Charles Weissmann

The transmissible agent that causes spongiform encephalopathies such as BSE or scrapie, the prion, is believed to be devoid of nucleic acid and identical with PrP*, a modified form of the normal host protein PrPC which is found predominantly on the surface of neurons. The "protein-only" hypothesis proposes that PrP*, when introduced into a healthy host, multiplies by causing conversion of PrPC into PrP*. Prusiner proposed that a protease-resistant form of PrP, PrPSc, which accumulates intracellularly in scrapie-infected organisms is PrP*; while this identification is still uncertain, a vast body of evidence supports the idea that the infectious agent consists of a conformationally altered form of PrP devoid of nucleic acid.

If indeed PrP is an essential component of the prion, then an animal devoid of PrP protein should be resistant to scrapie. We generated homozygous Prn-p^o/o ("PrP knockout") mice (H.-R.Büeler, M.Aguet). These animals developed normally and several behavioral tests revealed no differences to normal mice. Prn-p^o/o mice inoculated with scrapie prions remained free of scrapie during their natural life span and did not propagate prions while wild-type controls all died within 6 months. Surprisingly, heterozygous Prn-p^o/+ mice, which express PrPC at about half the normal level, lived for over a year without clinical symptoms, despite high levels of infectious agent and PrPSc in the brain early on.

Introduction of murine PrP transgenes rendered Prn-p^o/o mice highly susceptible to mouse but not to hamster prions, while the introduction of Syrian hamster PrP transgenes rendered them susceptible to hamster but to a much lesser extent to mouse prions (H.-R.Büeler). To investigate which domains of the PrP protein are required to mediate susceptibility to scrapie in Prn-p^o/o mice, we introduced into PrP knockout mice Prn-p transgenes encoding PrP with amino terminal deletions of various lengths and have so far found that PrP lacking 60 amino proximal residues, including the 5 so-called octa repeats, can still restore susceptibility to scrapie (M.Fischer, D.Shmerling). Interestingly, deletions extending further toward the carboxy terminus of PrP give rise to spontaneous behavioral disorders, in particular ataxia, and degeneration of specific areas of the cerebellum (D.Shmerling). While introduction of variant PrP genes has so far been carried out by transgenics (T.Rülicke, J.Götz, G.Stark), we have established a system allowing introduction of such genes using an adenovirus vector (E.Flechsig).

We have also introduced into Prn-p^o/o mice PrP genes under the control of promotors leading to overexpression in tissues other than that of the nervous system. Mice overexpressing PrP in splenocytes, but not mice overexpressing in T cells propagate scrapie infectivity. Thus, the presence of PrP is necessary but not sufficient to allow propagation of prions (A.Raeber, A.Sailer).

One important goal in prion research is the conversion of biosynthetic PrP into infectious PrP*. Others and also we (S.Sakaguchi) have shown that protease-resistant 35S-PrP (PrP^Sc) can be generated by incubating S35-labeled biosynthetic PrP with PrP^Sc, however it has not been possible to find out whether this material is infectious because of the large excess of infectivity associated with the PrP^Sc preparation used as "template".

A collaboration with A.Aguzzi and his colleagues (Neuropathology, Universitätsspital) is aimed at clarifying how prions introduced peripherally reach the central nervous system (F.Montrasio, S.Mirold) and a project to develop a mouse strain which allows controlled expression of PrP is underway (A.Cozzio).

Our experiments suggest that cattle or sheep devoid of the PrP gene might be viable and resistant to scrapie.

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Molecular Biology of Prion Diseases