Researchers solve 40-year-old mystery behind Sleeping Sickness

A parasite that lives in human blood has been pulling off a strange trick for decades. It covers itself in a dense coat of proteins so the immune system struggles to recognize it, yet it somehow avoids overproducing other nearby proteins made from the same stretch of genetic material. For about 40 years, that imbalance has puzzled scientists.

Now a team at the University of York says it has found the mechanism behind it. Writing in Nature Microbiology, the researchers identified a protein called ESB2 that helps the African trypanosome fine-tune the genetic instructions used to survive in the bloodstream. Rather than simply controlling which messages get made, the parasite appears to destroy some of them almost as soon as they appear.

That matters because the parasite, Trypanosoma brucei, causes sleeping sickness, a disease spread by tsetse flies in sub-Saharan Africa. Left untreated, the infection can move into the central nervous system and cause confusion, severe sleep disruption, and coma.

Dr. Joana Faria, senior author of the study and leader of the research group at York, put it this way: “We’ve discovered that the parasite’s secret to staying invisible isn’t just what it prints, but what it chooses to redact. By placing a ‘molecular shredder’ directly inside its ‘protein factory’, the parasite can edit its genetic manual in real-time.”

To stay alive in a mammalian host, the parasite coats itself in variant surface glycoprotein, or VSG. That protein shield is central to immune evasion. The study notes that VSG makes up 5 to 10 percent of the parasite’s total cellular mRNA and protein, a huge share for a single surface feature.

The odd part is that the same expression sites that encode VSG also carry several helper genes called ESAGs. These genes help with survival and host interactions, but they are needed at far lower levels. The mystery was how the parasite could produce massive amounts of the cloak protein while keeping those helper proteins more than 140-fold lower.

The York team traced the answer to a structure called the Expression Site Body, or ESB, a compartment in the nucleus where these genes are handled. There, ESB2 appears to act as an RNA endonuclease, a protein that cuts RNA. In effect, it trims away the messages tied to the helper genes while sparing the instructions for the protective coat.

“When we first saw the molecular shredder localised in the microscope, we knew we had found something special,” said Lianne Lansink, the study’s first author.

The researchers used proximity labeling mass spectrometry to map proteins around the ESB and identified three new components linked to this system: ESB2, ESB3 and ESAP1. Among them, ESB2 stood out because it carries a PIN domain, a feature usually associated with RNA cleavage.

Follow-up experiments backed that up. When the team depleted ESB2, ESB3 or ESAP1, the parasite showed a strong fitness cost. The effect was especially severe for ESB2. RNA sequencing then showed that knocking down those proteins caused a specific rise in ESAG transcripts from the active VSG expression site, in some cases by as much as 11-fold, while the main VSG transcript dropped only modestly.

That pattern pointed away from a simple transcription problem and toward post-transcriptional control, meaning the parasite was managing RNA after it had already been produced.

The team then showed that ESB2 behaves like an active nuclease in lab assays and that its catalytic activity is essential. A mutant version that disabled key residues could not rescue the parasite when the normal ESB2 was depleted. Overexpressing normal ESB2 pushed ESAG levels down, while overexpressing the inactive version pushed them up.

One detail made the finding even more interesting. ESB2 did not seem to recognize a special RNA sequence. Instead, the data suggest its activity depends on position. A reporter placed near the promoter stayed unchanged after ESB2 knockdown, while another placed farther downstream was strongly affected. That suggests ESB2 works through spatial restriction inside the ESB rather than by reading a specific sequence code.

The study also lays out a hierarchy. ESB2 depends on other proteins, including VEX2, ESAP1 and ESB3, to localize properly to the ESB. Without that upstream support, it loses its place in the nucleus and cannot do its job.

Faria said the result solves an old problem that had followed her since her postdoctoral days. “This discovery is a real full-circle moment for me,” she said. “The mystery of how this parasite manages the asymmetric expression of its genetic manual has been a cold case in the back of my mind since my days as a postdoc.”

The work marks the first major output from Faria’s lab at York. It was funded through a Sir Henry Dale Fellowship, a partnership between the Wellcome Trust and the Royal Society, and involved researchers from the United Kingdom, Portugal, the Netherlands, Germany, Singapore and Brazil.

The study does have limits. The researchers say it is still unclear exactly how ESB2 is activated along the active VSG expression site, whether it acts on precursor or mature RNA, and what explains the different levels of ESAG upregulation. Future work will also need to clarify whether partner proteins help guide or switch on ESB2.

This work gives researchers a more precise picture of how the sleeping sickness parasite balances stealth with survival inside the bloodstream.

By identifying ESB2 and the network that recruits it, the study points to a possible weak point in the parasite’s life cycle.

That does not amount to a treatment yet, but it offers a clearer target for future efforts to disrupt immune evasion in Trypanosoma brucei.

Research findings are available online in the journal Nature Microbiology.

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