[excerpt] Fox Chase Cancer Center researchers and their colleagues in Japan and San Francisco have obtained new insight into the molecular structure of prion particles responsible for mad cow disease and other degenerative neurological disorders. In new research to be published in this week's Online Early Edition of the Proceedings of the National Academy of Sciences (www.pnas.org), Fox Chase biophysicist Heinrich Roder, Ph.D., and colleagues describe a computer model of the structural core of prions, based on biophysical measurements of a fibrous form of a prion protein fragment. Prions are infectious protein particles linked to degenerative neurological diseases in animals and humans, such as mad cow disease (bovine spongiform encephalopathy or BSE) in cattle, scrapie in sheep and goats, and Creutzfeldt-Jakob disease (CJD) in humans.
For proteins, form really does equal function. Not only are they essential building blocks of the body, but proteins are also the workers of every cell, carrying out its specific functions. This function depends on the ultimate three-dimensional shape of the protein, a form achieved by folding flexible chains of amino acids until each is properly aligned so that the protein can do its job. Normally, the folding of proteins is highly efficient and specific, but sometimes the process goes awry, resulting in dangerous misfolded forms.
Prion diseases result from the conversion of a normal cellular protein into an alternative structure that forms threadlike fibers called amyloid fibrils. They accumulate in target tissues, such as brain tissue, where they cause the progressive degeneration of cognitive and motor functions and ultimately prove fatal.
Amyloid fibrils form as a result of mistakes in a protein's normal folding process. Each disease involving amyloid fibrils stems from the misfolding of a different protein that then packs into a similar structure. The formation of amyloid fibrils is linked to a wide range of diseases, not only BSE and CJD but also Alzheimer's disease, Type II diabetes and Parkinson's disease.
"Unraveling the molecular basis of this fundamental process is a necessary first step toward treating these diseases," Roder said. Unlike other amyloid fibrils, prion particles can interact with the normal host protein and transmit the disease from one individual to another. In some especially alarming cases, the disease can be transmitted from one species to another, as in the case of a new human variant of CJD linked to BSE.
"Despite the growing list of diseases known to involve deposits of fibrillar protein aggregates in and around cells, our understanding of the structural basis of these amyloid fibrils is rudimentary at best," said Roder. The large size and insolubility of these protein aggregates have limited the use of high-resolution techniques for structural studies, such as X-ray crystallography and nuclear magnetic resonance.
"We have been able to overcome some of these limitations by using NMR-based hydrogen-deuterium exchange experiments in conjunction with a solvent quenching protocol," Roder explained.
The new paper, entitled "NMR-Detected Hydrogen Exchange and Molecular Dynamics Simulations Provide Structural Insight into Fibril Formation of Prion Protein Fragment 106-126" will appear in the Dec. 9 issue of PNAS.
"Understanding the physical principles underlying the folding of proteins is a major challenge of molecular biophysics," said Roder. "Because aggregation of misfolded proteins can lead to disease, this basic knowledge has important implications for medicine as well as bioinformatics, biotechnology and cell biology."
Kazuo Kuwata, Ph.D., of the department of biochemistry and biophysics at Gifu University's School of Medicine in Japan is also a corresponding author of the study and has been a visiting scientist in Roder's Fox Chase laboratory. Other study authors include staff scientist Hong Cheng, Ph.D., of Roder's lab; Thomas L. James, Ph.D., of the department of pharmaceutical chemistry at the University of California at San Francisco; and Tomoharu Matumoto, Ph.D., and Kuniaki Nagayama, Ph.D., of the laboratory of ultrastructure research at Japan's National Institute for Physiologic Sciences.
