nooshin azari

Prion diseases are neurodegenerative disorders which ultimately results in the death of their victims. They are caused by structural transformation of cellular prion (PrPC) to its β-rich and anomalous isoform (PrPSc) and the accumulation of amyloid fibrillar deposits in the central nervous system. The precise mechanism underling this conversion is yet to be well understood. This study aimed to investigate the effect of non physiological temperatures on the misfolding mechanism of the human prion protein. The crystal structure of human prion protein (90-231), (PDB code: 2Lej) in pdb format was used as a starting structure in this study. Three model structures of this coordinate structure were used separately to simulate PrPC at 27, 37 and 47. Molecular dynamic simulations were then performed using double-precision MPI version of GROMACS 4.5.5 for 10 ns and the results were analyzed using SPSS software, SPDBV and VebLab programs. The change of temperature from 37 to 27 or 47 induced significant structural changes to PrPC. These tempratures caused PrPC to attain a more folded and less flexible tertiary structure compared to its native structure at 37. They, also, reduce protein-solvent hydrogen bonds and therefore increasing access of hydrophobic solvent to PrPC which may be behind the lower water solubility of PrPC and its increased resistance to proteolytic degradations. This study shows that changes of temperatures accelerate structural changes of PrPC and reduce its solubility while rendering it vulnerable to transition into PrPSc.

PrPC conversion to PrPSc isoform is the main known cause for prion diseases including Crutzfeldt-Jakob, Gerstmann-Sträussler-Sheinker syndrome and fatal familial insomnia in human. The precise mechanism underling this conversion is yet to be well understood. In the present work, using the coordinate file of PrPC (available on the Protein Data Bank) as a starting structure, separate molecular dynamic simulations were carried out at neutral and acidic pH in an explicit water box at 37°C and 1 atmosphere pressure for 10ns second period. Results showed that the acidic pH accelerates PrPC conversion to PrPSc by decreasing the protein gyration radius, flexibility and protein-solvent hydrogen bonds. In acidic conditions, PrPC attains a more folded and less flexible tertiary structure compared to its native structure at neutral pH; otherwise, the decrease of protein-solvent hydrogen bonds at acidic pH will enhance the hydrophobic character of PrPC that may exhibit association as multimeric assemblies. It can also lower water solubility and increase resistance to proteolytic degradations. Data indicated that there was no sensible protein denaturation during this conversion. It is hypothesized that the formation of slightly misfolded conformations with minor structural changes in secondary and/or tertiary structures are enough to menace scrapie formation in PrPC. Our findings show that scrapie formation seems to be a theoretically reversible process.