As long ago as 1985, scientists in the UK observed scared and aggressive cows showing movement disorders, lurching clumsily and finally breaking down. Shortly afterwards Gerald Wells and John Wilesmith described the Bovine Spongiforme Encephalopathy (BSE) due to the spongiform brain damage of the cow as an independent disease – prion research started.
The prion-hypothis, scepticism and a Nobel Prize
Stanley Ben Prusiner, a US-American doctor, postulated prions already in 1982, i. e. nucleic acid free, infectious proteins, as the trigger of numerous diseases such as Scrapie, BSE, Creutzfeldt-Jakob-disease, Kuru, the fatal familial insomnia as well as the Gerstmann-Sträussler-Scheinker-Syndrom (GSSS). At first the professional world cloaked with scepticism – it seemed to be hard to imagine that solely a protein would be responsible for all this suffering. Nowadays the prion hypothesis is considered relatively backed but additional influencing factors cannot be excluded. Prusiner was honored for his achievement with the Nobel Prize in medicine in 1997.
Helix or sheet
The prion-proteinC (PrPC, “cellular“) is found in healthy human brain and medulla in the form of spiral alpha-helices. By contrast the pathogenic infectious modification (PrPSC, “Scrapie“) is characterized by a deficient folding with bands – the beta-sheets. If PrPSC enters the body, the body’s own PrPC changes its spatial structure, new PrPSC develops which now accelerates the transformation of PrPC.
Due to its poor solubility, the protein PrPSC deposits – the affected cells perish, the brain degenerates building spongiform structures. For example Kuru is an according disease, the iatrogenic form of Creutzfeld-Jacob-Disease (iCJD) as well as BSE or Scrapie. Thus transgenic neurons which are not building PrPC cannot be infected with PrPSC.
In its classic form, the Creutzfeld-Jacob-Disease (sCJD) develops – due to the random spatial structure change – from PrPC to the infectious variation PrPSC. Genetic factors are considered the triggering factors for the familial form of Creutzfeld-Jacob-Disease (fCJD), the Gerstmann-Sträussler-Scheinker-Syndrom and the fatal familial insomnia.
A cellular communicator
But which function does the physiological prion-protein have within the body? A group of researchers at the Constance University has come one step closer to the secret: The protein supports communication between different cells during the embryonic development. Zebrafish embryos were not able to develop properly if the according gene coding for PrPC had been cut off. “This is how we were able to prove that PrP works as some kind of clue involved in building and preserving of cell-cell-contacts”, emphasizes Dr. Gonzalo Solis, member of the Constance research team, because without the aid of PrPC some proteins cannot reach the cellular ‘job site’.
Researchers at the Rudolf-Virchow-Center as well as the Institute for Virology at the Wuerzburg University decoded another piece of the puzzle in the complex mechanism of prion diseases: Dr. Vladimir Ermolayev and his team injected a test colorant into diseased nerve cells which binds to transporters. In functioning state those vehicles transport different proteins from the synapsis into the cell body. After onset of the symptoms of a prion disease, more than 45 percent of the cells weren’t able to do so. We have known about according processes in other neurodegenerative diseases like Alzheimer for quite some time. “The results are – especially for the diagnosis of prion diseases – very helpful, in particular since up to now we were only able to recognize a disease after it had been too late”, says Dr. Vladimir Ermolayev.
It all starts with an oxidation
But how is it possible that the structure of PrPSC can change spontaneously like for example in the classical form of the Creutzfeld-Jacob-Disease? Scientists at the Max-Planck-Institute for Biochemistry and the Ludwig-Maximilians-University in Munich went into this matter. The amino acid methionine, a component of the PrPC sequence plays a central role here. Methionine oxidizes easily by reactive oxygen species. Normally it affects only molecules on the PrPC surface. They are reduced again by the methionine sulfoxide reductase. Thus PrPC protects from reactive species such as hydrogen peroxide or free radicals.
If a cell is exposed to high oxidative stress or the repair system does not function well any more like with higher age, methionine inside the PrPC are affected as well. This has fatal consequences.
While methionine stabilizes the helix structure, the oxidized form supports the building of sheets and thus the change to PrPSC. “Nediljko Budisa, head of the research group “Molecular biochemistry at the Max-Planck-Institute for Biochemistry explains: “Although we are also discussing other mechanisms for the change into the malignant variation, we think that the oxidation of the amino acid methionine inside the prion protein plays a decisive role as well.”
A mutation with consequences
Genetic causes were postulated for the familial form of the Creutzfeldt-Jakob-Disease (fCJD), the Gerstmann-Sträussler-Scheinker-Syndrom and the fatal familial insomnia. A team of scientists at the Zurich University was able to support this hypothesis. They entered two point mutations into the prion gene of the mouse. The according exchange of amino acids resulted in a stiffening of the protein structure. In fact the transgenic animals developed both, plaques in the brain and symptoms of the prion diseases. Prof. Dr. Adriano Aguzzi at the Institute for Neuropathology: “The results confirm the hypothesis that prions consist solely of proteins and they suggest that the mutated protein is enough to cause an infectious disease.”
Stumbling block metabolism
In the iatrogenic form of the Creutzfeld-Jacob-Disease respectively with BSE or Scrapie, the infection with the protein PrPC has fatal consequences for the cellular metabolism. Christian Bach and a team of researchers at the Technical University Munich were able to prove that in affected cells the activities of more than 100 genes changed. “It affects several enzymes of the cholesterol biosynthesis”, explains Bach. Since the control mechanisms are affected as well, the cholesterol content leaps. “It is remarkable that only nerve cells react like that”, Prof. Dr. Hermann Schätzl at the Institute for Virology at the ‘Klinikum rechts der Isar’ (hospital in Munich). “Cells of the supporting frame on the other hand do not increase their cholesterol production”. It is a possibility that the increased cholesterol biosynthesis adds to the demise of neurons.
Much to be done
New research projects peruse mainly the ways of transmission of prion diseases. Perhaps the contagion happens by prion-associated proteins and not by the actual prion protein PrPSC? In addition it is planned to expand routine diagnostics for this disease. Researchers hope to find target structures for therapy once they understand the mechanisms going on in affected cells – there are many promising approaches.