With many putative rules that govern protein folding having been discovered, novel exciting directions are emerging from particular problems of neurodegenerative diseases. More than 15 severe maladies including prion, Alzheimer's, and Parkinson's diseases have been linked to specific protein aggregation. A common feature among all of these 'conformational' disorders is the conversion of a normal isoform of a protein into a specific -sheet rich polymeric amyloid form. Recent studies have demonstrated that a broad variety of proteins unrelated to any known conformational disease can adopt -sheet rich amyloid forms in vitro and in vivo. The fundamental questions are: how general the phenomenon of amyloidosis is (Figure 1); and if the amyloidosis is indeed more general than we thought before, what strategies presumably have evolved in nature to prevent amyloidosis?

Research in my lab is focused on the molecular mechanism of the conformational transitions of the prion and non-prion proteins and address basic issues of protein folding. They include the possibility of forming distinct long-lived states within the same primary structure, the position of the native state versus the abnormal states in the energetic landscape, and the overall complexity of the energy landscape of folding.

Our previous studies revealed that the folding of the prion protein (PrP) to its native a-helical conformation is under kinetic rather than thermodynamic control (Figure2). We found that abnormal b-sheet rich isoform is thermodynamically more stable than the native a-helical isoform. The conformational transition from the a-PrP to the b-PrP is separated by a large energetic barrier that is associated with unfolding and with a higher order kinetic process of oligomerization. Although the b-PrP is thermodynamically more stable than the a-PrP, the process of conformational transition is prevented over the protein's lifetime because of high energetic barrier. Thus, to avoid formation of non-native forms and to optimize the efficacy of the native folding pathway, thermodynamic and kinetic parameters should be subjects of natural evolution.

Figure 2. Free energy diagram for the conformational transition of the a-PrP to the b-PrP.

Our continuing work indicates that PrP is capable of forming structurally distinct -sheet rich abnormal isoforms in vitro, a b-oligomeric and amyloid forms. The b-oligomer is off the kinetic pathway to amyloid formation. The preference to fold into a particular abnormal isoform is dictated by experimental conditions. With the recognition that a common feature of all prion-associated maladies is the misfolding of PrP, discovery of multiple pathways of misfolding and formation distinct -sheet rich isoforms may explain the significant variation in prion disease phenotypes and the broad diversity of neuropathologic changes.

Recently, we have developed an in vitro conversion system that permits study of self-propagating conformational transition of PrP outside of the cellular environment (Figure 3). Currently we are using this tool to investigate (1) the extent to which the autocatalytic mechanism of replication is an essential element of the self-propagating process; (2) the degree to which sequence identity of interacting PrP isoforms is necessary for self-propagation; and (3) the role of the template in conformational diversity of self-propagating -sheet rich states.

Figure 3. In vitro conversion of recombinant a-helical monomeric PrP leads to the formation of b-sheet rich multimeric fibrils with properties of amyloid.

Considering the view that amyloid formation is a common property for many proteins, in future we will study the extent to which amyloidosis of non-prion proteins reproduces the basic hallmarks of self-propagating conformational transition of PrP. We will address the question of whether these specific features arise from the primary structure of PrP or are an inherent property of the amyloidogenic process.

Prion Resources and Links

A History of Discovery of Prion Diseases
The Prion Hypothesis by Stanly Prusiner
The Evidences Supporting the Prion Hypothesis
Mad Cow home page

The 1976 Nobel Prize to D. Carleton Gajdusek for his discoveries concerning "new mechanisms for the origin and dissemination of infectious diseases".
http://www.nobel.se/medicine/laureates/1976/press.html

The 1997 Nobel Prize to Stanley B Prusiner for his discovery of "Prions - a new biological principle of infection".
http://www.nobel.se/medicine/laureates/1997/press.html

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