The Transition of Toxoplasma gondii to Chronic Stage is Driven by a Positive Feedback Loop.

Toxoplasma gondii – also known as Toxo – is a parasite that affects a quarter of the world’s population. While Toxo survives inside most of its hosts, the active acute forms of the parasite are quickly suppressed by the immune system and turn into chronic, slow-growing cysts.

Symptoms of acute toxoplasmosis – the disease caused by Toxo – can be devastating, especially for those with weakened immune systems and pregnant women. During the AIDS epidemic, many people who carried Toxo developed encephalitis, in which dormant cysts in brain and muscle tissue reactivated. An active infection can also spread within a developing fetus, unchecked by the immune system, which is why people are advised not to change cat litter during pregnancy.

A new paper from Whitehead Institute member and MIT associate professor of biology Sebastian Loredo, published online April 20, 2023, in Nature MicrobiologyHelps explain how parasites differentiate or migrate between acute and chronic stages. The research, led by Lourido postdoctoral lab Haley Licon, identifies a new regulatory circuit that drives differentiation.

Lourido and colleagues previously found that a Toxo protein called BFD1 acts as a key regulator of chronic differentiation. This means that without BFD1, chronic infection does not occur. Also, the expression of BFD1 is sufficient to cause the parasites to switch to their chronic forms. In this publication, Licon found that another protein called BFD2 regulates BFD1 and helps Toxo adhere to chronic differentiation.

More than 1,000 genes alter expression during chronic differentiation, but only half of them appear to be directly controlled by BFD1. Lekun set out to try to understand how BFD1 causes parasites to become chronic.

“BFD1 appears to be a positive regulator of gene expression, which means that when it is turned on, it activates a host of other chronic phase genes,” she explained. “But we also know that it acts on other proteins that may influence the expression of genes needed for chronic cyst formation, which suggests that there is some sort of hierarchy in the regulation.” If one were to think of this hierarchy as a phone tree with hundreds of callers, BFD1 would be at the top of that tree.

In order to better understand the hierarchy of genes that control chronic infection, Lecon looked at a group of five genes directly controlled by BFD1 that she thought might be involved. Expression of each gene in Toxo cells was staged to test their effect on the differentiation hierarchy. Of the five candidates, only one protein behaved similarly to BFD1, and its loss prevented chronic differentiation from occurring. The lab named this protein BFD2.

Lacon confirmed that in parasites where BFD2 was completely removed, reproduction progressed normally during the acute phase, but not in the chronic phase. Even in mice, parasites were unable to form chronic stages when BFD2 was absent.

These results did not initially make sense, because BFD2 appears to control the same set of genes as BFD1. Licon chose to study BFD2 because it was directly regulated by BFD1; However, when I examined BFD1 expression in parasites lacking BFD2, I found that BFD1 expression was dependent on BFD2.

Lacon suggested that BFD1 and BFD2 participate in positive feedback. At the molecular level, BFD2 is an RNA-binding protein, and I found that it was necessary to make the BFD1 protein from messenger RNA. Like a chain reaction, a small amount of BFD2 will generate BFD1, which in turn helps Toxo produce more BFD2. The cycle continues until enough BFD1 is made to drive the transition to the chronic state.

“If a parasite is going to try and commit to being in a different state, it should avoid going backwards,” Loredo explained. “So, we’ve now found a positive feedback loop that provides the impetus to keep the parasite in a certain state. It’s a nice way of understanding that once a parasite gets a differentiation signal, it has this powerful force to help push it into the chronic stage and stay in the chronic stage.”

Although there are drugs that are effective against the acute form of Toxo, the chronic phase is resistant to these drugs, as well as our immune response. So Toxo infection can be controlled with such drugs, but not cured. Understanding the drivers of change between acute and chronic phases can yield insights into the most effective anti-Toxo therapy. “In principle, I think if Toxo can’t do these chronic phases, you might be able to get rid of them with drugs,” Lacon said.

Until then, Licon continues to work on discovering why BFD2 only works under certain conditions. “We’ve got our feet in the door, and now we’re going to continue to follow that path upwards in order to understand the greater complexity of what’s going on,” she said.