In vitro evaluation of the anti-prionic activity of newly synthesized congo red derivatives

"Transmissible Spongiform Encephalopathies" (TSE) are a group of degenerative progressive fatal disorders of the CNS, affecting both humans and animals. The main pathogenic event is the conversion of cellular prion protein from the normal, enzyme-sensitive (PrPsen), to the insoluble proteinase K-resistant isoform (PrPres). Since the new juvenile variant of Creutzfeldt-Jakob disease (vCJD) is probably due to the transmission of Bovine Spongiform Encephalopathy (BSE) prion protein to man, therapeutic and preventive compounds for animals and humans are urgently needed. Congo Red (benzidine-diazo-bis-1-naphthylamine-4-sulfonic acid sodium salt, CAS 573-58-0, CR), an azoic dye that inhibits amyloid deposition, and some newly synthesized derivatives, more lipophilic and less toxic, were tested for their anti-prionic activity, in different experimental models. Cell-free experiments using the synthetic peptide PrP 106-126, homologous to amino acid residues 106-126 of the human PrP, were run to determine the anti-amyloidogenic properties of some of the molecules. Peptide solutions containing each compound were incubated at 37 °C, for increasing times, to analyse the kinetics of aggregation of PrP 106-126 peptide. After incubation, the amount of non-aggregated peptide was measured by RP-HPLC. While CR enhanced the amyloidogenicity of PrP 106-126, derivatives "1a" and "1b" both showed the opposite behaviour, reducing aggregation by 15-20%. In other experiments using electron microscopy PrP 106-126 was assayed with the same molecules to assess the number and size of fibrils formed. CR showed its typical interaction, producing amyloid aggregates; "1a" did not interfere with fibril formation, while "1b" seemed to partially affect the structure of PrP 106-126 fibrils. Using a different cell-free model, it was investigated whether CR derivatives could reverse the protease-resistant PrPres, extracted from Syrian hamster infected brain, into the normal protease sensitive PrP sen. Samples containing fixed amounts of PrPres were incubated at 37 °C for 1 h with all the newly synthesized molecules, at concentrations ranging from 50 μg/mL to 750 μg/mL. After treatment with proteinase K, half of each sample was incubated with 3 mol/L guanidine thiocyanate in order to exclude over-stabilisation of the PrPres aggregates already observed with CR. The remaining amount of PrPres was assessed by Enhanced Chemoluminescence (ECL) Western blotting analysis. None of the compounds induced the reversion of PrPres to PrP sen; nevertheless, 6 of the 8 molecules interacted with PrP res molecules, over-stabilising the PrPres aggregates, from this aspect being similar to CR in activity. Finally, the inhibition of the generation of PrPres in the S12 clone of a mouse neuroblastoma cell line (N2a S12), persistently infected by the mouse adapted Chandler strain of scrapie, was evaluated. Increasing amounts of CR, "1a" and "1b" were added to the culture medium at each cell passage. After various days of treatment, the cells were collected, lysed, and the amount of PrPres was assayed by ECL Western blotting after PK treatment. As expected, there was a decrease in pathological PrP expression starting from the 4th day of treatment, with 5 and 10 μg/mL CR; PrPres completely disappeared after respectively 10 and 14 days of treatment. "1a" was strongly effective after 3 days of treatment at 5 and 10 μg/mL, but it was also highly toxic; at the concentration of 1 μg/mL, it had a mild inhibitory effect after 8 days. The reduction of PrPres was also evaluated by intracytoplasmic flow-cytometry immunofluorescence on CR- and "1a"-treated N2a S12 cells. CR induced a dose-related decrease of PrP expression from day 3 to 13 of treatment. At the concentrations of 2 and 1.5 μg/mL "1a" also strongly affected the expression of PrP starting from the 3rd day of treatment until the end of the experiment (day 13). These results confirm the importance of using an integrated system, based on different experimental models, to obtain useful information on the mechanism of action of anti-prionic compounds.