Parasite Speed Saves Malaria from Immune Capture

In Aesop’s fable “The Hare and the Tortoise,” the steady perseverance of the much slower tortoise pays off in his race against the considerably faster, but overconfident hare. As a parallel, the significantly quicker malaria parasite could be resigned to the same fate as the hare—with a little medicinal help of course—in its battle to avoid the much slower immune cells that reside within human skin. Now, investigators at the Centre for Infectious Diseases at Heidelberg University Hospital, the Centre for Molecular Biology at the University of Heidelberg (ZMBH), and the Heidelberg Institute for Theoretical Studies (HITS) have recently showed the molecular mechanisms that allow Plasmodium parasites to travel through the skin ten times faster than human immune cells.

The findings from the new study—published recently in PLOS Biology in an article titled “Inter-subunit interactions drive divergent dynamics in mammalian and Plasmodium actin filaments”—not only aids our understanding of a key component of all living cells, but they also provide information that could help in the discovery of new drugs.

“Cell motility is essential for protozoan and metazoan organisms and typically relies on the dynamic turnover of actin filaments,” the authors wrote. “In metazoans, monomeric actin polymerizes into usually long and stable filaments, while some protozoans form only short and highly dynamic actin filaments. These different dynamics are partly due to the different sets of actin regulatory proteins and partly due to the sequence of actin itself.”

Mosquitoes (left) inject malaria parasites (top middle) into skin. The parasites move very rapidly (bottom middle left) using a protein that is very similar to the one our cells (lower middle right) use to construct their form and contract: actin (right). Douglas and co-workers found that certain parts of the parasite protein are responsible for the different behavior of the actin in parasites. [Heidelberg University Hospital/ HITS/ ZMBH]

Actin is assembled into long rope-like structures called filaments. These filaments are important for the proper functioning of cells—such as muscle cells—and enable each of our movements. However, they also serve to enable immune system cells to move and capture invading pathogens. Likewise, they are of great importance for the movement of the malaria parasite.

“Strangely enough, malaria parasites are ten times nimbler than the fastest of our immune cells and literally outrun our immune defenses,” explains lead study investigator Ross Douglas, Ph.D., from the Heidelberg Centre for Infectious Diseases. “If we understand this important difference in movement, we can target and stop the parasite.”

A key issue in the current study is how the rate at which actin filaments are formed and broken down differs between parasites and mammals. It was known that certain sections of the actin protein differ between the parasite and mammals. To investigate the reasons behind the difference in speed, scientists replaced parts of the parasite protein with corresponding sections of protein from mammalian actin in the laboratory.

“We probed the interactions of actin subunits within divergent actin filaments using a comparative dynamic molecular model and explored their functions using Plasmodium, the protozoan causing malaria, and mouse melanoma-derived B16-F1 cells as model systems,” the authors penned. “Parasite actin tagged to a fluorescent protein (FP) did not incorporate into mammalian actin filaments, and rabbit actin-FP did not incorporate into parasite actin filaments. However, exchanging the most divergent region of actin subdomain 3 allowed such reciprocal incorporation. The exchange of a single amino acid residue in subdomain 2 (N41H) of Plasmodium actin markedly improved incorporation into mammalian filaments.

“When we made these changes in the parasite, we noticed that some parasites could not survive at all and others suddenly hesitated when they moved,” Dr. Douglas adds.

To investigate the underlying mechanism, the participating scientists performed experiments and had computer simulations ranging from modeling at the molecular level to observing the parasites in live animals. “High-performance computers were required for simulations to observe how the structure and dynamics of actin filaments change when individual sections are swapped,” remarks co-senior study investigator Rebecca Wade, Ph.D., who heads research groups at HITS and ZMBH that investigate protein interactions via computer simulations and mathematical modeling.

Amazingly, these findings could now be used to discover chemical compounds that selectively target parasite actin and affect either the building or breakdown of the filament. An example of this approach is tubulin, another type of protein which is involved in the building of the cytoskeleton via so-called microtubules. Medicines that target parasite microtubules—such as mebendazole—have been successfully used for decades to treat humans and animals for parasitic worms.

“In this way, it could be possible to effectively stop the entire parasite,” Dr. Douglas concludes.