Label-Free Microscopy Analyzes Dynamics of Cell Membranes

A new live-cell imaging technique will enable researchers to observe the formation and growth of cell membrane focal adhesions. Examining focal adhesions — cell membrane components that regulate adhesion and migration — is one of the keys to understanding how a cell proliferates, differentiates, and migrates.

Researchers at the Beckman Institute for Advanced Science and Technology and the Micro and Nanotechnology Laboratory at the University of Illinois have developed a new form of microscopy that allows them to observe the formation and evolution of cell membrane focal adhesions. Courtesy of Yue Zhuo/Brian Cunningham.

Photonic resonator outcoupler microscopy (PROM) is a label-free approach for dynamic, long-term, quantitative imaging of cell-surface interactions. PROM utilizes a photonic crystal biosensor surface to create a surface-bound electromagnetic field, which selectively illuminates only the extracellular matrix attached to the cell membrane and associated protein aggregates directly inside the cell membrane. The photonic crystal strictly limits lateral propagation of light while keeping light tightly bound to the biosensor surface, to enable high-resolution imaging of the cell membrane attachment footprint.

To demonstrate the ability of PROM to detect focal adhesion dimensions, researchers mapped the changes in the resonant reflected peak intensity from the biosensor surface. They observed similar spatial distributions between PROM images and fluorescence-labeled images of focal adhesion areas in dental epithelial stem cells and showed that the two imaging modalities have distinct spatial patterns, and thus provide complementary information about cell-surface activity.

Photonic resonator outcoupler microscopy (PROM) image that highlights focal adhesions of live dental stem cells. Courtesy of Yue Zhuo/Brian Cunningham.

“This is a new kind of biophysics method used to measure the peak intensity shift (PIS) of the spectra reflected from the biomaterials on a photonic crystal surface,” said researcher Yue Zhuo. "The PIS indicates the variation of cluster size in the focal adhesion area of the cell while it’s alive."

Researchers further demonstrated that cell-surface contacts and focal adhesion formation could be imaged by two orthogonal label-free modalities in PROM simultaneously, providing a general purpose tool for kinetic, high axial-resolution monitoring of cell membrane interactions.

As a label-free imaging approach, PROM does not have the limitations of fluorescence-based microscopy, such as photobleaching and stain cytotoxicity. Because PROM does not use fluorophores, and only uses low-intensity illumination, it does not limit the length of time that a live cell can be observed.

“In the future, we plan to use PROM to study stem cell differentiation, which can occur over the course of several weeks,” said Zhuo.

“PROM is providing real-time information about dynamic processes that occur specifically on cell membranes that is not available by any other method,” said professor Brian Cunningham. "Since so many biological processes are mediated through attachment of cells to surfaces, PROM provides a unique view of migration, chemotaxis, chemotoxicity, differentiation, biofilm formation, and division. We see PROM as an exciting new tool for cell biologists that can also be applied toward personalized anticancer drug selection, tissue engineering, and sensor-integrated tumor modeling.

PROM was developed by researchers at the Beckman Institute for Advanced Science and Technology and the Micro and Nanotechnology Laboratory at the University of Illinois.

The research was published in Light: Science & Applications (doi:10.1038/s41377-018-0001-5).