Tubulovesicular structures are not labeled using antibodies to prion protein (PrP) with the immunogold electron microscopy techniques.

Tubulovesicular structures (TVS) are disease-specific, intraneuronal particles found by thin-section electron microscopy in all of the transmissible spongiform encephalopathies. We used immunogold (both 10 nm immunogold and 1 nm immunogold silver enhanced) methods for ultrastructural localization of prion protein (PrP). In all scrapie models examined (263 K and 22CH in hamsters and 87V and ME7 in mice), TVS-containing processes were readily detected but neither these processes nor TVS themselves were decorated with gold particles. Even when amyloid plaques were observed in a close contact with TVS-containing neuronal processes, the processes remained unstained, while the plaques were decorated with gold particles. TVS located in areas adjacent to plaques in the 87V model and in areas of diffuse PrP immunolabelling in ME7 were also unlabelled with anti-PrP sera. Using immunogold techniques we were unable to label TVS with anti-PrP antibodies. As these technique proved to be sensitive enough to immunolabel not only amyloid plaques but also pre-amyloid accumulations of PrP, we strongly believe that the absence of staining reflects the structure of TVS and that they are not composed of PrP. That TVS are PrP negative may have several important implications for hypotheses about their nature. Principally, it does not support the suggestion that TVS are cross-sections of "thick tubules" visualized by touch-preparations of scrapie-affected mouse and hamster brains. If PrP is the infectious agent, as suggested by the prion hypothesis, the absence of stainable PrP in TVS would indicate that these are not the ultrastructural correlate of the agent. If however, TVS turn out to be more than merely a useful ultrastructural marker for the whole group of transmissible spongiform encephalopathies, it may suggest that PrP and the agent are two separate entities.