26 October 2011. When it comes to neurodegenerative diseases involving proteins that clump together, the consensus seems to be that small aggregates, or oligomers, are bad for cells, whereas larger ones, or amyloid fibrils, are less offensive. Although it’s not clear what the nasty oligomers actually look like or how they cause disease (see ARF Webinar), few researchers would contend that making them more resilient is a sensible route to therapy. But that’s exactly what the group of Véronique Perrier at the University of Montpellier in France has done. In a paper in the October 19 issue of The Journal of Neuroscience, Perrier and colleagues describe a small molecule that stabilizes oligomers of misfolded prion proteins, making them less deadly. “This is an interesting paper,” said Gregory Petsko at Brandeis University in Waltham, Massachusetts, who was not involved in this study. “I like the fact that they are trying to think outside the box.”
Perrier and colleagues propose that their results have implications for developing therapies and diagnostic tests for prion diseases and related conditions. But the conclusion has some researchers scratching their heads. “In Alzheimer’s, the idea for therapy has been to avoid the formation of oligomers, but here they are saying that more oligomers can reduce infectivity. It’s an interesting phenomenon, but I don’t know that this is something that would be common to other conditions,” said Gianluigi Forloni at the Mario Negri Institute in Milan, Italy, who also played no role in this study.
Prions are unique in that there are many different forms, or strains, of the abnormal prion protein, commonly referred to as prion scrapie (PrPsc). A study that compared the infectivity of an array of recombinant PrPsc proteins found that the varieties forming the most stable aggregates took longer to infect mouse brains, whereas the most labile needed the shortest incubation times (Colby et al., 2009). “I don’t know of similar studies having been done with other amyloid proteins,” said Petsko. “Other diseases don’t seem to work that way.”
According to the prion hypothesis, a PrPsc seed can trigger the conversion of normal prion protein in cells (PrPc) into the abnormally folded PrPsc, which then leads, via a process that involves many steps and various oligomeric intermediates, to amyloid fibrils. Some scientists have proposed a similar “seeding and nucleation” process for amyloid-β (Aβ) and other amyloidogenic proteins (see ARF related news story on Morales et al., 2011 and Langer et al., 2011). To detect whether the transition from PrPc to PrPsc has occurred, researchers typically look in brain tissue for a specific marker, a short peptide called PrP(27-30). Because PrPsc, unlike its cellular counterpart, is partially resistant to digestion, proteinase K leaves PrP(27-30) untouched. “When you screen for anti-prion drugs, most people have been looking for a decrease of this marker,” said Perrier. “In this study we did the opposite. We searched for compounds that favor the multimeric form of PrP(27-30).”
First author Adeline Ayrolles-Torro, Perrier, and colleagues used a two-step process. The first part was a “virtual screen.” They looked at models of PrP structure to find a region that could serve as a binding site for a small molecule, and from this, selected 32 commercially available compounds. Moving out of the virtual world, they tested those chemicals for blocking or promoting PrPsc aggregates in prion-infected nerve cells. One compound, P30, increased the amount of PrP(27-30) dimers and trimers, as detected by immunoblotting after proteinase K digestion.
To test their theory in vivo, the researchers incubated their compound with brain homogenates from PrPsc-infected mice and then injected the P30-treated extracts into the brains of healthy mice. The animals lived longer (175 days) than those injected with untreated homogenates (157 days).”It is not a big increase in survival, but sufficient to show that the compound has an effect,” said Perrier. Her group is now using those compounds to treat the prion-infected mice—a model that more closely mimics human disease. “They have done the first step,” said Forloni. “They will need to do more experiments to see whether this drug will be effective in reducing infectivity.” Although Perrier and colleagues have not done any experiments to find out how their compound might have helped increase survival, Perrier speculated that P30 might stabilize pre-amyloid forms of PrPsc, preventing them from proceeding further along the pathogenic pathway.
One limitation of the approach is its reliance on SDS PAGE to detect the oligomers, suggested Sylvain Lesne of the University of Minnesota, Minneapolis. He pointed out that other methods such as size-exclusion chromatography “give more information about the nature of the molecules that are in solution, under more physiological conditions.” With SDS PAGE and denaturation, there is a potential for artifacts, which is a concern for those studying Aβ as well (see Hepler et al., 2006). An unexplained finding is that P30 has much higher affinity for PrPsc and only causes PrPsc to form the more stable dimers and trimers, even though the region to which P30 binds is conserved between PrPc and PrPsc. “We think there is some interaction with the cellular PrP, but it is not sufficient to make oligomers,” said Perrier.
Could stabilizing oligomers be a broader therapeutic strategy? The European Medicines Agency may soon approve a drug for familial amyloid polyneuropathy that stabilizes native tetramers of transthyretin, preventing them from dissociating into monomers that can then form amyloid (see ARF related news story). Recently, Petsko's group, and researchers working independently at Dennis Selkoe’s lab at Brigham and Women’s Hospital, Boston, reported that α-synuclein, the protein that accumulates in Parkinson’s disease, normally exists as a an innocuous tetramer as well (see ARF related news story). “It is possible that stabilizing these tetramers might be a therapeutic approach,” said Petsko, "but whether this would ever lead to a therapy for Parkinson’s is another thing,” he cautioned.—Laura Bonetta.
Ayrolles-Torro A, Imberdis T, Torrent J, Toupet K, Baskakov I, Poncet-Montange G, Gregoire C, Roquet-Baneres F, Lehmann S, Rognan D, Pugniere M, Verdier J-M, Perrier V. Oligomeric-Induced Activity by Thienyl Pyrimidine Compounds Traps Prion Infectivity. The Journal of Neuroscience, October 19, 2011 • 31(42):14882–14892. Abstract