Exploration of the Main Sites for the Transformation of Normal Prion Protein (PrPC) into Pathogenic Prion Protein (PrPsc)

Abstract

Introduction: The functions and mechanisms of prion proteins (PrP^C) are currently unknown, but most experts believe that deformed or pathogenic prion proteins (PrP^Sc) originate from PrP^C, and that there may be plural main sites for the conversion of normal PrP^C into PrP^Sc. In order to better understand the mechanism of PrP^C transformation to PrP^Sc, the most important step is to determine the replacement or substitution site.

Material and methods: BALB/c mice were challenged with prion RML strain and from 90 days post-challenge (dpc) mice were sacrificed weekly until all of them had been at 160 dpc. The ultra-structure and pathological changes of the brain of experimental mice were observed and recorded by transmission electron microscopy.

Results: There were a large number of pathogen-like particles aggregated in the myelin sheath of the brain nerves, followed by delamination, hyperplasia, swelling, disintegration, phagocytic vacuolation, and other pathological lesions in the myelin sheath. The aggregated particles did not overflow from the myelin in unstained samples. The phenomenon of particle aggregation persisted all through the disease course, and was the earliest observed pathological change.

Conclusion: It was deduced that the myelin sheath and lipid rafts in brain nerves, including axons and dendrites, were the main sites for the conversion of PrP^C to PrP^Sc, and the PrP^Sc should be formed directly by the conversion of protein conformation without the involvement of nucleic acids.

Keywords: brain; mice; myelin sheath; particle aggregation; prion proteins.

Fig. 1

The myelinated nerves of mouse brain from three control groups including the normal control (A), the blank control (B), and the experimental control (C). The normal control received no treatment, the blank control was treated with physiological saline (0.85% NaCl), and the experimental control was treated with brain homogenates of normal mice. The myelin sheaths in these control groups were compact without stratification in the myelin, and subcellular structures like mitochondria were integrated

Fig. 2

Myelinated nerves of mouse brain from the test group at 90 days post challenge. Rightward arrows indicate structures of myelin sheath between two nerve fibres which had become fuzzy and deformed, a leftward arrow indicates that there were some black depositions accumulated into larger particles, and a downward arrow indicates the detached invaginated myelin plasmalemma and vacuoles

Fig. 3

The myelinated nerves of mouse brain from the test group at 97 days post challenge. The myelin had become curved and deposition or polymerisation of dispersed particles was obvious in the myelin sheath. Rightward arrows indicate that the cytoplasmic membrane was no longer integrated and leftward arrows indicate that stratification in the myelin sheath was more obvious where there were black pathogen-like particles accumulated

Fig. 4

The myelinated nerves of mouse brain from the test group at 104 days post challenge. The plasma membrane had dissolved and spread out like mud (rightward arrow). In addition, there were bubbly structures (leftward arrow). Degenerative changes at the border between two myelin sheaths were more obvious (rightward arrow) and adjacent mitochondria are also damaged (downward arrow)

Fig. 5

The myelinated nerves of mouse brain from the test group at 111 days post challenge. The rightward arrow points to the bubbly structures, the upward arrow points suspicious pathogenic fibres, and the downward arrows point to the polymerised bubble with polymeric particles

Fig. 6

The myelinated nerves of mouse brain from the test group at 118 days post challenge. Some abnormal pathogenic structures like local extension, bending, and black particle deposition were widespread in the myelin sheath. The downward arrow indicates membrane vacuole, while the rightward arrow indicates tubulovesicular structure

Fig. 7

The myelinated nerves of mouse brain from the test group at 125 days post challenge. The stratification in the myelin sheath had increased as indicated by downward arrows, and some myelin sheaths had swollen and become diffused as indicated by the rightward arrow. 1200x

Fig. 8

The myelinated nerves of mouse brain from the test group at 132 days post challenge. Upward arrows indicate that the myelin sheath had collapsed and spread to the cytoplasm and the infolded myelin sheath had further developed into phagocytic vacuoles and rightward arrows indicate that multi-membrane vacuoles had formed in some myelin sheaths and the aggregated black particles were dissolving in these vacuoles

Fig. 9

The myelinated nerves of mouse brain from the test group at 139 days post challenge. The myelin sheath is stratified obviously and many more particle aggregations or depositions were observed. The upward arrow indicates that one myelin sheath contained another downward arrows indicate particle polymerisation, and rightward arrows indicate myelin sheath lamination

Fig. 10

The myelinated nerves of mouse brains from the test group at 146 days post challenge. The whole myelin sheath cavity is filled with vacuoles and layered myelin fibres, and there is typical particle deposition, myelin stratification, and vacuolisation. The downward arrow indicates the myelin in the form of a four-layer pipe, upward arrows indicate the myelin sheath separated from the membrane, rightward arrows indicate particle polymerisation and deposition, and leftward arrows indicate myelin sheath lamination. 4000x

Fig. 11

The myelinated nerves of mouse brain from the test group at 153 days post challenge. The cytoplasm had completely dissolved and separated from the membrane as indicated by leftward arrows. Upward arrows indicate that the detached axonal membrane had formed an inner tongue. 3000x

Fig. 12

The myelinated nerves of mouse brain from the test group at 160 days post challenge. There were autophagocytic vacuoles and multimembrane vesicles in the myelin sheath, polymeric particles had become still more blurred, and the aggregated black particles in the myelin had almost dissolved. The downward arrow indicates the large dissolved plaques and the rightward arrow indicates that macrophagocyte nuclei were marginally assembled inside cells