Fig 1.
Schematic representation of cellular prion protein (PrPC).
The N-terminal domain of PrPC is unstructured and possesses distinctive sequences identified as octapeptide repeats (see main text for details). These octarepeat regions contain hystidine residues (in blue) able to bind monovalent and divalent cations, such as copper ions Cu+ and Cu2+ (orange dots). The C-terminus contains a single disulphide bridge (in red) and 2 glycosylation sites. The asparagine residues involved in the glycosylation of the protein are represented in green. The overall structure of the C-terminus is composed of 2 short antiparallel beta sheet strands, namely β1 and β2 (in yellow) and 3 alpha helices, indicated as α1, α2, and α3. A third beta sheet strand has been recently identified and named β0 (in yellow).
Fig 2.
Schematic representation of the mechanism of cellular prion protein (PrPC)-mediated S-nitrosylation of N-methyl-d-aspartate receptor (NMDAR).
One mechanism controlling NMDAR as well as other membrane ion channels involves direct modulation by nitric oxide (NO). Catalytic amounts of copper can act as electron acceptors promoting the reaction of NO with thiols, providing inhibitory S-nitrosylation (RSNO) of NMDAR. The RSNO formation can take place only after one-electron oxidation from the free radical NO to NO+ by transition metal. In brief, glutamate is released from the presynaptic terminal of neurons and activates NMDAR on the postsynaptic terminal. NMDAR activation and opening generates Na+ and Ca2+ influx and K+ efflux. In the cytosol, upon entrance, Ca2+ ions bind to different proteins, among these, calmodulin (CaM). The CaM bound to Ca2+ triggers neuronal nitric oxide synthase (nNOS) and copper-transporting ATPase 1 (Atp7a). Activation of nNOS leads to NO release in the synaptic cleft. Activation of Atp7a in the trans-Golgi network (TGN) ensues in Cu2+ release in the synaptic space. Transient free Cu2+ ions are immediately bound by copper-binding proteins like PrPC, which is highly expressed in both pre- and postsynaptic terminals. PrPC has high affinity for both Cu2+ and Cu+ and can be found in lipid raft domains, which also contain NMDAR. NO can react with extracellular cysteine thiols of NMDAR subunits GluN1 and GluN2A, leading to cysteine S-nitrosylation (SNO-Cys). The S-nitrosylation inhibits NMDAR activation by closing the channel. The chemical reaction between NO and cysteine thiol requires the presence of an electron acceptor such as Cu2+. PrPC coordinates Cu2+ ions, which support the reaction of NO with thiols, leading to the S-nitrosylation of GluN1 and GluN2A and therefore NMDAR inhibition.