Prion disease is a molecular hall of mirrors, in which a normal cellular protein, PrPC, is refolded by the pathogenic PrP “scrapie” protein, PrPSc. The very idea of a disease transmitted without bacterial or viral infection or without an inherited or somatic genetic mutation was resisted by many biologists after prion disease (and the name itself) was proposed by ASCB member Stanley Prusiner in 1982. But further research, a greater understanding of the powers and perils of protein folding, and the outbreak in the late 1980s of “mad cow disease” or bovine spongiform encephalopathy (BSE) led to wider scientific acceptance, crowned by Prusiner’s 1997 Nobel Prize in Medicine. Nearly a decade later though, there is still no treatment, cure, or even consensus on the best therapeutic strategy for prion disease in humans, domestic livestock, or wild animals, which are thought to be the disease reservoir for PrPSc.
There isn’t even a consensus on what the normal protein, PrPC, does, according to University of Toronto prion researcher David B. Williams. There is, for example, an established line of PrPC –null mice that seem to thrive and reproduce easily while remaining impervious to prion disease. This raised the question of whether prion disease could be treated by drugs that block or lower the expression of normal PrPC. Williams and co-workers were intrigued by recent reports that FK506, an immunosuppressive drug in wide use for organ transplantation as an inhibitor of the FKBP family of peptidyl prolyl isomerases, significantly increased survival in mouse models with active prion disease. These were promising results but the mechanism of action was controversial. Did FK506 affect the progression of prion disease by blocking calcineurin activation of T-cells, by up-regulating autophagy, or by reducing the expression of normal PrPC? In a new paper published in the March 1 issue of Molecular Biology of the Cell, Williams, Pawel Stocki, Maxime Sawicki, and colleagues in Toronto and at the University of Alberta clearly show that FK506’s protective effect is coming from a sharp down regulation in PrPC production. The drug does this by interfering with translocation of the protein into the ER.
“Prior to our paper, it was known that FK506 has various effects on the prion protein,” says Williams. “But there was quite a controversy as to what was actually happening so we confirmed that there was a reduction in amount and then we went on to show how this occurs in some detail.” FK506 could be affecting prion protein production at a number of points, according to Williams, “Reduced transcription or translation were possibilities as were degradation of the protein in lysosomes or in the cytosol or even shed from the cell surface. What we did was pinpoint the actual process where FK506 is causing the degradation of the prion protein—its inability to get into the ER. It’s FK506 affecting translocation.”
The results suggest to Williams a different avenue of attack for prion disease. Certainly new strategies are needed. Other promising drug approaches that bind either the normal or the disease prion must struggle to devise ways across the blood brain barrier Attempts to use antibodies against prions in the brain have raised concerns about neurotoxicity. Promising drugs effective against one strain of prion disease in mice have yielded disappointing results in trials with mice carrying the human Cruetzfeldt-Jakob Disease strain.
The novelty of using FK506 to knock down PrPC expression before the proteins can be misfolded by the disease prion has advantages, says Williams. There are clearly many different prion strains within a species, let alone across species boundaries. Flattening PrPC expression should in principle work against all strains, he believes, even in those carried by deer and elk populations that transmit disease prions to livestock like sheep or cattle.
But the drug itself, a powerful immune suppressant approved for use in organ transplants, has strong side effects. Mice treated with FK506 at the doses required to extend survival following prion infection were very sick indeed, Williams says. The next step here would be to test less immunosuppressive variants of FK506, which his lab has shown still reduce PrPC expression by inhibiting translocation, but don’t exist yet in sufficient quantity for large animal trials.
But it’s the precise role of the FKBP enzymes on the translocation of the prion protein that still eludes Williams. “That’s why I say that we don’t know the fine details yet. FK506 is an inhibitor of the FKBP family of peptidyl prolyl isomerases so there must be an FBKP involved somewhere. We tried really hard to knock down each of the 12 FKBP proteins within the ER and cytosol to pinpoint the mechanism but we had difficulty recapturing the phenotype due to toxicity issues. Only in the case of one of the ER family members, FKBP10, did we see that its depletion reduced PrPC expression in mouse cells. Surprisingly, when we looked at the mechanism, it was not due to a block in translocation but rather it was due to degradation after entry into the ER. These findings indicate that FKBP enzymes are involved at more than one stage of prion protein biogenesis, both during and after translocation, and that interfering with their function is an exciting avenue in the development of new treatments for prion diseases.”
About the Author:
John Fleischman was the ASCB Senior Science Writer from 2000 to 2016. Best unpaid perk of the job? Working with new grad students and Nobel Prize winners.
