New high-speed imaging techniques are allowing scientists to show how a single cell mobilizes its resources to activate its immune response, a new research study shows.
Howard R. Petty, Ph.D., professor and biophysicist at the University of Michigan Health System’s Kellogg Eye Center, has dazzled his colleagues with movies of fluorescent-lit calcium waves that pulse through the cell, issuing an intracellular call-to-arms to attack the pathogens within.
He explains that these high-speed images provide a level of detail about cell signaling that simply wasn’t possible just a few years ago.
In the April 15 issue of the Proceedings of the National Academy of Sciences, Petty provides more detail on cell signaling, depicting what he calls the “molecular machinery” underlying the immune response. He has identified a sequence of amino acids (LTL) that controls the calcium wave pathway and, crucially, the ability of immune cells to destroy targets.
The findings are important because they could eventually lead scientists to design drugs based on the amino acid motif.
“Our clinical goal,” explains Petty, “is to characterize the immune cell’s signaling function so that we can interrupt it or somehow intervene when it begins to misfire.” The process has implications for treating autoimmune diseases such as arthritis, multiple sclerosis, and the eye disorder uveitis.
Through images of phagocytosis, the process by which a cell engulfs and then destroys its target, Petty is able to track the movement of calcium waves as they send signals to key players in the immune response. The “calcium wave” is a stream of calcium ions coming into the cell, which is detected by the fluorescence emission of a calcium-sensing dye.
As a cell membrane begins to surround its target, two calcium waves begin to circulate. When the target is completely surrounded, one wave traveling around the cell’s perimeter splits in two, with the second wave encircling the phagosome or sac-like compartment. This second wave allows the digestive enzymes to enter the phagosome and finally destroy the target.
When Petty introduced a mutation in the gene (FcyRIIA) that controls phagocytosis, he found that the calcium wave simply circled the cell and bypassed the phagosome altogether. As a result, the immune cell could engulf, but could not carry out the destruction of its target. This led him to conclude that the LTL sequence orchestrates the cell signaling process.
The sequence may also have a role in directing other cell activities, for example signaling the endoplasmic reticulum to form a spindle that connects the phagosome and the outer cell membrane. “The spindle seems to act as an extension cord that signals the calcium wave into the phagosome to finish the attack,” suggests Petty.
Petty explains that many of these findings are possible thanks to high-speed imaging techniques that enable him to merge knowledge of physics with cell and molecular biology. He uses high sensitivity fluorescence imaging with shutter speeds 600,000 times faster than video frames.
“Before the advent of high-speed imaging, you could not ask many of these questions because we had no way to see the movement of calcium waves,” he says. “With conventional imaging you ended up with a blur of calcium.” By contrast, Petty’s images resemble the movement of a comet across the night sky.
In the study reported in PNAS, Petty used leucocytes as a model for the process. The amino acid sequence is in the region of the gene FcyRIIA. He is currently studying the same phenomena in the eye, where phagocytosis disposes of the regularly-shed remnants of photoreceptor cells.