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

Persistent ER stress causes GPI anchor deficit to convert a GPI-anchored prion protein into pro-PrP via the ATF6-miR449c-5p-PIGV axis

Endoplasmic reticulum (ER) stress and unfolded protein response are cells' survival strategies to thwart disruption of proteostasis. Tumor cells are continuously being challenged by ER stress. The prion protein, PrP, normally a glycosylphosphatidylinositol (GPI)-anchored protein exists as a pro-PrP retaining its GPI-peptide signal sequence in human pancreatic ductal cell adenocarcinoma (PDAC). Higher abundance of pro-PrP indicates poorer prognosis in PDAC patients. The reason why PDAC cells express pro-PrP is unknown. Here, we report that persistent ER stress causes conversion of GPI-anchored PrP to pro-PrP via a conserved ATF6-miRNA449c-5p-PIGV axis. Mouse neurons and AsPC-1, a PDAC cell line, express GPI-anchored PrP. However, continuous culture of these cells with the ER stress inducers thapsigargin or brefeldin A results in the conversion of a GPI-anchored PrP to pro-PrP. Such a conversion is reversible; removal of the inducers allows the cells to re-express a GPI-anchored PrP. Mechanistically, persistent ER stress increases the abundance of an active ATF6, which increases the level of miRNA449c-5p (miR449c-5p). By binding the mRNA of PIGV at its 3'-UTRs, miR449c-5p suppresses the level of PIGV, a mannosyltransferase pivotal in the synthesis of the GPI anchor. Reduction of PIGV leads to disruption of the GPI anchor assembly, causing pro-PrP accumulation and enhancing cancer cell migration and invasion. The importance of ATF6-miR449c-5p-PIGV axis is recapitulated in PDAC biopsies as the higher levels of ATF6 and miR449c-5p and lower levels of PIGV are markers of poorer outcome for patients with PDAC. Drugs targeting this axis may prevent PDAC progression.

A point mutation in GPI-attachment signal peptide accelerates the development of prion disease

A missense variant from methionine to arginine at codon 232 (M232R) of the prion protein gene accounts for ~ 15% of Japanese patients with genetic prion diseases. However, pathogenic roles of the M232R substitution for the induction of prion disease have remained elusive because family history is usually absent in patients with M232R. In addition, the clinicopathologic phenotypes of patients with M232R are indistinguishable from those of sporadic Creutzfeldt-Jakob disease patients. Furthermore, the M232R substitution is located in the glycosylphosphatidylinositol (GPI)-attachment signal peptide that is cleaved off during the maturation of prion proteins. Therefore, there has been an argument that the M232R substitution might be an uncommon polymorphism rather than a pathogenic mutation. To unveil the role of the M232R substitution in the GPI-attachment signal peptide of prion protein in the pathogenesis of prion disease, here we generated a mouse model expressing human prion proteins with M232R and investigated the susceptibility to prion disease. The M232R substitution accelerates the development of prion disease in a prion strain-dependent manner, without affecting prion strain-specific histopathologic and biochemical features. The M232R substitution did not alter the attachment of GPI nor GPI-attachment site. Instead, the substitution altered endoplasmic reticulum translocation pathway of prion proteins by reducing the hydrophobicity of the GPI-attachment signal peptide, resulting in the reduction of N-linked glycosylation and GPI glycosylation of prion proteins. To the best of our knowledge, this is the first time to show a direct relationship between a point mutation in the GPI-attachment signal peptide and the development of disease.

Accumulated precursors of specific GPI-anchored proteins upregulate GPI biosynthesis with ARV1

We previously reported that glycosylphosphatidylinositol (GPI) biosynthesis is upregulated when endoplasmic reticulum-associated degradation (ERAD) is defective; however, the underlying mechanistic basis remains unclear. Based on a genome-wide CRISPR-Cas9 screen, we show that a widely expressed GPI-anchored protein CD55 precursor and ER-resident ARV1 are involved in upregulation of GPI biosynthesis under ERAD-deficient conditions. In cells defective in GPI transamidase, GPI-anchored protein precursors fail to obtain GPI, with the remaining uncleaved GPI-attachment signal at the C-termini. We show that ERAD deficiency causes accumulation of the CD55 precursor, which in turn upregulates GPI biosynthesis, where the GPI-attachment signal peptide is the active element. Among the 31 GPI-anchored proteins tested, only the GPI-attachment signal peptides of CD55, CD48, and PLET1 enhance GPI biosynthesis. ARV1 is prerequisite for the GPI upregulation by CD55 precursor. Our data indicate that GPI biosynthesis is balanced to need by ARV1 and precursors of specific GPI-anchored proteins.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2019-2020

This review is the tenth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2020. Also included are papers that describe methods appropriate to analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. The review is basically divided into three sections: (1) general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, quantification and the use of arrays. (2) Applications to various structural types such as oligo- and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals, and (3) other areas such as medicine, industrial processes and glycan synthesis where MALDI is extensively used. Much of the material relating to applications is presented in tabular form. The reported work shows increasing use of incorporation of new techniques such as ion mobility and the enormous impact that MALDI imaging is having. MALDI, although invented nearly 40 years ago is still an ideal technique for carbohydrate analysis and advancements in the technique and range of applications show little sign of diminishing.

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

Glycosylphosphatidylinositol (GPI) is a posttranslational glycolipid modification of proteins that anchors proteins in lipid rafts on the cell surface. Although some GPI-anchored proteins (GPI-APs), including the prion protein PrP^C, have a glycan side chain composed of N-acetylgalactosamine (GalNAc)−galactose−sialic acid on the core structure of GPI glycolipid, in vivo functions of this GPI-GalNAc side chain are largely unresolved. Here, we investigated the physiological and pathological roles of the GPI-GalNAc side chain in vivo by knocking out its initiation enzyme, PGAP4, in mice. We show that Pgap4 mRNA is highly expressed in the brain, particularly in neurons, and mass spectrometry analysis confirmed the loss of the GalNAc side chain in PrP^C GPI in PGAP4-KO mouse brains. Furthermore, PGAP4-KO mice exhibited various phenotypes, including an elevated blood alkaline phosphatase level, impaired bone formation, decreased locomotor activity, and impaired memory, despite normal expression levels and lipid raft association of various GPI-APs. Thus, we conclude that the GPI-GalNAc side chain is required for in vivo functions of GPI-APs in mammals, especially in bone and the brain. Moreover, PGAP4-KO mice were more vulnerable to prion diseases and died earlier after intracerebral inoculation of the pathogenic prion strains than wildtype mice, highlighting the protective roles of the GalNAc side chain against prion diseases.

Siglec Signaling in the Tumor Microenvironment

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are a family of receptors that recognize sialoglycans - sialic acid containing glycans that are abundantly present on cell membranes. Siglecs are expressed on most immune cells and can modulate their activity and function. The majority of Siglecs contains immune inhibitory motifs comparable to the immune checkpoint receptor PD-1. In the tumor microenvironment (TME), signaling through the Siglec-sialoglycan axis appears to be enhanced through multiple mechanisms favoring tumor immune evasion similar to the PD-1/PD-L1 signaling pathway. Siglec expression on tumor-infiltrating immune cells appears increased in the immune suppressive microenvironment. At the same time, enhanced Siglec ligand expression has been reported for several tumor types as a result of aberrant glycosylation, glycan modifications, and the increased expression of sialoglycans on proteins and lipids. Siglec signaling has been identified as important regulator of anti-tumor immunity in the TME, but the key factors contributing to Siglec activation by tumor-associated sialoglycans are diverse and poorly defined. Among others, Siglec activation and signaling are co-determined by their expression levels, cell surface distribution, and their binding preferences for cis- and trans-ligands in the TME. Siglec binding preference are co-determined by the nature of the proteins/lipids to which the sialoglycans are attached and the multivalency of the interaction. Here, we review the current understanding and emerging conditions and factors involved in Siglec signaling in the TME and identify current knowledge gaps that exist in the field.

A knockout cell library of GPI biosynthetic genes for functional studies of GPI-anchored proteins

Over 100 kinds of proteins are expressed as glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) on the cell surface in mammalian cells. GPI-APs possess unique properties in terms of their intracellular trafficking and association with lipid rafts. Although it is clear that GPI-APs play critical roles in various biological phenomena, it is poorly understood how the GPI moiety contributes to these mechanisms. More than 30 genes are involved in the correct biosynthesis of GPI-APs. We here constructed a cell library in which 32 genes involved in GPI biosynthesis were knocked out in human embryonic kidney 293 cells. Using the cell library, the surface expression and sensitivity to phosphatidylinositol-specific phospholipase C of GPI-APs were analyzed. Furthermore, we identified structural motifs of GPIs that are recognized by a GPI-binding toxin, aerolysin. The cell-based GPI-knockout library could be applied not only to basic researches, but also to applications and methodologies related to GPI-APs.

Mammalian GPI-anchor modifications and the enzymes involved

Glycosylphosphatidylinositol (GPI) is a glycolipid added to the C-terminus of a large variety of proteins in eukaryotes, thereby anchoring these proteins to the cell surface. More than 150 different human proteins are modified with GPI, and GPI-anchored proteins (GPI-APs) play critical roles in embryogenesis, neurogenesis, immunity, and fertilization. GPI-APs are biosynthesized in the endoplasmic reticulum (ER) and transported to the plasma membrane via the Golgi apparatus. During transport, GPI-APs undergo structural remodeling that is important for the efficient folding and sorting of GPI-APs. Asparagine-linked glycan-dependent folding and deacylation by PGAP1 work together to ensure that correctly folded GPI-APs are transported from the ER to the Golgi. Remodeling of the GPI lipid moiety is critical for the association of GPI-APs with lipid rafts. On the cell surface, certain GPI-APs are cleaved by GPI cleavage enzymes and released from the membrane, a key event in processes such as spermatogenesis and neurogenesis. In this review, we discuss the enzymes involved in GPI-AP biosynthesis and the fate of GPI-APs in mammalian cells, with a focus on the assembly, folding, degradation, and cleavage of GPI-APs.

Glycosylphosphatidylinositol-Anchor Synthesis in Plants: A Glycobiology Perspective

More than 200 diverse secretory proteins from Arabidopsis thaliana carry a glycosylphosphatidylinositol (GPI) lipid anchor covalently attached to their carboxyl-terminus. The GPI-anchor contains a lipid-linked glycan backbone that is preassembled in the endoplasmic reticulum (ER) of plants and subsequently transferred to distinct proteins, which provides them with specific features. The GPI-anchored proteins exit the ER and are transported through the Golgi apparatus to the plasma membrane. In the Golgi, the glycan moiety can be further modified by the specific attachment of sugar residues. While these biosynthetic steps are already quite well understood in mammals and yeast, comparatively little is known in plants. In this perspective, we discuss the current knowledge about the biosynthesis of the GPI-anchor glycan moiety in the light of recent findings for mammalian GPI-anchor glycan modifications.