This is how dead and dangerous prions spread to the brain

Infectious proteins called prions – which cause devastating brain diseases, including "mad cow" disease and Creutzfeldt-Jakob disease – can, in rare cases, be transmitted through contaminated food, of medical instruments or blood.

But if anyone is exposed to prions, how do infectious proteins get to the brain?

New study reveals that, surprisingly, prions found in the blood do not seem to spread to the brain through a single suspected pathway – that is, crossing the blood-brain barrier, the network of blood vessels serving as a filter in the brain. [10 Things You Didn’t Know About the Brain]

The findings, published today (Nov. 29) in the journal PLOS Pathogens, add to the existing evidence that prions probably reach the brain in another way – through the nerves – in the same way that Herpes and rabies can invade the brain.

Research could one day lead to the development of treatments to prevent prions from spreading to the brain, even after exposing a person to dangerous proteins orally or through the bloodstream, said the authors of the study.

Enter the brain

Prion diseases lead to a progressive worsening of symptoms, including changes in memory, personality and behavior; decline of cognitive function; and difficulty of coordination, according to the National Institutes of Health. There is no cure for these diseases, which are usually fatal in a few months or years.

Although the spread of prion diseases is extremely rare, there have been some notable cases. In the UK, there have been nearly 200 cases of a prion disease called variant Creutzfeldt-Jakob disease, linked to the consumption of contaminated beef in the 1980s and 1990s. In cattle , the disease is sometimes called mad cow disease.

In addition, a few hundred people worldwide contracted Creutzfeldt-Jakob disease when they received treatments contaminated with growth hormone in the 1950s to 1980s.

Previous studies have shown that prions travel along the nerves to spread to the central nervous system, which includes the brain and spinal cord. However, some studies in animals have shown that prions can also cross the blood-brain barrier, and it's unclear whether this pathway contributes to the brain's infection.

In the new study, researchers at the University Hospital Zurich in Switzerland used genetically modified mice with a highly permeable blood-brain barrier, meaning that substances could easily cross this barrier.

The researchers found that these mice did not develop the disease faster than mice with a normal blood-brain barrier. After the researchers had infected the mouse's blood with prions, the symptoms appeared in both groups of mice and their mortality rate was similar.

"These results suggest that the passage of prions across the blood-brain barrier may not be relevant to the development of the disease," and that effective treatments should aim to stem the spread of prions, the researchers said.

Originally published on Science live.