Glowing phagocytosis

Usually, when we think about biotechnology, it’s in the context of agriculture, and occasionally in the context of medicine, but biotechnology is useful for a lot more. It can be used to study complex cellular and developmental processes with results that can be stunningly beautiful, and sometimes silly.
Margaret Clarke researches the soil amoeba Dictyostelium discoideum using biotechnology. Dr. Clarke is officially retired, but as a dedicated scientist, she’s continuing her work. She visited Iowa State yesterday and today.

In particular, Dr. Clarke studies phagocytosis – literally “cell eating”. These amoeba are single celled organisms that eat bacteria (and just about any bacteria-sized particle that might be nutritious). Phagocytosis is the process of forming a cup that engulfs the prey, drawing the prey into the phagocyte, and digesting the prey.
Her work has important applications in human medicine, as the phagocytosis process takes place in special phagocytic cells that are part of the immune system of humans and other animals. Learning how phagocytosis works in amoeba can help us to understand how it works in the immune system.
Before biotechnology came around, Dr. Clarke used biochemical analysis to determine what sorts of compounds were at work in actively phagocytising cells. She was able to find out that both actin and myosin were present. While this was important information (people used to think myosin was only present in muscle tissue), she wasn’t able to get any information about exactly where, when, and how these proteins were produced in the phagocytosis process – especially since it happens so quickly!
Finally, about 15 years ago, people started using biotechnology to label proteins in living cells with fluorescent proteins. Dr. Clarke saw that she could use these fluorescent labels to actually see where, when, and how actin and other compounds accumulated and dissapated in amoeba that were phagocytising. Dr. Clarke labeled different proteins with either red or green fluorescent proteins, learning much about the phagocytosis process. Even better – she caught them on film!

The images above are from Dr. Clarke’s 2006 paper Phagocyte meets prey: Uptake, internalization, and killing of bacteria by Dictyostelium amoebae. This transgenic amoeba are expressing RFP-LimEΔ, which is the gene for red fluorescent protein joined to a gene for a protein that binds to actin. Where ever actin is expressed, the amoeba will be a brighter red. It is eating GFP labeled E. coli.
As you can see, Dictyostelium are greedy little guys! They’ll try to eat just about anything, including things that are too large for them, like this amoeba trying to eat a yeast cell to the right (for the video of this frustrated little guy, see Honorable Mention #5 at the Olympus Bioscapes 2009 competition).

In addition to its awesome phagocytosis techniques, Dictyostelium has an amazing life cycle. When food becomes scarce, the amoeba will put out cAMP,a chemical that causes them to aggregate. About 100,000 individual amoeba group together and start to form what’s called a “migrating slug.” The “slug” will move through the soil toward the surface where it will develop into a fruiting body and eventually put out spores that will become single celled amoeba again.
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Clarke M, & Maddera L (2006). Phagocyte meets prey: uptake, internalization, and killing of bacteria by Dictyostelium amoebae. European journal of cell biology, 85 (9-10), 1001-10 PMID: 16782228