A bispecific immunotweezer prevents soluble PrP oligomers and abolishes prion toxicity
Fig 4
scPOM-bi prevents the formation of soluble, PK resistant oligomers.
(A) DLS showed the presence of soluble oligomers (red shades in histograms, reported as percentage) upon addition of the POM1 toxic antibody to recombinant mPrP in vitro. Subsequent addition of POM2 did not remove the oligomers or inhibit toxicity. Smaller species comparable to monomeric forms (blue) were detected in solution when POM1 was in complex with ΔmPrPC90-230, lacking the flexible tail, and when POM2 was added to mPrP prior to POM1 addition. Similarly small species were found when the neuroprotective scPOM-bi was added to mPrP; the bispeficic was also capable of removing the soluble oligomers generated by POM1. DLS data is shown for 3 time points after complex formation. (n = 5 for scPOM1:mPrP, scPOM2:mPrP and scPOM-bi:mPrP; n = 3 for scPOM1:mPrP then scPOM2, scPOM2:mPrP then scPOM1 and scPOM1:mPrP then scPOM-bi) (B) DLS can only detect soluble material. To investigate the presence of insoluble aggregates we formed the mPrP:Ab complexes in vitro, centrifuged them and analyzed the resulting supernatant with PAGE/Western blot. Soluble material was only detected in toxic combinations (POM1:mPrP or POM1:mPrP followed by POM2, red and orange). The percentage of mPrP and antibody in solution (normalized against isolated PrP or antibody) is shown; data from quantification of band intensity on SDS-PAGE (images in S5 Fig—n = 7 for all samples tested). (C) In order to characterize both soluble oligomers and insoluble aggregates we formed the mPrP:Ab complexes in vitro and deposited the resulting material on microscopy slides. Confocal microscopy indicates that toxic antibody combinations (e.g. POM1:mPrP or POM1:mPrP followed by POM2, red and orange) generate species with smaller average size than protective antibody combinations. The surface area of the detected species is reported on the y axis, the horizontal line represents the average. Differences can also be appreciated by visual inspection of the confocal microscopy images (S4 Fig—scPOM1:mPrP n = 166, scPOM2:mPrP n = 1136, scPOM-bi:mPrP n = 204, scPOM1:mPrP then scPOM2 n = 444, scPOM2:mPrP then scPOM1 n = 74 and scPOM1:mPrP then scPOM-bi n = 1767). D) The soluble oligomers generated by POM1 showed increased resistance to in vitro degradation by proteinase K at 2μg/ml (red). Such resistance was abolished when POM1 bound a mPrP construct lacking the FT (light red) or in non-toxic antibodies (shades of blue). Data from quantification of PK resistant bands on western blot, normalized against isolated PrP (images in S6 Fig—scPOM1:mPrP n = 5, scPOM1:ΔPrP n = 3, scPOM2:mPrP n = 4, scPOM-bi:mPrP n = 4).