How one parasite stays in shape

CAMBRIDGE, MA  - The pervasive, potentially deadly parasite Toxoplasma gondii (T. gondii) is common in human populations around the world. A single-celled organism one one-hundredth the length of a human cell, T. gondii is built to be tough enough to survive in the vast, hostile environment of its hosts’ bodies. New research from Whitehead Member Sebastian Lourido and postdoctoral researcher Clare Harding has identified an important part of the skeletal scaffolding that gives T. gondii cells their strength and stability.

T. gondii rely on a stable cellular skeleton to maintain their structural integrity in the wide-ranging conditions of the digestive system, central nervous system, and bloodstream in their host — as well as many other environments where they may travel over their complex lifecycles. If the skeletal structures crumble, they cannot survive. The parasite’s outer membrane is supported by an inner armor plating of membrane-bound sacs called alveoli. These in turn are supported by thick cord-like molecules called microtubules that run two thirds the length of the parasite. The microtubules provide structure to the cell and give the parasite the ability to twist and constrict itself in order to squeeze through protective barriers in the human body. Researchers suspected that some kind of protein must glue the microtubules to the alveoli, keeping the whole structure firmly connected, but the protein had not been identified.

 
Harding and colleagues at the University of Glasgow previously observed that the GAPM proteins, found in the alveoli, appeared to play a role in maintaining the cell’s structure. In a paper published online in Nature Communications on January 23, Lourido and Harding identified the protein GAPM1a, along with related proteins GAPM2a and GAPM3, as the linchpins of T. gondii’s sturdy cytoskeleton. These proteins help to tether the microtubules in place along the inner membrane.

In order to verify GAPM1a’s role, the researchers added tags to the proteins that prompted the cell to break them down in a controlled fashion until they were effectively eliminated. As the protein disappeared, the cell’s shape and structure changed. The untethered microtubules became disorganized and began to degrade, and the parasite became shorter and rounder, also losing volume. Similar effects occurred with the elimination of the protein GAPM2a, and the researchers predict that GAPM3 is similarly essential for structural integrity.

This structural chaos hobbled the parasite’s reproduction. Typically, once inside its host, a T. gondii cell reproduces by constructing two daughter cells within itself. However, the structure that supports the membrane also provides necessary scaffolding for the construction of the daughter cells. With GAPM1a gone and the cell deformed, reproduction stalled. The researchers observed the parasites repeatedly generating nuclei for daughter cells that they never managed to complete. Simultaneously, the parent cells lost all pretense of structure, becoming more and more disorganized.

The researchers also observed that loss of GAPM1a affects the parasites’ ability to move under certain circumstances. In the lab, T. gondii’s movement is often observed as two-dimensional gliding along flat surfaces, which mainly tests the function of parasite motors and is not affected by GAPM degradation. However, the parasites normally move in three dimensions: for example, between cells in a tissue. This type of movement, which is the predominant form during infection, is negatively impacted by GAPM degradation. These findings suggest that the microtubules are necessary for 3D movement.

The dire effects of eliminating GAPM1a demonstrate the delicate anatomy of the T. gondii parasite. The cytoskeleton is sturdy enough to withstand many hostile environments, yet delicate enough to fall apart if the tiniest protein tether comes undone.

This study was supported by the Wellcome Trust (103972/Z/14/Z and 087582/Z/08/Z), the European Research Council (ERC-2012-StG 309255-EndoTox), the National Institutes of Health (1DP5OD017892 and 1R21AI123746), and the Institute for Collaborative Biotechnologies (W911NF-09-0001) from the US Army Research Office.

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Sebastian Lourido’s primary affiliation is with Whitehead Institute for Biomedical Research, where his laboratory is located and all his research is conducted. He is also an assistant professor of biology at Massachusetts Institute of Technology.

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Harding, C. et al. “Alveolar proteins stabilize cortical microtubules in Toxoplasma gondii." Nature Communications, January 23, https://doi.org/10.1038/s41467-019-08318-7