Simple 'Wiring' Diagram Underlies Immune Cells' Complex Movement

A white blood cell can be a sensitive hunter. When it “smells” chemicals that signal invaders, it crawls over and engulfs them. This motion requires a complex, coordinated set of steps: the cell establishes which way is "forward," and an interior bucket brigade works to extend the cell's interior protein scaffold in the direction it needs to go.

Steven Altschuler, PhD, and Lani Wu, PhD, both Associate Professors in UT Southwestern’s Green Center for Systems Biology.

Researchers have long studied how such processes are coordinated, but data combined from these diverse studies suggests an "everything-connects-to-everything" network, said Steven Altschuler, PhD, and Lani Wu, PhD, both Associate Professors in UT Southwestern’s Green Center for Systems Biology.

Their results were published Friday in the journal Cell.

The researchers focused on human neutrophils, white blood cells that respond rapidly to chemical triggers, and concentrated on the process of polarization, which defines the direction of travel. Neutrophils react in stages to a chemical trigger: polarization starts within seconds, develops fully within 2-3 minutes, and can be maintained for more than 10 minutes.

Neutrophils use the proteins actin and myosin – similar to proteins that muscles use to generate force – to pull themselves toward a chemical attractant. The neutrophil cytoskeletal system has three distinct "modules." The front module promotes actin assembly in the leading edge, the back module controls contraction, and the microtubule module acts as a "bank" that stores and distributes parts.

Previous work has shown that these modules "talk" to each other through biochemical links. However, when or where the talk between these modules actually occurs had not been clear.

"You can look at a lot of telephone wires in a neighborhood, and that shows you who's connected, but it doesn't tell you how the information is flowing," Dr. Wu said.

In the current study, led by a multidisciplinary team of two postdocs, Yanqin Wang, PhD, stained neutrophils with fluorescent markers for proteins that indicate activity in each of the three modules: actin, α-tubulin and p-MLC2 (a component of myosin). Chin-Jen (Jeremy) Ku, PhD, developed computational methods to quantify the intensity and localization of each fluorescent stain to indicate the degree of activity and polarization in the system.

They also used six pharmacological compounds to enhance or inhibit each module independently to measure the crosstalk between the modules.

"Interestingly, persistent crosstalk is arranged in a surprisingly simple circuit," Dr. Altschuler said. "A linear cascade from front module to back module to microtubules influences the degree of activity, and a feed-forward network in the reverse direction influences cell polarity."

The researchers found that they could understand the cells' behavior without the complete biochemical understanding of the underlying processes, an approach that may prove useful in studying other cellular systems.

They also found that the typical view of how cells respond to a signal may not be accurate. Rapid signal response has usually been understood as arising from a static pattern of crosstalk between subsystems. In contrast, they found, the crosstalk between the subsystems changed rapidly over the course of 10 minutes.

"Now that we understand the information flow for the neutrophils, we see that it's very simple, and we can see that it's a way for the neutrophil to maintain persistent behavior, even though there are a lot of stimuli around them," Dr. Altschuler said. "The front part wants to chase the intruders, and the back says 'Stay the course.' These two information currents allow the front to be flexible but still get the job done.

"You can see this under the microscope – a neutrophil will chase a bacterium while there are others all around it, and it doesn't pay any attention to the others,” he added.