargo moves is crucial for unravelling the mysteries of a living cell, from its function and metabolism to its ultimate fate. Until now, scientists have relied on fluorescent microscopy to image intracellular cargoes and how they are localised within the cell’s cytoskeleton. However, traditional technology is able to observe only a limited number of cargoes and is limited by the photobleaching of fluorescent labels. Visualising the transport of cargo travelling along the intricate cellular scaffold using fluorescence-based methods has proven extremely challenging, with the lack of a label-free microscopic technique hindering our ability to understand cellular cargo transport.

CL-iSCAT microscopy addresses these challenges, allowing for label-free, real-time observation of cargo trafficking in the submicron cellular environment. One feature that sets CL-iSCAT apart is its dual-modality system that integrates fluorescence imaging with iSCAT microscopy. This combination enabled separate observation of specifically labelled cargoes or subcellular structures against countless unmarked cargoes moving along the microtubular networks. The integration of the two complementary techniques is expected to facilitate innovative research in cell biology, thus deepening our understanding of biological phenomena that occur within the cells.

“Through the achievement of observing live cells at an ultrahigh resolution independent of fluorescence, we have established a novel paradigm for elucidating the intricate details of biological processes,” said Cho Minhaeng, Director of the IBS Center for Molecular Spectroscopy and Dynamics.

Professor Hong Seok-Cheol, co-corresponding author of the study, added, “The development of an imaging technology enabling high-resolution and rapid observation of biological processes allows for an in-depth understanding of life from a molecular dynamics perspective. Our new approach of long-term visualisation holds great potential for groundbreaking medical discovery.”

With this new tool at their disposal, the IBS researchers were able to selectively monitor the dynamic movement of active cargoes within living cells. They used time-differential image analysis to precisely monitor the movements of hundreds of cargoes simultaneously over an extended period of time. By utilising the huge localisation data acquired from all cargo positions, they demonstrated the ability of CL-iSCAT microscopy to reconstruct the spatial distribution of microtubular networks in a high spatial resolution beyond the diffraction limit, allowing it to perform with a resolution of down to 15 nm.

According to the researchers, the potential of the new method is far-reaching. One of the grand challenges of our time is to investigate viral infection and monitor the effects of antiviral vaccines and drugs in real time; since the size of typical viruses is a few tens of nanometres, it will be possible for the CL-iSCAT to visualise the whole process from the onset of viral infection to cell death.

The research team has also observed that cellular traffic phenomena remarkably mirror the real-life roadway traffic observed in human society. They uncovered many intriguing transport phenomena taking place during cargo movement, including traffic jams within a cell, collective migration, and hitchhiking for efficient cargo transport in uncharted cellular territories.

“It is particularly fascinating to discover several typical traffic events experienced by city commuters in the highly complex cellular world but at micrometre scales,” Cho said. “In the future, we aim to delve deeper into the efficient transport strategies adopted by cells to overcome these challenges in transportation and their relevance to cellular phenomena.”

Image caption: Schematic representation of a cellular traffic pattern drawn with the road network in and around Seoul. Image credit: Institute for Basic Science.