Unfolding the mystery of misfolding PRIONS

Just mention the term prion and you’ll draw a blank look from most people. But say mad cow, and you’ll get a reaction.
Mad cows have spongy brains. They fall over. And, if you eat their parts, you might get the disease, too. That’s what most people remember of the mad cow crisis that swept Britain in the late 1980s. Thousands of cattle were destroyed before the epidemic was contained.
The culprit turned out to be cattle feed which contained ground up sheep offal contaminated with scrapies, a neurodegenerative disease of sheep. Scientists eventually figured out misfolded prions were behind the infection that had crossed the species barrier from sheep to cows and, sadly, to humans. In humans, it is called Variant Creutzfeldt-Jakob Disease.
In May 2003, Canada’s first official case of mad cow or BSE (bovine spongiform encephalopathy) popped up on a farm in Alberta. Overnight trade borders closed, stifling a $4.1 billion dollar a year beef export business. While barely a dozen mad cows have appeared in the ensuing years, the BSE crisis here cost billions in lost trade and lost jobs.
The Alberta government responded with a $35 million fund to support research into BSE and prion related diseases through Alberta Ingenuity and the Alberta Prion Research Institute. The federal government poured support into PRIONet, a member of the Network of Centres of Excellence. The University of Alberta set up the Centre for Prions and Protein Folding Diseases.
Over the past few years, the scientists involved with these groups have collaborated on some of the most important prion research in the world.
This year, I have had the opportunity to interview some of these scientists, thanks to a journalism award from the Canadian Institutes for Health Research.
What I find remarkable is the breadth of their research projects and the advancements they are making here. What started with one mad cow in Alberta has evolved into an intriguing investigation of a host of neurodegenerative diseases that impact all of us.
Two of the many talented researchers working in this area are Dr. David Westaway and Dr. David Wishart. Both are professors at the University of Alberta, although Dr. Westaway arrived from Britain via the University of Toronto. He is director of the Centre for Prions and Protein Folding Diseases.

Dr. David Wisehart

Dr. Wishart is a bio-informatics specialist and he was the lead scientist on the Human Metabolome Project. He describes prions as “small proteins that we all have. They’re in every living organism as far as we can tell, from yeast all the way to humans.”
The exact purpose of prions is not known, although they are thought to play a protective role. The normal or cellular prion protein is harmless. But sometimes things go wrong.
“Occasionally, if the protein has been mutated or if some external agent modifies the prion protein, it changes shape. Once it changes shape, it starts doing something bad. It actually starts self-assembling into what are called fibrils. And these fibrils start filling up brain cells, leading to brain cell death,” explains Dr. Wishart.
His most recent work uses a variety of techniques, including nuclear magnetic resonance spectroscopy, mass spectronomy and circular dichroism, to observe the prion protein at an atomic scale. The point is to observe the folding process in real time and hopefully find a way to stop it.
“Prions are naturally helical, meaning they look like a bunch of springs stuck together. But, when they misfold,” Dr. Wishart says, “they turn from a helical protein to something that’s called a beta strand or a sheet. And the beta sheet is in a sense a bunch of ribbons. And, in fact, silk is an example of a fibre that is made up almost exclusively of beta sheets. So when prions go from the helical to a beta sheet, they say that they convert or they misfold. This is something that proteins will often do. Normally, the body gets rid of them but, in the case of prions, when they misfold, they aggregate and they actually become toxic.”
It’s this aggregation the gives rise to the term “infectious”. A chain reaction starts and cannot be stopped.

Dr. David Westaway

According to Dr. Westaway, a normal prion or protein is a solo operator. As such, the outside of the molecule is covered with chemical charges that like water.
“The positive and negative charges interact with the water molecule and the protein molecule stays under control. It’s basically dissolved in water and everything is fine and dandy. But, in the context of disease, the proteins start to assemble into aggregates and very often this aggregation property is somehow linked to the fact that the non-water loving part of the molecule gets turned inside out. So, in chemical jargon, the hydrophobic parts of the molecule, instead of being hidden inside, come to the outside. The hydrophobic parts of a molecule like to interact with the hydrophobic parts of other molecules. So you start to get an assembly where the contact between the molecules is a bit more like an oily interface that pushes water out of the way.”
This initiates a domino effect where the proteins build up on one another.
In his lab, Dr. Wishart has been exploring how the prion converts or misfolds and has identified that the tail end of the molecule seems to be the part that gets disrupted or unfolded first.
“It aggregates first in what we call dimers or pairs, then in tetramers or sets of four and then, ultimately, in octamers, or groups of eight molecules. These aggregates are all hung together near the back end from last to about 70 residues. So they produce this insoluable, tightly massed core that can’t be cut… that can’t be broken down. Then these octamers eventually start forming fibrils or threads.”
Dr. Wishart goes on to say that it appears the tetramer and octamer, the groups of four and eight proteins, become highly toxic and form the principle seed that leads to infection.
It’s an exciting discovery that opens up the door to developing a means of stopping the misfolding process.
Dr. Westaway suggests that once you define the misfolding process in molecular terms, “then you can create an anti-molecule to stop it from happening. It’s what is called smart therapy.”
Over in his lab at the Centre for Prions and Protein Folding Diseases, Dr. Westaway has uncovered at least two important pieces to the prion puzzle. These are chaperone and shadoo proteins.
Chaperones are helper molecules. Sometimes when proteins start folding into the right shape, they get stuck, so along comes a chaperone to smooth it out so it goes into the right shape. This is well known in the science of cell biology for proteins inside the cell.
Says Westaway, “The field of chaperones is well known in the science of cell biology for proteins inside the cell. But prion disease is a bit more cutting edge because it seems there may well be chaperone-type activity involved in the wrong way, in helping a good protein go bad. Some of the crucial events of refolding or misfolding may not be going on inside the cell. In fact, they may be going on outside the cell. This is an important frontier that will bear close scrutiny in the coming years.”
A student in Westaway’s lab, Joe Watts, confirmed that this protein exists. It is quite abundant in the brain and has a lot of features similar to normal prion proteins.
“We think that shadoo may be part of a family of a molecules on the surface of brain cells that help brain cells deal with damage,” Dr. Westaway explains. “We have looked at what happens to the shadoo protein in an animal that has a prion disease and we were very surprised to get a very simple answer: that the shadoo protein starts to disappear when animals are replicating prions. It is what we call a tracer. We didn’t expect to make this discovery but, somehow when the protein is disappearing, it’s telling you that prions are replicating.”
He goes on to speculate that in a disease state, the shadoo proteins are being cannibalized by what he calls chopping proteins known as proteases. These get rid of proteins that are no longer needed by the body. What triggers the sudden attack warrants further investigation.
So what is it that brings home all this talk about how prions fold?
Catching Variant Creutzfeldt-Jakob Disease from a mad cow is still a very rare event. The odds in Canada are one in a million. However, there are other protein folding diseases that are all too common: Alzheimer’s, Parkinson’s and Lou Gehrig’s Disease.
The research of Westaway and Wishart goes a long way toward informing discovery on this front.
A provocative article, published in the journal Nature in early 2009, suggests that normal prion proteins, thought to be the protectors of nerve cells, are involved in the killing of brain cells. It appears the misfolded protein aggregates that cause Alzheimer’s bind to the normal prion proteins to initiate the killing.
Dr. Westaway is adamant. “We’ll check that out, and other labs will check that and very likely some important new knowledge will come out of looking at those ideas with a fine tooth comb.” √

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One Comment on “Unfolding the mystery of misfolding PRIONS”

  1. Michell Says:

    The best explanation I have come across, thank you!

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