Blocking key host pathways may stop Cryptosporidium infections

Researchers at the Francis Crick Institute have mapped the human metabolic pathways that Cryptosporidium, an intestinal parasite, requires to survive. Shutting down these pathways may offer a new way to treat patients while avoiding the development of drug resistance.

The parasite Cryptosporidium invades and reproduces inside epithelial cells in the gut, causing severe diarrhoea, which is particularly dangerous for children in endemic regions and people with weakened immune systems. Despite its major impact on global public health, there are no fully effective treatments.

In a study published today in Cell, the researchers aimed to understand what this parasite requires from the host epithelial cell, in order to identify new options for interventions.

They conducted a genome-scale screening experiment, based on CRISPR genetic editing, which involved systematically disabling nearly every protein-coding gene, individually, from human intestinal cells. The cells were then infected with Cryptosporidium and imaged to see how each gene influenced parasite survival.

A metabolic tipping point

The researchers found that the genes that are involved in making cholesterol, an essential component of all human cells, appeared to have opposing effects – some boosting infection and others blocking it.

This balance hinged on a molecule midway through the cholesterol production line, called squalene. Removing genes before squalene production blocked infection, while removing genes after squalene production boosted infection.

Squalene is secreted from glands in our skin, where it is known to play a protective role, particularly against oxidative stress. The team discovered that squalene plays a similar role in the intestine: when squalene levels are high, reactive oxygen species (a hallmark of oxidative stress) are low and vice versa.

The primary way that humans control oxidative stress is through a molecule called glutathione. This antioxidant is vital to limiting oxidative damage and nearly all organisms have the capacity to make it. Surprisingly, the team discovered that, while the Cryptosporidium parasite uses glutathione, it cannot make its own. This leaves the parasite dependent on glutathione from the intestinal cell and particularly vulnerable to oxidative stress.

Cryptosporidium's Achilles heel

Finally, the team tested whether Cryptosporidium's dependencies on host metabolism could be targeted with drugs.

Squalene production can be directly inhibited with lapaquistat, a drug originally developed for treatment of high cholesterol, and then later abandoned. Lapaquistat directly inhibits squalene synthesis, whereas statins, the front-line drugs for lowering cholesterol act indirectly and only partially decrease squalene levels.

Within a mouse model of disease, lapaquistat reduced the infection and completely blocked intestinal damage, suggesting that the drug could be repurposed to fight Cryptosporidium.

Cryptosporidium infections can be life-threatening, especially in children, so we urgently need new treatments.

There is already extensive safety data for lapaquistat, making it easier to fast-track clinical trials for Cryptosporidium. And because the drug targets a host pathway, and not a single parasite protein, there should be a much larger barrier to the parasite developing drug resistance."

Adam Sateriale, Group Leader of the Cryptosporidiosis Laboratory

Bishara Marzook, Postdoctoral Fellow in the Cryptosporidiosis Laboratory and first author, said: "At some point in its evolution, Cryptosporidium lost the ability to produce glutathione and instead hijacks the host's production of this essential molecule. This could be a clever way for Cryptosporidium to save its energy for other processes, but we've shown it could also be its downfall.

"We now have a huge dataset of how almost every single gene in human epithelial cells affects Cryptosporidium infections. Beyond cholesterol synthesis, we're now starting to look at other host properties which affect the parasite, opening new doors for research."