The protein that makes you mad

Jun 26 2006

In recent years our feeding habits have been the focus of ongoing polemics. Everybody will remember the "mad cow" crisis when the sales of veal plummeted for fear of contagion, thousands of animals were sacrificed and beef imports, especially from the United Kingdom - the focus of the epidemic - were curtailed.

The origin of the crisis was the feeding of cattle with animal feed that was contaminated by a new pathogenic agent - a prion. These cows, after a long incubation period, died of dementia. The prion entered the human food chain without evident symptoms being observed - the pathogen was able to jump the species gap to humans. This created unprecedented alarm amongst the public at large and gave rise to great interest in these diseases - Transmissible Spongiform Encephalopathies (TSEs) and in the great protagonist of the story, the prion.

Prions are proteins - without DNA - capable of causing rare neurodegenerative diseases, currently without a cure, by means of a novel process different from that of the “classic" virus and bacteria. The prion presents two distinct forms: a “healthy" one present in all animals (PrPc) and another, pathogenic one (PrPsc) which is the causative agent of the ailment. When PrPsc enters a living being, through feeding, it looks for the PrPc as a host and transforms it into a new PrPsc. So, the PrPc disappears and the PrPsc accumulates.

Although the PrPc exists in practically all the tissues of the body, the TSEs only appear in the brain. This is why neurones die and spaces or cavities appear which give rise to the typically “spongy" appearance. What route does the prion follow from the mouth to the brain? This is the great mystery. The first obstacle it encounters is the acidity of the gastric juices, then the action of the enzymes that break up the foods we swallow and then it has to pass through the digestive tract wall. The cells containing the PrPc could be the entry gate but … what cells are they? Are the same in different animal species? Nobody knows.

Answering these questions has been the aim of this thesis and, in order to achieve this objective, we carried out a “sweep" along the digestive tract of rats, primates and of Pyrenean cows - the most affected breed in Navarre. We employed a number of techniques enabling the location of PrPc- using antibodies specifically targeting it. These antibodies, amongst other items, were marked with fluorescent molecules so the positive cells could be subsequently visualised with a microscope.

The results showed that PrPc appears in the endocrine cells that are dispersed throughout the digestive tract. Endocrine cells produce hormones, biochemical messengers that are secreted into the blood and control the correct functioning of the body. This suggests that the conversion of PrPc to PrPsc may occur in these cells and arrive at the brain in the blood. But, in the digestive tract there are dozens of different endocrine cells and not all have PrPc. To identify which ones have and which do not, we detected, simultaneously, the PrPc and another substance, characteristic of each cell type and observed if the marking appeared in the same cells or not. By means of this system we were able to confirm that, curiously, the PrPc appeared in the same cells in all three animal species.

Finally, we also detected PrPc in the nervous system of the wall of the digestive tract. In this case, the disorder may be propagated to the brain through the signals between neurones - the synapses - that link both organs.