Biochemical Characterization of the Scrapie Infectious Agent

Scrapie is a degenerative brain disease of sheep that may be transmitted to laboratory rodents. It has been shown to be similar to human diseases such as Creutzfeld Jackob disease (CJD), Gersmann Straussler Syndrome (GSS) and kuru. A landmark property of these diseases is that, although transmissible, no nucleic acid, the material known to transmit genetic information, has been identified as part of the infectious agent. A protein, PrP6 =, is the only candidate for a component of the infectious agent which has been identified so far. The scrapie infectious agent has been denominated prion to distinguish these infectious agents from vi ruses and other conventional pathogens, a 11 of which contain nucleic acid.

This protein, PrP9 =, has specific properties that distinguish it from a normal brain protein, PrP=. The exact difference between the two proteins is unkown. It could be either chemical, meaning a specific chemi ca 1 group can be missing from one of them, or it could be conformational, i.e. a different folding of the molecule. The hypothesis under this proposal is that another molecule exists in the prion beside PrP6 =. If this is the case than the difference between the normal protein and the pathological one can be explained by a change in conformation of the normal protein caused by its binding to the other, as yet unidentified, component. If another component exists than it must be tightly bound to PrP•= since infectivity seems to be inseparable from this protein. No molecule fitting such a description has been found to date but this may be attributed to the fact that the putative second component of the prion is very low in concentration; a very likely possibility in view of the large number of PrP6 = molecules needed to transmit infectivity.

We intend to use methods that are as general as possible in order that no molecule, which may be a second component present in low concentration, will be excluded from the search, and so that any non- reactive chemical groups will be protected from the reagents to be used. Chemical reagents will be used that will bind PrP8 = to neighbor molecules, therefore introducing a “tag” to the putative second component of the prion. We will also use specific reagents such as enz ymes, which will digest specific molecules which have been shown to be in aggregation with PrP9 = in the brains of scrapie infected animals. This will allow us to see if such treatments will restore to the pathological protein its normal properties. Our preliminary results with enzymes that digest specific sugar polymers suggest that this may well be the case. Our long term goal, besides in c reasing our understanding of the pathological mechanisms of scrapie and the related human disor- ders, is to apply our increasing knowlege of the pr-ion disorders to other r-elated diseases such as Alzheimer-s ‘ disease (AD). AD and CJD , the human paral1e1s of scrapie, share many clinica1 and pathological features. Both diseases are dementing disorders that occur later in life, but they differ with respect to their incidence and peak age of onset. AD is 5000 times more common than CJD. The lack of blood or- CSF laboratory tests for CJD or AD sometimes causes confusion with the differential diagnosis.

Sugar- polymers may serve as the link between the prion disorders and other amyloidogenic diseases, such as AD . Amyloids are deposits, found in the brains of people suffering f ram these diseases, that are specific for these diseases. Sugar polymers have been found in the brain amyloid deposits of patients affected with AD as wel1 as in scrapie and CJD infected brains. The difference remains in the protein precipitating with the sugar polymer which seems to be different for each kind of disorder. In AD, the protein in the amyloid is A4 not PrPSc. The properties of both proteins whilst in the amyloid aggregate are very similar. Both come from a normal gene that at the same time produce a normal membrane protein with unknown function. In both cases, A4 and PrP, the protein in the deposits have different properties from that of the normal one, both being insoluble and resistant to proteases.

While CJD and scrapie are transmissible to experimental animals, transmission of AD has not yet been successful. Research is therefore restricted. It is likely that studies on other CNS degenerative diseases, caused by prions may provide important, new investigative approaches to AD.