The absence of infectivity‐associated, protease‐resistant prion protein (PrPSc) in the brains of spontaneously sick transgenic (Tg) mice overexpressing PrP linked to Gerstmann–Sträussler Scheinker syndrome, and the failure of gene‐targeted mice expressing such PrP to develop disease spontaneously, challenged the concept that mutant PrP expression led to spontaneous prion production. Here, we demonstrate that disease in overexpressor Tg mice is associated with accumulation of protease‐sensitive aggregates of mutant PrP that can be immunoprecipitated by the PrPSc‐specific monoclonal antibody designated 15B3. Whereas Tg mice expressing multiple transgenes exhibited accelerated disease when inoculated with disease‐associated mutant PrP, Tg mice expressing mutant PrP at low levels failed to develop disease either spontaneously or following inoculation. These studies indicate that inoculated mutant PrP from diseased mice promotes the aggregation and accumulation of pre‐existing pathological forms of mutant PrP produced as a result of transgene overexpression. Thus, while pathological mutant PrP possesses a subset of PrPSc characteristics, we now show that the attribute of prion transmission suggested by previous studies is more accurately characterized as disease acceleration.

In this report we document the results of several independent studies testing the sensitivity, specificity and reliability of the Prionics Western blotting (PWB) procedure to detect bovine and ovine disease-specific, protease-resistant prion protein (PrP(Sc)). Validation of the technique was obtained by blind analysis of samples from cattle affected with bovine spongiform encephalopathy (BSE), clinically normal animals or cattle with neurological diseases unrelated to BSE. Overall, very high sensitivity, specificity and reliability was observed. It became clear that sampling of the correct brain region and the method used for protein extraction are important factors for correct diagnosis. Furthermore, we tested the usefulness of the PWB technique as an instrument for surveillance purposes. We analyzed animals from a culling scheme as well as older animals from abattoirs to determine the number of subclinical BSE cases detectable by histopathological examination, immunohistochemistry for PrP(Sc) and PWB. In both studies, BSE-affected animals with no overt clinical symptoms were detected. These results demonstrate the usefulness of the PWB procedure in surveillance systems serving as a rapid diagnostic tool to identify animals subclinically infected with BSE.

The analysis of mice deficient in the myelin-associated glycoprotein (MAG) or Fyn, a nonreceptor-type tyrosine kinase proposed to act as a signaling molecule downstream of MAG, has revealed that both molecules are involved in the initiation of myelination. To obtain more insights into the role of the MAG-Fyn signaling pathway during initiation of myelination and formation of morphologically intact myelin sheaths, we have analyzed optic nerves of MAG-, Fyn-and MAG/Fyn-deficient mice. We observed a slight hypomyelination in optic nerves of MAG mutants that was significantly increased in Fyn mutants and massive in MAG/Fyn double mutants. The severe morphological phenotype of MAG/Fyn mutants, accompanied by behavioral deficits, substantiates the importance of both molecules for the initiation of myelination. The different severity of the phenotype of different genotypes indicates that the MAG-Fyn signaling pathway is complex and suggests the presence of compensatory mechanisms in the single mutants. However, data are also compatible with the possibility that MAG and Fyn act independently to initiate myelination. Hypomyelination of optic nerves was not related to a loss of oligodendrocytes, indicating that the phenotype results from impaired interactions between oligodendrocyte processes and axons and/or impaired morphological maturation of oligodendrocytes. Finally, we demonstrate that Fyn, unlike MAG, is not involved in the formation of ultrastructurally intact myelin sheaths. Key words: double knock-out mutant; hypomyelination; Fyn; MAG; oligodendrocyte; optic nerve; spinal cordMyelin sheaths electrically insulate axons, and the generation of action potentials is thus confined to myelin-free regions of axons, the nodes of Ranvier. The resulting saltatory conduction of nerve impulses increases the speed at which information is propagated along axons. Given the functional importance of myelin, it is of particular interest to understand the differentiation of myelinating glial cells, the interaction between myelin-forming glial cells and axons, and the formation and maintenance of myelin at the molecular level.One of the molecules involved in the formation and maintenance of myelin is the myelin-associated glycoprotein (MAG), a member of the immunoglobulin superfamily (Arquint et al., 1987;Lai et al., 1987;Salzer et al., 1987). The analysis of MAG-deficient mice has revealed abnormal interactions between myelin-forming glial cells and axons and a variety of ultrastructural abnormalities of myelin sheaths in the CNS, and degeneration of myelin and axons in the peripheral nervous system (PNS) (Li et al., 1994;Montag et al., 1994) (for review, see Schachner and Bartsch, 2000). Of particular interest for the present study is a delayed myelination of developing optic nerves of MAG-deficient mice (Montag et al., 1994) and a hypomyelination of optic nerves of adult null mutants . Myelination in the PNS, in contrast, was not retarded in the absence of MAG (Montag et al., 1994). All these results demonstrate that MAG performs...

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