Time-lapse cell imaging reveals dynamic activity

Living cells are miniature worlds bustling with activity. A new, advanced imaging method can track cells over long periods of time using only light – no dye or chemicals required – to reveal dynamics and provide insight into how cells function, develop and interact.

Researchers from the University of Illinois and collaborators described the method, phase correlation imaging, in a study published in the journal Scientific Reports. The study also used PCI to look at how elements of the cell’s internal skeleton structure guide transportation within the cell.

A composite PCI image shows mass density in a cell (red) overlaid with the correlation time map highlighting activity (green).
A composite PCI image shows mass density in a cell (red) overlaid with the correlation time map highlighting activity (green).

“The cell is a very dynamic system,” said Gabriel Popescu, an electrical and computer engineering professor and the leader of the study. “The cytoskeleton is continuously remodeling, there are vesicles that are continuously transported throughout the cell, cells communicate with one another by moving mass around. Most cell-imaging methods take a snapshot and miss this activity. It’s like looking at one frame of a football game. You get some information, but not the whole story.”

It’s difficult for researchers to track what’s going on inside a living cell because cells and their internal structures are transparent, Popescu said. Researchers use chemicals like colored or fluorescent stains to see structures inside cells, but they can disrupt activity or even poison the cell and only work for a limited time, making it difficult to track normal cellular activity.
PCI uses two beams of light to precisely measure mass within the cell. Because PCI only uses light, the researchers can scan a cell culture over and over, creating a timeline of movement within the cell and highlighting hotspots of activity. PCI can scan a whole cell culture yet allows researchers to zoom in to one particular area of a cell to watch its dynamic activity.

“Each image is a map of the mass density in the cell. We can track at each point how fast the mass is moving around,” said study co-author Lihong Ma, a professor from Zhejiang Normal University in China who visited Popescu’s laboratory for a year during the study. “We found that what is happening dynamically in the cell is sometimes very different from the structure itself. Places where the cell is more dense are not necessarily more dynamic – actually, we see it is the opposite.”

Using PCI, the researchers saw that the nucleus of the cell – the compartment that houses DNA and serves as the cell’s information and command center – is actually very compartmentalized in terms of where activity takes place. They were also able to tell the difference between cells that are dormant but can become active again when stimulated, called quiescent cells, and older cells that have stopped dividing, called senescent cells. The distinction is important, because quiescent cells are crucial for healing and repair after injury, but are difficult to see without specific chemical labels.

“The quiescent cells are like war horses at the front, calmly waiting for the triggers from their commander, which in a cellular context are external or internal stimuli. On the other hand, the senescent cells have already retired from their job with no intention for further activity,” said Arindam Chakraborty, a U. of I. postdoctoral researcher in the lab of cell and developmental biology professor Supriya Prasanth. Both Chakraborty and Prasanth are co-authors of the paper.

The researchers used PCI to study the filaments that provide the cells’ structural supports, called the cytoskeletons, to see how they are involved in transporting mass around the cells. They were able to disrupt individual components of the cytoskeleton in glial cells and study how each change affected the cell.

“We could clearly see how the PCI map changes totally,” said Martha Gillette, a cell and developmental biology professor and a study co-author, “telling exactly where and at what scale activity is occurring and where it is absent when the cytoskeleton is disrupted. This information only emerges because we incorporate that dynamic time component. With other methods, you can’t observe that long. The cells will die very quickly.”

The researchers are beginning to use PCI to study neurons. They hope to witness emergent behavior, such as neurons in a culture beginning to talk to each other and stem cells developing into neurons.

Popescu also hopes to apply PCI to three-dimensional imaging techniques to understand dynamics in multiple dimensions and to study thicker tissues.

“The next step is to apply these dynamic maps to large tissues, such as embryos, brain slices and whole organisms like worms, to watch development,” Popescu said. “Combining this with other tools for imaging will be very exciting because then you can watch how ensembles of cells work together. I expect that these correlation maps will give us information on the viability of embryos for in vitro fertilization, for example, or the connectivity and signaling in brain tissue.”

The National Science Foundation supported this work. Popescu and Gillette are affiliates of the Beckman Institute for Advanced Science and Technology at Illinois.

Contact: Gabriel Popescu, Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, 217/333-4840, gpopescu@illinois.edu.

Writer: Liz Ahlberg Touchstone, Physical Sciences Editor, University of Illinois News Bureau, 217/244-1073, eahlberg@illinois.edu.

Image by Gabriel Popescu.