α2,3 linkage of sialic acid to a GPI anchor and an unpredicted GPI attachment site in human prion protein

Abstract

Prion diseases are transmissible, lethal neurodegenerative disorders caused by accumulation of the aggregated scrapie form of the prion protein (PrPSc) after conversion of the cellular prion protein (PrPC). The glycosylphosphatidylinositol (GPI) anchor of PrPC is involved in prion disease pathogenesis, and especially sialic acid in a GPI side chain reportedly affects PrPC conversion. Thus, it is important to define the location and structure of the GPI anchor in human PrPC. Moreover, the sialic acid linkage type in the GPI side chain has not been determined for any GPI-anchored protein. Here we report GPI glycan structures of human PrPC isolated from human brains and from brains of a knock-in mouse model in which the mouse prion protein (Prnp) gene was replaced with the human PRNP gene. LC–electrospray ionization–MS analysis of human PrPC from both biological sources indicated that Gly229 is the ω site in PrPC to which GPI is attached. Gly229 in human PrPC does not correspond to Ser231, the previously reported ω site of Syrian hamster PrPC. We found that ∼41% and 28% of GPI anchors in human PrPCs from human and knock-in mouse brains, respectively, have N-acetylneuraminic acid in the side chain. Using a sialic acid linkage-specific alkylamidation method to discriminate α2,3 linkage from α2,6 linkage, we found that N-acetylneuraminic acid in PrPC's GPI side chain is linked to galactose through an α2,3 linkage. In summary, we report the GPI glycan structure of human PrPC, including the ω-site amino acid for GPI attachment and the sialic acid linkage type.

Loss of the N-acetylgalactosamine side chain of the GPI-anchor impairs bone formation and brain functions and accelerates the prion disease pathology

“…1D). Consistent with this, GPI-APs purified from brain, kidney, and skeletal muscle were reported to be modified with the GalNAc side chain (5)(6)(7)51,52). The tissue-specific expression of PGAP4 suggests that the GPI-GalNAc side chain regulates certain specific but non-essential functions of GPI.…”

“…Therefore, combined with a previous report (40), it is suggested that asialo forms of GPI-GalNAc side chain (Gal-GalNAc-GPI and GalNAc-GPI) has protective function from prion diseases. Because human PrP C GPI is more likely to be sialylated (approximately 40% in human brain (7)) than mouse PrP C GPI, augmented desialylation on the GPI-GalNAc side chain may be an effective novel therapeutic for treating human prion diseases. Elucidation of the relationship between structure and function of the GPI-GalNAc side chain will facilitate our understanding of the pathology of prion diseases and the development of therapeutics based on the structure of the GPI-GalNAc side chain.…”

Glycosylphosphatidylinositol-Anchor Synthesis in Plants: A Glycobiology Perspective

“…In mammalian brain, galactosylated and sialylated GPI-anchors are more abundant than in other tissues. However, not all GPI-anchored proteins are modified to the same extent with some proteins having only GalNAc instead of additional galactose and sialic acid modifications ( Kobayashi et al, 2020 ). Sialylation of the human prion protein side chain may contribute to the pathology of prion disease ( Bate et al, 2016 ).…”

“…The Golgi-resident galactosyltransferase B3GALT4 catalyzes the transfer of a galactose from UDP-galactose to the side chain GalNAc residue ( Wang et al, 2020 ). The GPI sialyltransferase was not identified yet, but recently it was found that in prion protein the sialic acid N-acetylneuraminic acid (Neu5Ac) is present in α2,3-linkage ( Kobayashi et al, 2020 ).…”

Siglec Signaling in the Tumor Microenvironment

“…They both harbor a glycosylphosphatidylinositol anchor (GPI anchor) and can be released from the cell membrane by the action of phospholipases. Interestingly, the GPI anchor, consisting of a phospholipid tail, a glycan core and a phosphoetanolamine linker to which the protein is attached, contains a glycolipid that can be sialylated as well (100)(101)(102). Therefore, CD24 and CD52 are glycoproteins attached to a glycolipid, which makes their context highly interesting for future studies.…”