Prions are novel, transmissible pathogens that differ from viroids, viruses, parasites, fungi, and bacteria, both with respect to the diseases they cause and their structure . They are capable of causing degenerative diseases of the centra1 nervous system both in animals anal in humans. Kuru, Creutzfeldt-Jakob’s disease (CJD) and Gerstmann-Straussler-Sheinker’s (GSS) syndrome illustrate the acquired, sporadic, and genetic manifestation of the -human prion diseases. These transmissible pathogens also cause Scrapie of sheep and goats. Bovine spongiform enphalopathy,(BSE) in cows and transmissible mink encephalopathy, and chronic wasting disease (CWD) of captive mule deer and elk which are thought to result from the ingestion of Scrapie infested animal products (Pruisner, 1991). In addition to these diseases, certain neuropathological changes associated with scrapie disease seem to correlate well with similar changes observed in Alzheimer’s patients (Duguid 1989). These similarities have been attributed to similar alterations in gene expression in both of the diseased states (Duguid, 1989).
First, in addition to the unique diseases that they cause, prions also exhibit certain novel molecular and structural properties which further differentiate them from other infectious pathogens. The unusual biological properties were first unraveled through experiments performed on scrapie -infested sheep. The isolated scrapie agent from these sheep seem to exhibit certain unusual properties. One such property is the scrapie-agents resistance to nuclease digestion. After being subjected to several rounds of digestion with various nucleases including micrococcal nuclsase, nuclease - P, and deoxyribonucleases 1 and 2, there still seemed to be no decrease in prions ability to infect, such as in scrapies. Limited proteolysis ability,
however, was found with proteinase K (Stahl and Prusiner, 1991). Furthermore, the scrapie agents (prions) resistance to UV irradiation at 254 nm also indicates a photochemistry very different from that of viruses- (Bellinger-Kawahara et. al. 1988). The resistance of the scrapie agent to UV irradiation is compared to that of other viruses and viroids in table 1. Lastly, Studies on the scrapie agent in murine spleen with the use of rate-zonal sucrose gradients and gel electrophoresis, has led to the discovery that the scrapie agent has a small molecular weight ranging around 50,000 daltons or less (Prusiner, 1982). The various molecular properties of the scrapies agent (prions), as discussed above, clearly indicate prions uniqueness from either viruses or viroids. The different novel properties of the scrapie agent are summarized in table 2.
Having now understood some of the basic differences between a prion and other infectious pathogens, it is now important to focus on what aspects of the prion protein make it an infectious pathogen. Subcellular fractions from hamster brain enriched for scrapie infectivity were purified by homegenation, detergent extraction’s, nuclease digestion, limited proteolysis with proteinase K and centrifugation on discontinues sucrose gradients (Bolton et. al., 1982). Analyses of the resulting fractions led to the discovery of a major band migrating between 27 and 30 KDa. This protein was subsequently labeled PrP 27-30 (Stahl and Prusiner, 1991). Interestingly, the PrP 27-30 is not found in similar fractions purified from normal brain (Bolton et. al., 1982). Through further studies it was found that PrP 27-30 is the disease-specific protease resistant core of a larger protein of 33-35 KDa called PrPc which is protease-sensitive (Prusiner, 1991). While it is known that both the normal (protease-sensitive) PrPc and protease-resistant PrPsc are encoded by the same endogenous gene, the nature of the disease- associated modification of PrP is not understood. It has been proposed, however, that PrPsc, when introduced into a normal cell, causes the conversion of PrPc or its precursor into PrPsc by an unknown process which may involve conformational or chemical modifications, during or after it synthesis (Caughey and Raymond, 1991). It is believed that these chemical modifications may specifically target post translational modifications since a variety of experimental evidence indicates that PrPsc is formed during a posttranslationa1 event from PrPc or a precursor (Stahl and Prusiner, 1991). It. is believed PrPsc ability to cause conversion of PrPc to PrPsc when introduced to a new host cell may be the mode of exogenous infection of prions from one animal to another.
Prions are composed largely, if not entirely, of an abnormal isoform of the prion protein (PrPsc), which is a normal cellular protein occurring at high concentration in a diseased central nervous system. It is possible that spontaneous mutations in the prion gene can give rise to the isoform(PrPsc), without any sort of exogenous infection, which may in turn cause sporadic and hereditary human diseases (Gabizon et. al., 1989). One such human disease is Creutzfeldt-Jakob disease. One study has found, through allele--specific oligonucleotide hybridization studies, that a codon 200 lysine mutation of the prior-protein gene is consistently found among Jews with the Creutzfeldt-Jakob disease, which strongly supports a genetic pathogenesis for their illness (Hsiao et. al., 1991). This disease occurs more than 100 times more frequently among Libyan Jews than in the worldwide population which researchers attribute to the codon 200 haplotype of the prion protein gene found among all the Libyan Jews with the disease (Hsiao et. al., 1991). Another example of how a mutated PrP gene can give rise to a diseased state is Gerstmann-Straussler-Sheinker’s disease. This disease is tightly linked to a proline to leucine change in codon 102 of one of the alleles of the PrP gene. This mutation was not observed in 100 normal individuals or in 15 individuals suffering from other forms of inherited or sporadic prion diseases (Weissman, 1991). It is believed that the amino-acid substitution allows spontaneous conversion of PrPc into PrPsc with a frequency high enough to allow for the expression of this disease in the lifetime of the individual (Prusiner, 1991).
Another potentially important finding, for humans, in prion related research is the fact that certain similarities have been found in relation to neuropathologica1 changes in scrapie and Alzheimer’s disease (Diedrich 1991). Furthermore, it is believed that these neuropathologica1 changes shared by scrapie and Alzheimer’s disease can be attributed to similar changes in brain gene expression (Duguid, 1989). To begin, some of the central nervous system lesions shared in both conditions include astrocytosis, amyloid deposition, vacuolation, neuron loss, and neuroaxona1 dystrophy (Diedrich,1991). These analogous pathological lesions in scrapie and Alzheimer’s disease, as mentioned in the previous paragraph, implies a convergent pathogenic mechanism which at the molecular level involves the activation or repression of a defined set of genes whose altered expression is responsible for the neurodegenerative disease (Duguid 1989). To support this statement it has been found through comparison of mRNA populations both in control and diseased brain tissue by differential screening of a cDNA library that there are two mRNA’s, apolipoprotein E (apo E) and cathepsin D (CD), that seem to increase in abundance in both diseases (Diedrich 1991). In addition, it has also been documented through past research that there is also an increased expression of mRNA’s leading to the production of glial fibrillary acidic protein (GFAP), the B chain of apha-crystallin, metallothionein 2, and sulfated glycoprotein 2 in both scrapie and Alzheimer’s disease (Diedrich, 1991).
One theory as to the purpose of the increase in apo E expression is that it may represent an attempt by astrocytes to repair or limit damage in the central nervous system (Duguid, 1989). In diseased states such as in Alzheimer’s or scrapies disease, the astrocytes may be assuming the role of macrophages in attempting to repair damage (Diedrich, 1987). In addition, it is also believed a possible reason for the increased cathepsin D in astrocytes, is that it may represent a convergent pathway in amyloid formation or processing even though there is a difference in the amyloid-forming proteins of the various diseases (Diedrich, 1991). The amyloid proteins that are produced in both diseases may be just pathological by-products inadvertently produced by astrocytes or it may be a stimulus which triggers astrocytes to limit damage or repair injury in the central nervous system.
It has also been found, as mentioned earlier, that early in the course of infection, in animals with scrapie agent present, astrocytes increase in number and size and their processes seem to enlarge and become more numerous (Duguid, 1989). This transformation of the astrocytes is accompanied by an increase in the amount of GFAP mRNA and protein. This accumulation GFAP seem to localize more in processes surrounding small blood vessels, the ventricles, foci in the hippocampus, thalamus, deeper layers of neocortex, and the cerebellum (Duguid, l989). In Alzheimer’s disease, the GFAP mRNA is increased in neuritic plaques and in astrocytic processes infiltrating the plaques from the astrocytes at the perimeter (Duguid, l989). Like the theories proposed to help explain the role of apolipoprotein E and cathepsin D in the neurodegenerative state, other theories have also been formulated to help explain the increased expression of the mRNA leading to glial fibrillary acidic protein (GFAP) in both scrapie and Alzheimer’s disease. One theory views the striking localization of GFAP mRNA in astrocytic processes as visual evidence for a possible solution utilized by glial cells to the problem of trying to produce protein at sites some distance away from the cell body (Diedrich ,1991). Both astrocytes and oligodendrocytes seem to transport mRNA’s for GFAP and myelin basic protein to sites of protein synthesis in processes and membranes (Diedrich, 1991).
Based on current research it is clear that prions are unlike other infectious pathogens both in structure and in some of the wide variety of diseases they elicit. A number of current experiments also implicate PrPsc as the infectious and pathogenic isoform of the prion protein. It is believed that the infectious prions mode of infection between animals is by causing a conversion of PrPc into PrPsc by possibly targeting post-translational modifications in the host cell. However, a spontaneous mutation of the PrP gene may also induce the conversion of PrPc to PrPsc without any exogenous infection.
Although our understanding of PrPsc and PrPc has progressed immensely in the past few years, there still remains many questions to be answered about this novel infectious pathogen. For example, the mechanism by which prions replicate still remains elusive. Gaining a better understanding of the different events that feature in prion replication should help us better understand the basis for its specific mode of action as an infectious pathogen. By also continuing research into the changes in gene expression of mRNA’s in neurodegenerative diseases, such as the experiments performed comparing the neuropathologica1 changes between Alzheimer’s and scrapies, can lead to deeper insight into the primary causes of not only these diseases but of other central nervous system degenerative disorders such as Parkinson’s disease. Common themes and patterns which may emerge from this type of research may provide a better understanding of the primary causes of the various neurodegenerative disorders. For example, through the research conducted both on scrapie and Alzheirner’s disease, some important themes leading to the respective neurodegenerative diseases have been discovered. One important theme that seems to emerge from these analysis is the importance of astrocyte activation in the formation of lesions (Diedrich, 1987). Activated astrocytes have been found to be associated with amyloid plaque deposits in not only scrapie but also in Alzheimer’s disease as well (Diedrich, 1987).
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