β-cleavage of the prion protein is specific to the human eye and has implications for the spread of infectious prions and ocular disorders

Abstract

Purpose : Recently, we reported β-cleaved prion protein (PrP^C) in human ocular tissues. Here, we explored whether this is unique to the human eye, and its implications in ocular disorders.

Methods : The ratio of β- and α-cleaved PrP^C in ocular tissues and fluids was determined by Western blotting (WB). Processing of green-fluorescent-protein (GFP)-tagged PrP^C was evaluated in human retinal (ARPE-19) and neuronal (BE(2)-M17) cell-lines respectively.

Results : PrP^C was β-cleaved in human ocular tissues, and soluble N-terminal fragment (N2) was released in the aqueous and vitreous humor (AH & VH). In contrast, PrP^C was α-cleaved in human brain and retinal tissue of diurnal and nocturnal animals. Insertion of the GFP tag between N-terminal residues 37 and 38 of PrP^C interfered with β-cleavage in ARPE-19 cells, and was cytotoxic. In BE(2)-M17 cells, PrP^C-GFP was α-cleaved, and was nontoxic. Surprisingly, neither exposure to light, oxidizing agents, inflammatory stimuli, or anti-oxidants altered β-cleavage of PrP^C in ARPE-19 cells.

Conclusions : β-cleavage of PrP^C is obligatory in human ocular tissues, and N2 likely protects the cells from oxidative stress, an established function of this fragment. Paradoxically, β-cleaved C-terminus (C2) of PrP^C is susceptible to conversion by the pathogenic PrP-scrapie (PrP^Sc) form while α-cleaved fragment (C1) is resistant, increasing the susceptibility of the human eye to prion infection. Moreover, C2 includes the binding site for β1-integrin and amyloid-β (Aβ), molecules implicated in ocular disorders. These observations suggest re-evaluation of animal studies on prion infection of the eye, and underscore the significance of PrP^C in ocular disorders.

This is a 2021 ARVO Annual Meeting abstract.