In this study, we have set up two experimental scrapie infections on hamsters by cerebral inoculations of mouse-adapted scrapie agent 139A and ME7. Typical neuropathological abnormalities of TSE and deposits of PrPSc have been observed in the brains of the infected hamsters, confirming again that mouse-adapted scrapie agent can overcome species barrier to infect hamsters. The PrP proteins of mouse and hamster share a great deal of homology in amino acids sequences and in tertiary structures, with only seven amino acids differences. Our data here provide the evidences that hamster's PrPC supplies as the substrate for replication of mouse-derived PrPSc. A number of sCJD strains can transmit more efficiently to the human PrP transgenic mouse lines than to wild type mice, which shows shorter incubation times and higher susceptibility . However, it makes an exception that replacement of the murine PrP gene with bovine PrP gene led surprisingly to longer incubation period for BSE in the transgenic mice than in the wild type mice despite the increase in identity between the host and donor PrP . It seems that except for the consistence of PRNP sequences, other unknown factors will affect on the host susceptibility. Like other TSE transmissions among different species , the clinical manifestations in the infected hamsters emerge extremely late after long incubation times. It reflects an inefficient conversion from hamster's PrPC to PrPSc by exotic mouse prion protein during the first passage. Successive passages of the new strains in hamsters in future may decrease and fix the respective incubation periods.
Different incubation times and clinical courses in hamsters by infection of agents 139A and ME7 imply that besides the amino acid homology between mouse and hamster, scrapie strain is another element for the transmission across species. Such phenomena have been described elsewhere . The amounts of the infectious agents in this study seem not to be the essential reason, since the levels of PrPSc of two strains, regardless in mice brains as the inoculum or in hamster brains as the product, are quite comparable. One speculation may lie on the differences in their unknown tertiary structures of those two prion strains, leading to the differences possibly in molecular level during conversion from PrPC to PrPSc.
The hamsters infected with mouse-scrapie agent 139A and ME7 possess similar pathogenic and pathological changes. Instead of the predominantly monoglycosyl PrPres in the original mouse-adapted strains, the PrPres formed in the hamster brains infected with agents 139A and ME7 are predominant diglycosylated, which show the same glycosylation patterns as that of a hamster-adapted scrapie strain 263 K. Apart from the glycosylating profiles of PrPSc, other main biochemical features of the two newly-formed PrPSc in hamsters, i.e. immunoreactivity, PK-resistance, solubility and stability in GdnHCl, are highly comparable with that of agent 263 K. Those data indicate that two kinds of the newly-formed PrPSc in hamster brains lose their original molecular characteristics in mouse brains, while obtain new properties that show markedly hamster-specific.
Our data emphasize the host microenvironment affects obviously the molecular features of the new PrPSc generated during transmission across species. Other previous studies have shown that TSE strains alter their characteristics during the passage in a foreign species and the changed features maintained stably with the serial passage, besides the latent period became shorter than the first passage [16, 17]. It is belived that inter-species prion disease transmission is frequently the acquisition of new strain properties, in which transgenic mouse and protein misfolding cyclic amplification (PMCA) approaches provide a facile means of generating and characterizing novel prion strains . However, some kinds of prion strains never sacrifice the original molecular properties when infecting onto other species. The famous example is BSE agent, which keeps its main biochemical and molecular characteristics after causing infection on human (vCJD), cats (FSE) and other ungulates (exotic ungulate encephalopathy) . The exact reason for such difference among prion strains remains unclear. It is speculated that interactions with chaperones or other cellular factors, depending on prion variant, will be at least part of some species barriers . Nevertheless, this mysterious phenomenon lies at least on prion strain, host PrPC and host microenvironment.
During the course of TSE, a kind of protein fibril, referred as SAF, is usually observed in brain tissues . SAFs are abnormal structures uniquely associated with prion diseases of many species. The major, even exclusive component for SAF is PrPSc . The structure of SAF in vitro can be destroyed easily by many physical and chemical agents, including GdnHCl in this study. Although it is still not settled whether the infection of prion needs a fixed morphological structure like virus, it is certain that maintenance of SAF in inoculum is not indispensable. Our previous study  and others  have repeatedly addressed GdnHCl-treated brain extracts from scrapie infected animals, in which the fibriform structures of SAFs are undetectable, still maintain its infectivity. Unfortunately, the SAF structures of the original mouse-adapted strains 139A and ME7 are hard to be observed, possibly because of long-term stored specimen. Comparison of the SAF structures of the one TSE agent from different infected species will help to understand the potential ultra-structural changes in during interspecies transmission.
One of the features of prion diseases is that they usually have extremely long incubation. In line with the results of our previous study of the experimental bioassays on hamsters with scrapie agent 263 K , SAF and PrPSc have been observed in brains infected with agent 139A and ME7 during their incubation periods, which are much earlier than the appearance of clinical symptoms. Those accord well with the natural phenomenon of almost all infectious diseases that the pathogens are usually detectable earlier than the appearance of clinical symptom. Interestingly, SAFs are observed much earlier than the PrPSc in the brain tissues from the two infections in our experimental condition. Whether it is a general feature for TSEs is not known. If it were, it would highlight that the nascent constructs of PrPSc in the early stage of disease may not be stable enough for resisting the routinely concentrated PK digestion. Nevertheless, our data stress again that assays for SAF and PrPSc in brains are useful biomarkers for screening TSE before onset of symptoms.
Conformational stability of PrPres has also been used to differentiate TSE strains. When PrPC converts to PrPSc, the increased component of β-sheet structure makes the prion protein more stable to resist the effectiveness of GdnHCl and PK . According to our data, the resistibility of the three hamster-adapted scrapie strains to GdnHCl and PK is almost similar, though strain 139A-ha is slightly weak. This similarity elucidates that a consistency of PrPres form in hamsters in the molecular level.