Selective Precipitation of Prions by Polyoxometalates: A Goal towards an Ante-Mortem test against Prions and Revealing the 3D-Structure of Infectious Prions 

Prions are improperly folded infectious proteinacious particles which are devoid of nucleic acids. The PrP^Sc replicates the normal cellular prion protein (PrP^C) into beta rich, infectious form (PrP^Sc) by an unknown mechanism. Prion diseases are fatal, and active research is pursued by several research groups to suppress activity of prion particles. However, we are very much interested in developing early (antemortem) diagnosic tests. Currently, infectious prions can be diagnosed only by autopsy, hence it is vital to diagnose the prions in the early stages (before dysfunction of central nervous system begins). The main difficulty in probing the presence of prions in bodily fluids is that their concentration level is extremely low during the developing stages of disease[Safar, 2006 #14]. Though the currently employed conformation dependent immunoassay (CDI) method relies on concentration level of PrP^Sc([PrP^Sc] > [PrP^C]), complexity has been increased to detect the presence of prions in body fluids. Attempts to increase the sensitivity of the CDI method by screening with many chemical compounds led to the accidental discovery that polyoxometalates (POMs) (eg: phosphotungstate) selectively precipitate infectious prions in the presence of other cellular prion protein [Lee, 2005 #16; Safar, 1998 #36](Figure 1). The sensitivity of the CDI method was enhanced several fold following this discovery, and optimization of precipitation level is the focus of ongoing project within the group.

Figure 1. Schematic representation of selective precipitation PrPSc.

Initial observations from the preliminary studies suggest that prion protein shows some structural, charge, and size selectivity on polyoxometalates, which in turn in some cases (POMs), an optimum precipitation is observed at a particular concentration and with larger POMs infectious PrP^Sc interacts quite distinctly [Lee, 2005 #16] (eg: rather precipitation, solubilization of prion is effected). However, the nature of the interaction between the PrP^Sc and POMs, as well as determination of stoichimetric ratio between the PrP^Sc and the corresponding POMs need to addressed.

In addition, we are also interested in developing 2D and 3D crystals of infectious prions whose tertiary and/or quaternary structure is not known due to its insoluble nature, which hampers structural studies. By exploiting the enhanced solubility demonstrated by larger POMs, crystal growth will be effected, and using several spectroscopic techniques like NMR, CD, optical spectroscopy, AFM and EM, structural information of the infectious prions can be elucidated in addition to the single crystal X-ray crystallography study. We are currently looking for suitable large POMs which can retain the infectivity of PrP^Sc after solubiliztion. Revealing the structure of the infectious prions will greatly assist the research community to develop the antibodies to recover from fatal prion diseases.