Prion diseases are a group of related neurodegenerative diseases, including bovine spongiform encephalopathy (commonly called “Mad Cow disease”), scrapie and Creutzfeldt Jacob Disease (CJD), which are believed to be transmitted by a protein rather than by a microbe. According to the prion hypothesis, which was proposed by Stanley Prusiner in 1982, this infectious agent is an abnormally folded form of a normal protein that is present in all cells. The abnormal protein is toxic because it is insoluble and prone to aggregate in clumps which interfere with the functioning of nerve cells. These aggregates break down, so that the fragments become “seeds” which act as templates that induce more of the normal prion molecules adopt the abnormal conformation.
New research led by Tricia Serio, an assistant professor of molecular cell biology and biochemistry at Brown University in Providence, Rhode Island, has identified a protein which appears to be crucial for self-replication of the abnormal prion protein and for propagation of the “seed” in the budding yeast Saccharomyces cerevisiae. The work is published today in the online open-access journal PLoS Biology.
The protein implicated is heat shock protein (HSP) 104. HSPs are a large family of proteins present in all cells, which have previously been implicated in prion disease pathogenesis. HSP levels increase in response to exposure of the cells to elevated temperatures. They also act as molecular chaperones, assisting newly-formed proteins to fold up into their proper three-dimensional conformation, and preventing them from accumulating in clumps. It is this latter role which appears crucial for the inheritance of abnormally folded prion protein during division of yeast cells.
Serio’s team constructed a mutated form of the HSP104 gene, and a fusion of the gene encoding green fluorescent protein (GFP) and the gene encoding the yeast prion protein Sup35 . Both constructs were then inserted into yeast cells; because it was fused to GFP, the Sup35 molecules emitted a green fluorescence, enabling them to be visualized easily.
Examination of the yeast cells containing the mutated, non-functional HSP104 gene showed that Sup35 still formed toxic aggregates. However, it was found that there was a dramatic reduction in the motility of the aggregates within the cytoplasm. They remained clumped together, fragmenting at a far slower rate than they normally would, such that, during cell division, the abnormal prion was not transmitted to daughter cells. This also occurred when HSP104 was inhibited by addition of a chemical called guanidine hydrochloride.
The authors conclude that HSP104 is crucial for the seeding mechanism. They propose a model whereby disassembly of prion protein clumps is dependent on HSP104 function. In the figure below, which illustrates the model, the normal soluble form of Sup35 molecule is shown in green; the abnormal form of Sup35, which is insoluble and prone to aggregation, is shown in black; and HSP104 is shown as a barrel-shaped structure.
In 2005, the same team found that the abnormally folded Sup35 can induce normal Sup35 molecules to adopt the toxic conformation within a single cycle of cell division. It is because of this ability to rapidly seed the formation of new aggregates that a prion disease infection spreads quickly through the brain of an infected organism. Thus, the new findings could lead to the development of drugs which prevent or slow the spread of infection throughout the brain. Furthermore, it was recently discovered that amyloid-beta protein, which is associated with Alzheimer’s Disease, can also be propagated by the same seeding mechanism; the findings may, therefore, also lead to treatments that slow the progression of Alzheimer’s, and other neurodegenerative diseases in which there is an intracellular accumulation of toxic, insoluble proteins.
Satpute-Krishnan, P. et al. (2007). HSP104-dependent remodelling of prion complexes mediates protein-only inheritance. PLoS Biology 5: e24 DOI:10.1371/journal.pbio.0050024.
Satpute-Krishnan, P. & Serio, T. R. (2005). Prion protein remodelling confers an immediate phenotypic switch. Nature 437: 262-265.