Melanin or a Melanin-Like Substance Interacts with the N-Terminal Portion of Prion Protein and Inhibits Abnormal Prion Protein Formation in Prion-Infected Cells

ABSTRACT

Prion diseases are progressive fatal neurodegenerative illnesses caused by the accumulation of transmissible abnormal prion protein (PrP). To find treatments for prion diseases, we searched for substances from natural resources that inhibit abnormal PrP formation in prion-infected cells. We found that high-molecular-weight components from insect cuticle extracts reduced abnormal PrP levels. The chemical nature of these components was consistent with that of melanin. In fact, synthetic melanin produced from tyrosine or 3-hydroxy-l-tyrosine inhibited abnormal PrP formation. Melanin did not modify cellular or cell surface PrP levels, nor did it modify lipid raft or cellular cholesterol levels. Neither did it enhance autophagy or lysosomal function. Melanin was capable of interacting with PrP at two N-terminal domains. Specifically, it strongly interacted with the PrP region of amino acids 23 to 50 including a positively charged amino acid cluster and weakly interacted with the PrP octarepeat peptide region of residues 51 to 90. However, the in vitro and in vivo data were inconsistent with those of prion-infected cells. Abnormal PrP formation in protein misfolding cyclic amplification was not inhibited by melanin. Survival after prion infection was not significantly altered in albino mice or exogenously melanin-injected mice compared with that of control mice. These data suggest that melanin, a main determinant of skin color, is not likely to modify prion disease pathogenesis, even though racial differences in the incidence of human prion diseases have been reported. Thus, the findings identify an interaction between melanin and the N terminus of PrP, but the pathophysiological roles of the PrP-melanin interaction remain unclear.

IMPORTANCE The N-terminal region of PrP is reportedly important for neuroprotection, neurotoxicity, and abnormal PrP formation, as this region is bound by many factors, such as metal ions, lipids, nucleic acids, antiprion compounds, and several proteins, including abnormal PrP in prion disease and the Aβ oligomer in Alzheimer's disease. In the present study, melanin, a main determinant of skin color, was newly found to interact with this N-terminal region and inhibits abnormal PrP formation in prion-infected cells. However, the data for prion infection in mice lacking melanin production suggest that melanin is not associated with the prion disease mechanism, although the incidence of prion disease is reportedly much higher in white people than in black people. Thus, the roles of the PrP-melanin interaction remain to be further elucidated, but melanin might be a useful competitive tool for evaluating the functions of other ligands at the N-terminal region.