Complement protein C1q recognizes a conformationally modified form of the prion protein.

Several studies have suggested the implication of the classical complement pathway in the early stages of prion disease pathogenesis. To explore this hypothesis, surface plasmon resonance spectroscopy was used to test the ability of human C1q to recognize mouse PrP immobilized on a sensor chip. In this configuration, C1q bound avidly to PrP, with a K(D) of 5.4 nM (k(on) = 2.4 x 10(5) M(-1) s(-1); k(off) = 1.3 x 10(-3) s(-1)). The isolated C1q globular domain also bound to immobilized PrP, although with a higher K(D) (238 nM), due to a decreased k(on) (4.2 x 10(3) M(-1) s(-1)). Interaction was strongly enhanced by Cu(2+) ions, with a 10-fold increase in overall binding in the presence of 10 microM CuSO(4), without significant modification of the kinetic parameters. In contrast, using the same technique, no interaction was detected between immobilized C1q and soluble PrP. Likewise, gel filtration and chemical cross-linking analyses yielded no evidence for an interaction between these proteins in solution. Comparative analysis of the antigenic reactivity of soluble and immobilized PrP was performed by ELISA and surface plasmon resonance spectroscopy, respectively, using anti-PrP monoclonal antibodies. This analysis provides evidence that immobilized PrP undergoes a major conformational change in the sequence stretch 141GNDWEDRYYRENMYRYPNQ159 located in its C-terminal globular domain. It is concluded that immobilized PrP undergoes structural modifications that possibly mimic the conformational changes occurring during conversion to the pathological isoform and that C1q represents a natural sensor of these changes. Pathological implications of this recognition property are discussed in light of recent reports.

Prion protein (PrPc) immunocytochemistry and expression of the green fluorescent protein reporter gene under control of the bovine PrP gene promoter in the mouse brain.

Expression of the cellular prion protein (PrP(c)) by host cells is required for prion replication and neuroinvasion in transmissible spongiform encephalopathies. As a consequence, identification of the cell types expressing PrP(c) is necessary to determine the target cells involved in the cerebral propagation of prion diseases. To identify the cells expressing PrP(c) in the mouse brain, the immunocytochemical localization of PrP(c) was investigated at the cellular and ultrastructural levels in several brain regions. In addition, we analyzed the expression pattern of a green fluorescent protein reporter gene under the control of regulatory sequences of the bovine prion protein gene in the brain of transgenic mice. By using a preembedding immunogold technique, neuronal PrP(c) was observed mainly bound to the cell surface and presynaptic sites. Dictyosomes and recycling organelles in most of the major neuron types also exhibited PrP(c) antigen. In the olfactory bulb, neocortex, putamen, hippocampus, thalamus, and cerebellum, the distribution pattern of both green fluorescent protein and PrP(c) immunoreactivity suggested that the transgenic regulatory sequences of the bovine PrP gene were sufficient to promote expression of the reporter gene in neurons that express immunodetectable endogenous PrP(c). Transgenic mice expressing PrP-GFP may thus provide attractive murine models for analyzing the transcriptional activity of the Prnp gene during prion infections as well as the anatomopathological kinetics of prion diseases.

PrP expression and replication by Schwann cells: implications in prion spreading.

Prion infection relies on a continuous chain of PrP(c)-expressing tissues to spread from peripheral sites to the central nervous system (CNS). Direct neuroinvasion via peripheral nerves has long been considered likely. However, the speed of axonal flow is incompatible with the lengthy delay prior to the detection of PrP(Sc) in the brain. We hypothesized that Schwann cells could be the candidate implicated in this mechanism; for that, it has to express PrP(c) and to allow PrP(Sc) conversion. We investigated in vivo localization of PrP(c) in sciatic nerve samples from different strains of mice. We demonstrated that PrP(c) is mainly localized at the cell membrane of the Schwann cell. We also studied in vitro expression of PrP(c) in the Schwann cell line MSC-80 and demonstrated that it expresses PrP(c) at the same location. More specifically, we demonstrated that this glial cell line, when infected in vitro with the mouse Chandler prion strain, both produces the PrP(Sc) till after 18 passages and is able to transmit disease to mice, which then develop the typical signs of prion diseases. It is the first time that infection and replication of PrP(Sc) are shown in a peripheral glial cell line.