Glycosylation differences between the normal and pathogenic prion protein isoforms

Application of N-Terminal Labeling Methods Provide Novel Insights into Endoproteolysis of the Prion Protein in Vivo

Alternative α- and β-cleavage events in the cellular prion protein (PrPC) central region generate fragments with distinct biochemical features that affect prion disease pathogenesis, but the assignment of precise cleavage positions has proven challenging. Exploiting mouse transgenic models expressing wild-type (WT) PrPC and an octarepeat region mutant allele (S3) with increased β-fragmentation, cleavage sites were defined using LC-MS/MS in conjunction with N-terminal enzymatic labeling and chemical in-gel acetylation. Our studies profile the net proteolytic repertoire of the adult brain, as deduced from defining hundreds of proteolytic events in other proteins, and position individual cleavage events in PrPC α- and β-target areas imputed from earlier, lower resolution methods; these latter analyses established site heterogeneity, with six cleavage sites positioned in the β-cleavage region of WT PrPC and nine positions for S3 PrPC. Regarding α-cleavage, aside from reported N-termini at His110 and Val111, we identified a total of five shorter fragments in the brain of both mice lines. We infer that aminopeptidase activity in the brain could contribute to the ragged N-termini observed around PrPC's α- and β-cleavage sites, with this work providing a point of departure for further in vivo studies of brain proteases.

Analysis of Native and Permethylated N-Glycan Isomers Using MGC-LC-MS Techniques

Glycosylation is an important post-translational modification that affects many critical cellular functions such as adhesion, signaling, protein stability, and function, among others. Abnormal glycosylation has been linked to many diseases. As such, the investigation of glycans and their roles in disease pathway and progression is important. Glycan analysis can be challenging, however, due to such factors as the heterogeneity of glycans and isomers as well as the poor ionization efficiency provided by mass spectrometry analyses. This chapter presents efficient methods that overcome these and other challenges for the analysis of native and permethylated N-glycan isomers in biological samples. Instructions regarding the packing of the MGC column, the N-glycan sample prep, and the LC-MS conditions are also provided.

Reactive astrocytes associated with prion disease impair the blood brain barrier

BACKGROUND

Impairment of the blood-brain barrier (BBB) is considered to be a common feature among neurodegenerative diseases, including Alzheimer's, Parkinson's and prion diseases. In prion disease, increased BBB permeability was reported 40 years ago, yet the mechanisms behind the loss of BBB integrity have never been explored. Recently, we showed that reactive astrocytes associated with prion diseases are neurotoxic. The current work examines the potential link between astrocyte reactivity and BBB breakdown.

RESULTS

In prion-infected mice, the loss of BBB integrity and aberrant localization of aquaporin 4 (AQP4), a sign of retraction of astrocytic endfeet from blood vessels, were noticeable prior to disease onset. Gaps in cell-to-cell junctions along blood vessels, together with downregulation of Occludin, Claudin-5 and VE-cadherin, which constitute tight and adherens junctions, suggested that loss of BBB integrity is linked with degeneration of vascular endothelial cells. In contrast to cells isolated from non-infected adult mice, endothelial cells originating from prion-infected mice displayed disease-associated changes, including lower levels of Occludin, Claudin-5 and VE-cadherin expression, impaired tight and adherens junctions, and reduced trans-endothelial electrical resistance (TEER). Endothelial cells isolated from non-infected mice, when co-cultured with reactive astrocytes isolated from prion-infected animals or treated with media conditioned by the reactive astrocytes, developed the disease-associated phenotype observed in the endothelial cells from prion-infected mice. Reactive astrocytes were found to produce high levels of secreted IL-6, and treatment of endothelial monolayers originating from non-infected animals with recombinant IL-6 alone reduced their TEER. Remarkably, treatment with extracellular vesicles produced by normal astrocytes partially reversed the disease phenotype of endothelial cells isolated from prion-infected animals.

CONCLUSIONS

To our knowledge, the current work is the first to illustrate early BBB breakdown in prion disease and to document that reactive astrocytes associated with prion disease are detrimental to BBB integrity. Moreover, our findings suggest that the harmful effects are linked to proinflammatory factors secreted by reactive astrocytes.

Role of sialylation of N-linked glycans in prion pathogenesis

Mammalian prion or PrPSc is a proteinaceous infectious agent that consists of a misfolded, self-replicating state of the prion protein or PrPC. PrPC and PrPSc are posttranslationally modified with N-linked glycans, which are sialylated at the terminal positions. More than 30 years have passed since the first characterization of the composition and structural diversity of N-linked glycans associated with the prion protein, yet the role of carbohydrate groups that constitute N-glycans and, in particular, their terminal sialic acid residues in prion disease pathogenesis remains poorly understood. A number of recent studies shed a light on the role of sialylation in the biology of prion diseases. This review article discusses several mechanisms by which terminal sialylation dictates the spread of PrPSc across brain regions and the outcomes of prion infection in an organism. In particular, relationships between the sialylation status of PrPSc and important strain-specific features including lymphotropism, neurotropism, and neuroinflammation are discussed. Moreover, emerging evidence pointing out the roles of sialic acid residues in prion replication, cross-species transmission, strain competition, and strain adaptation are reviewed. A hypothesis according to which selective, strain-specified recruitment of PrPC sialoglycoforms dictates unique strain-specific disease phenotypes is examined. Finally, the current article proposes that prion strains evolve as a result of a delicate balance between recruiting highly sialylated glycoforms to avoid an "eat-me" response by glia and limiting heavily sialylated glycoforms for enabling rapid prion replication.

Reactive astrocytes associated with prion disease impair the blood brain barrier

Background

Impairment of the blood-brain barrier (BBB) is considered to be a common feature among neurodegenerative diseases, including Alzheimer's, Parkinson's and prion diseases. In prion disease, increased BBB permeability was reported 40 years ago, yet the mechanisms behind the loss of BBB integrity have never been explored. Recently, we showed that reactive astrocytes associated with prion diseases are neurotoxic. The current work examines the potential link between astrocyte reactivity and BBB breakdown.

Results

In prion-infected mice, the loss of BBB integrity and aberrant localization of aquaporin 4 (AQP4), a sign of retraction of astrocytic endfeet from blood vessels, were noticeable prior to disease onset. Gaps in cell-to-cell junctions along blood vessels, together with downregulation of Occludin, Claudin-5 and VE-cadherin, which constitute tight and adherens junctions, suggested that loss of BBB integrity is linked with degeneration of vascular endothelial cells. In contrast to cells isolated from non-infected adult mice, endothelial cells originating from prion-infected mice displayed disease-associated changes, including lower levels of Occludin, Claudin-5 and VE-cadherin expression, impaired tight and adherens junctions, and reduced trans-endothelial electrical resistance (TEER). Endothelial cells isolated from non-infected mice, when co-cultured with reactive astrocytes isolated from prion-infected animals or treated with media conditioned by the reactive astrocytes, developed the disease-associated phenotype observed in the endothelial cells from prion-infected mice. Reactive astrocytes were found to produce high levels of secreted IL-6, and treatment of endothelial monolayers originating from non-infected animals with recombinant IL-6 alone reduced their TEER. Remarkably, treatment with extracellular vesicles produced by normal astrocytes partially reversed the disease phenotype of endothelial cells isolated from prion-infected animals.

Conclusions

To our knowledge, the current work is the first to illustrate early BBB breakdown in prion disease and to document that reactive astrocytes associated with prion disease are detrimental to BBB integrity. Moreover, our findings suggest that the harmful effects are linked to proinflammatory factors secreted by reactive astrocytes.

Deficiency in ST6GAL1, one of the two α2,6-sialyltransferases, has only a minor effect on the pathogenesis of prion disease

Prion diseases are a group of fatal neurodegenerative diseases caused by misfolding of the normal cellular form of the prion protein or PrPC, into a disease-associated self-replicating state or PrPSc. PrPC and PrPSc are posttranslationally modified with N-linked glycans, in which the terminal positions occupied by sialic acids residues are attached to galactose predominantly via α2-6 linkages. The sialylation status of PrPSc is an important determinant of prion disease pathogenesis, as it dictates the rate of prion replication and controls the fate of prions in an organism. The current study tests whether a knockout of ST6Gal1, one of the two mammalian sialyltransferases that catalyze the sialylation of glycans via α2-6 linkages, reduces the sialylation status of PrPSc and alters prion disease pathogenesis. We found that a global knockout of ST6Gal1 in mice significantly reduces the α2-6 sialylation of the brain parenchyma, as determined by staining with Sambucus Nigra agglutinin. However, the sialylation of PrPSc remained stable and the incubation time to disease increased only modestly in ST6Gal1 knockout mice (ST6Gal1-KO). A lack of significant changes in the PrPSc sialylation status and prion pathogenesis is attributed to the redundancy in sialylation and, in particular, the plausible involvement of a second member of the sialyltransferase family that sialylate via α2-6 linkages, ST6Gal2.

Cryo-EM of prion strains from the same genotype of host identifies conformational determinants

Prion strains in a given type of mammalian host are distinguished by differences in clinical presentation, neuropathological lesions, survival time, and characteristics of the infecting prion protein (PrP) assemblies. Near-atomic structures of prions from two host species with different PrP sequences have been determined but comparisons of distinct prion strains of the same amino acid sequence are needed to identify purely conformational determinants of prion strain characteristics. Here we report a 3.2 Å resolution cryogenic electron microscopy-based structure of the 22L prion strain purified from the brains of mice engineered to express only PrP lacking glycophosphatidylinositol anchors [anchorless (a) 22L]. Comparison of this near-atomic structure to our recently determined structure of the aRML strain propagated in the same inbred mouse reveals that these two mouse prion strains have distinct conformational templates for growth via incorporation of PrP molecules of the same sequence. Both a22L and aRML are assembled as stacks of PrP molecules forming parallel in-register intermolecular β-sheets and intervening loops, with single monomers spanning the ordered fibril core. Each monomer shares an N-terminal steric zipper, three major arches, and an overall V-shape, but the details of these and other conformational features differ markedly. Thus, variations in shared conformational motifs within a parallel in-register β-stack fibril architecture provide a structural basis for prion strain differentiation within a single host genotype.

Prions are protein-based pathogens that can spread within and between hosts without carrying a pathogen-specific nucleic acid genome. Given this protein-based propagation mechanism, a long-standing mystery in the prion disease field has been the molecular basis of distinct, faithfully propagating strains in a single type of mammalian host. Here we provide a direct, high-resolution cryo-EM-based comparison of the structures of two highly infectious prion strains isolated from the brains of mice of a single genotype. We show in detail how these two prion strains are protein filaments of mouse PrP molecules that display distinct conformational templates for growth on their tips. Our results identify purely conformational, rather than sequence-based, underpinnings of infectious and deadly prion strains.

High sensitivity glycomics in biomedicine

Many analytical challenges in biomedicine arise from the generally high heterogeneity and complexity of glycan- and glycoconjugate-containing samples, which are often only available in minute amounts. Therefore, highly sensitive workflows and detection methods are required. In this review mass spectrometric workflows and detection methods are evaluated for glycans and glycoproteins. Furthermore, glycomic methodologies and innovations that are tailored for enzymatic treatments, chemical derivatization, purification, separation, and detection at high sensitivity are highlighted. The discussion is focused on the analysis of mammalian N-linked and GalNAc-type O-linked glycans.

Chronic Wasting Disease (CWD) in Cervids and the Consequences of a Mutable Protein Conformation

Chronic wasting disease (CWD) is a prion disease of cervids (deer, elk, moose, etc.). It spreads readily from CWD-contaminated environments and among wild cervids. As of 2022, North American CWD has been found in 29 states, four Canadian provinces and South Korea. The Scandinavian form of CWD originated independently. Prions propagate their pathology by inducing a natively expressed prion protein (PrPC) to adopt the prion conformation (PrPSc). PrPC and PrPSc differ solely in their conformation. Like other prion diseases, transmissible CWD prions can arise spontaneously. The CWD prions can respond to selection pressures resulting in the emergence of new strain phenotypes. Annually, 11.5 million Americans hunt and harvest nearly 6 million deer, indicating that CWD is a potential threat to an important American food source. No tested CWD strain has been shown to be zoonotic. However, this may not be true for emerging strains. Should a zoonotic CWD strain emerge, it could adversely impact the hunting economy and game meat consumers.

Glycomic and Glycoproteomic Techniques in Neurodegenerative Disorders and Neurotrauma: Towards Personalized Markers

The proteome represents all the proteins expressed by a genome, a cell, a tissue, or an organism at any given time under defined physiological or pathological circumstances. Proteomic analysis has provided unparalleled opportunities for the discovery of expression patterns of proteins in a biological system, yielding precise and inclusive data about the system. Advances in the proteomics field opened the door to wider knowledge of the mechanisms underlying various post-translational modifications (PTMs) of proteins, including glycosylation. As of yet, the role of most of these PTMs remains unidentified. In this state-of-the-art review, we present a synopsis of glycosylation processes and the pathophysiological conditions that might ensue secondary to glycosylation shortcomings. The dynamics of protein glycosylation, a crucial mechanism that allows gene and pathway regulation, is described. We also explain how-at a biomolecular level-mutations in glycosylation-related genes may lead to neuropsychiatric manifestations and neurodegenerative disorders. We then analyze the shortcomings of glycoproteomic studies, putting into perspective their downfalls and the different advanced enrichment techniques that emanated to overcome some of these challenges. Furthermore, we summarize studies tackling the association between glycosylation and neuropsychiatric disorders and explore glycoproteomic changes in neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, Huntington disease, multiple sclerosis, and amyotrophic lateral sclerosis. We finally conclude with the role of glycomics in the area of traumatic brain injury (TBI) and provide perspectives on the clinical application of glycoproteomics as potential diagnostic tools and their application in personalized medicine.

Cellular prion protein in human plasma-derived extracellular vesicles promotes neurite outgrowth via the NMDA receptor-LRP1 receptor system

Exosomes and other extracellular vesicles (EVs) participate in cell–cell communication. Herein, we isolated EVs from human plasma and demonstrated that these EVs activate cell signaling and promote neurite outgrowth in PC-12 cells. Analysis of human plasma EVs purified by sequential ultracentrifugation using tandem mass spectrometry indicated the presence of multiple plasma proteins, including α₂-macroglobulin, which is reported to regulate PC-12 cell physiology. We therefore further purified EVs by molecular exclusion or phosphatidylserine affinity chromatography, which reduced plasma protein contamination. EVs subjected to these additional purification methods exhibited unchanged activity in PC-12 cells, even though α₂-macroglobulin was reduced to undetectable levels. Nonpathogenic cellular prion protein (PrP^C) was carried by human plasma EVs and essential for the effects of EVs on PC-12 cells, as EV-induced cell signaling and neurite outgrowth were blocked by the PrP^C-specific antibody, POM2. In addition, inhibitors of the N-methyl-d-aspartate (NMDA) receptor (NMDA-R) and low-density lipoprotein receptor–related protein-1 (LRP1) blocked the effects of plasma EVs on PC-12 cells, as did silencing of Lrp1 or the gene encoding the GluN1 NMDA-R subunit (Grin1). These results implicate the NMDA-R–LRP1 complex as the receptor system responsible for mediating the effects of EV-associated PrP^C. Finally, EVs harvested from rat astrocytes carried PrP^C and replicated the effects of human plasma EVs on PC-12 cell signaling. We conclude that interaction of EV-associated PrP^C with the NMDA-R–LRP1 complex in target cells represents a novel mechanism by which EVs may participate in intercellular communication in the nervous system.

PMCA-Based Detection of Prions in the Olfactory Mucosa of Patients With Sporadic Creutzfeldt-Jakob Disease

Sporadic Creutzfeldt-Jakob disease (sCJD) is a rare neurodegenerative disorder caused by the conformational conversion of the prion protein (PrP^C) into an abnormally folded form, named prion (or PrP^Sc). The combination of the polymorphism at codon 129 of the PrP gene (coding either methionine or valine) with the biochemical feature of the proteinase-K resistant PrP (generating either PrP^Sc type 1 or 2) gives rise to different PrP^Sc strains, which cause variable phenotypes of sCJD. The definitive diagnosis of sCJD and its classification can be achieved only post-mortem after PrP^Sc identification and characterization in the brain. By exploiting the Real-Time Quaking-Induced Conversion (RT-QuIC) assay, traces of PrP^Sc were found in the olfactory mucosa (OM) of sCJD patients, thus demonstrating that PrP^Sc is not confined to the brain. Here, we have optimized another technique, named protein misfolding cyclic amplification (PMCA) for detecting PrP^Sc in OM samples of sCJD patients. OM samples were collected from 27 sCJD and 2 genetic CJD patients (E200K). Samples from 34 patients with other neurodegenerative disorders were included as controls. Brains were collected from 26 sCJD patients and 16 of them underwent OM collection. Brain and OM samples were subjected to PMCA using the brains of transgenic mice expressing human PrP^C with methionine at codon 129 as reaction substrates. The amplified products were analyzed by Western blot after proteinase K digestion. Quantitative PMCA was performed to estimate PrP^Sc concentration in OM. PMCA enabled the detection of prions in OM samples with 79.3% sensitivity and 100% specificity. Except for a few cases, a predominant type 1 PrP^Sc was generated, regardless of the tissues analyzed. Notably, all amplified PrP^Sc were less resistant to PK compared to the original strain. In conclusion, although the optimized PMCA did not consent to recognize sCJD subtypes from the analysis of OM collected from living patients, it enabled us to estimate for the first time the amount of prions accumulating in this biological tissue. Further assay optimizations are needed to faithfully amplify peripheral prions whose recognition could lead to a better diagnosis and selection of patients for future clinical trials.

High-resolution structure and strain comparison of infectious mammalian prions

Within the extensive range of self-propagating pathologic protein aggregates of mammals, prions are the most clearly infectious (e.g., ∼10⁹ lethal doses per milligram). The structures of such lethal assemblies of PrP molecules have been poorly understood. Here we report a near-atomic core structure of a brain-derived, fully infectious prion (263K strain). Cryo-electron microscopy showed amyloid fibrils assembled with parallel in-register intermolecular β sheets. Each monomer provides one rung of the ordered fibril core, with N-linked glycans and glycolipid anchors projecting outward. Thus, single monomers form the templating surface for incoming monomers at fibril ends, where prion growth occurs. Comparison to another prion strain (aRML) revealed major differences in fibril morphology but, like 263K, an asymmetric fibril cross-section without paired protofilaments. These findings provide structural insights into prion propagation, strains, species barriers, and membrane pathogenesis. This structure also helps frame considerations of factors influencing the relative transmissibility of other pathologic amyloids.

Sporadic Creutzfeldt-Jakob disease: Real-Time Quaking Induced Conversion (RT-QuIC) assay represents a major diagnostic advance

Sporadic Creutzfeldt-Jakob disease (sCJD) is a rare and fatal neurodegenerative disorder with an incidence of 1.5 to 2 cases per million population/year. The disease is caused by a proteinaceous infectious agent, named prion (or PrPSc), which arises from the conformational conversion of the cellular prion protein (PrPC). Once formed, PrPSc interacts with the normally folded PrPC coercing it to undergo similar structural rearrangement. The disease is highly heterogeneous from a clinical and neuropathological point of view. The origin of this variability lies in the aberrant structures acquired by PrPSc. At least six different sCJD phenotypes have been described and each of them is thought to be caused by a peculiar PrPSc strain. Definitive sCJD diagnosis requires brain analysis with the aim of identifying intracerebral accumulation of PrPSc which currently represents the only reliable biomarker of the disease. Clinical diagnosis of sCJD is very challenging and is based on the combination of several clinical, instrumental and laboratory tests representing surrogate disease biomarkers. Thanks to the advent of the ultrasensitive Real-Time Quaking-Induced Conversion (RT-QuIC) assay, PrPSc was found in several peripheral tissues of sCJD patients, sometimes even before the clinical onset of the disease. This discovery represents an important step forward for the clinical diagnosis of sCJD. In this manuscript, we present an overview of the current applications and future perspectives of RT-QuIC in the field of sCJD diagnosis.

Methodologies for Measuring Protein Trafficking across Cellular Membranes

Nearly all proteins are synthesized in the cytosol. The majority of this proteome must be trafficked elsewhere, such as to membranes, to subcellular compartments, or outside of the cell. Proper trafficking of nascent protein is necessary for protein folding, maturation, quality control and cellular and organismal health. To better understand cellular biology, molecular and chemical technologies to properly characterize protein trafficking (and mistrafficking) have been developed and applied. Herein, we take a biochemical perspective to review technologies that enable spatial and temporal measurement of protein distribution, focusing on both the most widely adopted methodologies and exciting emerging approaches.

Biological Functions of the Intrinsically Disordered N-Terminal Domain of the Prion Protein: A Possible Role of Liquid-Liquid Phase Separation

The mammalian prion protein (PrPC) is composed of a large intrinsically disordered N-terminal and a structured C-terminal domain, containing three alpha-helical regions and a short, two-stranded beta-sheet. Traditionally, the activity of a protein was linked to the ability of the polypeptide chain to adopt a stable secondary/tertiary structure. This concept has been extended when it became evident that intrinsically disordered domains (IDDs) can participate in a broad range of defined physiological activities and play a major functional role in several protein classes including transcription factors, scaffold proteins, and signaling molecules. This ability of IDDs to engage in a variety of supramolecular complexes may explain the large number of PrPC-interacting proteins described. Here, we summarize diverse physiological and pathophysiological activities that have been described for the unstructured N-terminal domain of PrPC. In particular, we focus on subdomains that have been conserved in evolution.

PMCA-generated prions from the olfactory mucosa of patients with Fatal Familial Insomnia cause prion disease in mice

Background:

Fatal Familial Insomnia (FFI) is a genetic prion disease caused by the D178N mutation in the prion protein gene (PRNP) in coupling phase with methionine at PRNP 129. In 2017, we have shown that the olfactory mucosa (OM) collected from FFI patients contained traces of PrPSc detectable by Protein Misfolding Cyclic Amplification (PMCA).

Methods:

In this work, we have challenged PMCA-generated products obtained from OM and brain homogenate of FFI patients in BvPrP-Tg407 transgenic mice expressing the bank vole prion protein to test their ability to induce prion pathology.

Results:

All inoculated mice developed mild spongiform changes, astroglial activation, and PrPSc deposition mainly affecting the thalamus. However, their neuropathological alterations were different from those found in the brain of BvPrP-Tg407 mice injected with raw FFI brain homogenate.

Conclusions:

Although with some experimental constraints, we show that PrPSc present in OM of FFI patients is potentially infectious.

Funding:

This work was supported in part by the Italian Ministry of Health (GR-2013-02355724 and Ricerca Corrente), MJFF, ALZ, Alzheimer’s Research UK and the Weston Brain Institute (BAND2015), and Euronanomed III (SPEEDY) to FM; by the Spanish Ministerio de Economía y Competitividad (grant AGL2016-78054-R [AEI/FEDER, UE]) to JMT and JCE; AM-M was supported by a fellowship from the INIA (FPI-SGIT-2015-02).

Time of Detection of Prions in the Brain by Nanoscale Liquid Chromatography Coupled to Tandem Mass Spectrometry Is Comparable to Animal Bioassay

Prions cause transmissible and inevitably fatal neurological diseases in agriculturally important animals, including bovine spongiform encephalopathy in domestic cattle, scrapie in sheep and goats, and chronic wasting disease in cervids. Because animals are largely asymptomatic throughout the course of the disease, early detection of prion disease is important. Hamsters were peripherally (ip) inoculated with hamster-adapted (Sc237) prions. By week 13 of a 14-week disease course, clinical signs appeared. A multiple-reaction-monitoring-based method was used to quantitate the amount of proteinase-K-digested prions (PrP 27-30) and the extent of methionine 213 oxidation present in the brains of infected hamsters. Detectable amounts of PrP 27-30 were present in all animals after 4 weeks. The extent of methionine 213 oxidation decreased over time. When we compared our quantitation results to those from other researchers using bioassay, we observed that consistent detection of PrP 27-30 by mass spectrometry occurs at a time when prions are reliably detected by bioassay.

Site-specific analysis of N-glycans from different sheep prion strains

Prion diseases are a group of neurodegenerative diseases affecting a wide range of mammalian species, including humans. During the course of the disease, the abnormally folded scrapie prion protein (PrP^Sc) accumulates in the central nervous system where it causes neurodegeneration. In prion disorders, the diverse spectrum of illnesses exists because of the presence of different isoforms of PrP^Sc where they occupy distinct conformational states called strains. Strains are biochemically distinguished by a characteristic three-band immunoblot pattern, defined by differences in the occupancy of two glycosylation sites on the prion protein (PrP). Characterization of the exact N-glycan structures attached on either PrP^C or PrP^Sc is lacking. Here we report the characterization and comparison of N-glycans from two different sheep prion strains. PrP^Sc from both strains was isolated from brain tissue and enzymatically digested with trypsin. By using liquid chromatography coupled to electrospray mass spectrometry, a site-specific analysis was performed. A total of 100 structures were detected on both glycosylation sites. The N-glycan profile was shown to be similar to the one on mouse PrP, however, with additional 40 structures reported. The results presented here show no major differences in glycan composition, suggesting that glycans may not be responsible for the differences in the two analyzed prion strains.

To date, prion diseases remain a controversy amongst scientists. Although we know now it is the abnormal form of the prion protein (PrP^Sc) that causes the disease, many questions are still left unanswered. To understand the cellular mechanism of these diseases, we should first and foremost try to fully understand the prion protein itself. Even though many findings have been made regarding the structure of the protein, a large part of it is still unknown. Since the prion protein is actually a glycoprotein, to resolve its structure we need to put our focus not only on the protein part of the glycoprotein but also on the glycan structures as well. Here we compared two different sheep prion strains and although no major differences have been found between the glycan structures, this analysis may help the understanding of the role glycans have in prion diseases.

N-Linked glycosylation profiles of therapeutic induced senescent (TIS) triple negative breast cancer cells (TNBC) and their extracellular vesicle (EV) progeny

Triple negative breast cancer (TNBC) has poor clinical outcomes and limited treatment options. Chemotherapy, while killing some cancer cells, can result in therapeutic-induced-senescent (TIS) cells. Senescent cells release significantly more extracellular vesicles (EVs) than non-senescent cells. Recently, N- and O-linked glycosylation alterations have been associated with senescence. We aimed to profile the N-linked glycans of whole cells, membrane, cytoplasm and EVs harvested from TIS TNBC cells and to compare these to results from non-senescent cells. TIS was induced in the Cal51 TNBC cells using the chemotherapeutic agent paclitaxel (PTX). Ultra-performance liquid chromatography (UPLC) analysis of exoglycosidase digested N-linked glycans was carried out on TIS compared to non-treated control cells. LC-Mass spectrometry (MS) analysis of the N-linked glycans and lectin blotting of samples was carried out to confirm the UPLC results. Significant differences were found in the N-glycan profile of the Cal51 membrane, cytoplasm and EV progeny of TIS compared to non-senescent cells. Protein mass spectrometry showed that the TIS cells contain different glycan modifying enzymes. The lectin, calnexin demonstrated a lower kDa size (∼58 kDa) in TIS compared to control cells (∼90 kDa) while Galectin 3 demonstrated potential proteolytic cleavage with 32 kDa and ∼22 kDa bands evident in TIS compared to non-senescent control cells with a major 32 kDa band only. TIS CAL51 cells also demonstrated a reduced adhesion to collagen I compared to control non-senescent cells. This study has shown that therapeutic-induced-senescent TNBC cells and their EV progeny, display differential N-glycan moieties compared to non-senescent Cal51 cells and their resultant EV progeny. For the future, N-glycan moieties on cancer senescent cells and their EV progeny hold potential for (i) the monitoring of treatment response as a liquid biopsy, and (ii) cancer senescent cell targeting with lectin therapies.

Accommodation of In-Register N-Linked Glycans on Prion Protein Amyloid Cores

Although prion protein fibrils can have either parallel-in-register intermolecular β-sheet (PIRIBS) or, probably, β-solenoid architectures, the plausibility of PIRIBS architectures for the usually glycosylated natural prion strains has been questioned based the expectation that such glycans would not fit if stacked in-register on each monomer within a fibril. To directly assess this issue, we have added N-linked glycans to a recently reported cryo-electron microscopy-based human prion protein amyloid model with a PIRIBS architecture and performed in silico molecular dynamics studies to determine if the glycans can fit. Our results show that triantennary glycans can be sterically accommodated in-register on both N-linked glycosylation sites of each monomer. Additional simulations with an artificially mutated β-solenoid model confirmed that glycans can be accommodated when aligned with ∼4.8 Å spacing on every rung of a fibril. Altogether, we conclude that steric intermolecular clashes between glycans do not, in themselves, preclude PIRIBS architectures for prions.

Prion protein glycans reduce intracerebral fibril formation and spongiosis in prion disease

Posttranslational modifications (PTMs) are common among proteins that aggregate in neurodegenerative disease, yet how PTMs impact the aggregate conformation and disease progression remains unclear. By engineering knockin mice expressing prion protein (PrP) lacking 2 N-linked glycans (Prnp180Q/196Q), we provide evidence that glycans reduce spongiform degeneration and hinder plaque formation in prion disease. Prnp180Q/196Q mice challenged with 2 subfibrillar, non-plaque-forming prion strains instead developed plaques highly enriched in ADAM10-cleaved PrP and heparan sulfate (HS). Intriguingly, a third strain composed of intact, glycophosphatidylinositol-anchored (GPI-anchored) PrP was relatively unchanged, forming diffuse, HS-deficient deposits in both the Prnp180Q/196Q and WT mice, underscoring the pivotal role of the GPI-anchor in driving the aggregate conformation and disease phenotype. Finally, knockin mice expressing triglycosylated PrP (Prnp187N) challenged with a plaque-forming prion strain showed a phenotype reversal, with a striking disease acceleration and switch from plaques to predominantly diffuse, subfibrillar deposits. Our findings suggest that the dominance of subfibrillar aggregates in prion disease is due to the replication of GPI-anchored prions, with fibrillar plaques forming from poorly glycosylated, GPI-anchorless prions that interact with extracellular HS. These studies provide insight into how PTMs impact PrP interactions with polyanionic cofactors, and highlight PTMs as a major force driving the prion disease phenotype.

Modeling PrPSc Generation Through Deformed Templating

Deformed templating is the process by which self-replicating protein conformations with a given cross-β folding pattern can seed formation of an alternative self-replicating state with different cross-β folding pattern. In particular, uninfectious but propagative PrP amyloid can transform into a bona fide infectious conformer, PrP^Sc through deformed templating. The process can take many rounds of replication (if taking place in vitro) or even several passages of the evolving PrP conformers through successive brains if in vivo, through experimental transmission. In all cases, deformed templating involves a forced conversion in which there is a mismatch between the template and the substrate and/or the templating environment, typically a recombinant PrP amyloid, adept at converting recombinant PrP under denaturing conditions (e.g., presence of chaotropic agents), encountering a glycosylated, GPI-anchored PrP^C substrate under physiological conversion conditions. Deformed templating is characterized by emergence of intermediate conformers that exhibit biochemical characteristics that are intermediate between those of the initial PrP amyloid and the final PrP^Sc conformers. Here, we took advantage of the recent elucidation of the structure of a PrP amyloid by cryo-EM and the availability of a physically plausible atomistic model of PrP^Sc that we have recently proposed. Using modeling and Molecular Dynamics (MD) approaches, we built a complete molecular modelization of deformed templating, including an atomistic model of a glycosylated intermediate conformer and a modified model of PrP^Sc. Among other unanticipated outcomes, our results show that fully glycosylated PrP can be stacked in-register, and how 4-rung β-solenoid (4RβS) PrP architectures can share key structural motifs with parallel-in register intermolecular sheet (PIRIBS) PrP amyloids. Our results shed light on the mechanisms of prion replication.

Role of sialylation in prion disease pathogenesis and prion structure

Mammalian prion or PrP^Sc is a proteinaceous infectious agent that consists of a misfolded, self-replicating state of a sialoglycoprotein called the prion protein or PrP^C. Sialylation of the prion protein, a terminal modification of N-linked glycans, was discovered more than 30 years ago, yet the role of sialylation in prion pathogenesis is not well understood. This chapter summarizes current knowledge on the role of sialylation of the prion protein in prion diseases. First, we discuss recent data suggesting that sialylation of PrP^Sc N-linked glycans determines the fate of prion infection in an organism and control prion lymphotropism. Second, emerging evidence pointing out at the role N-glycans in neuroinflammation are discussed. Thirds, this chapter reviews a mechanism postulating that sialylated N-linked glycans are important players in defining strain-specific structures. A new hypothesis according to which individual strain-specific PrP^Sc structures govern selection of PrP^C sialoglycoforms is discussed. Finally, this chapter explain how N-glycan sialylation control the prion replication and strain interference. In summary, comprehensive review of our knowledge on N-linked glycans and their sialylation provided in this chapter helps to answer important questions of prion biology that have been puzzling for years.

A soluble derivative of PrPC activates cell-signaling and regulates cell physiology through LRP1 and the NMDA receptor

Cellular prion protein (PrP^C) is a widely expressed glycosylphosphatidylinositol-anchored membrane protein. Scrapie prion protein is a misfolded and aggregated form of PrP^C responsible for prion-induced neurodegenerative diseases. Understanding the function of the nonpathogenic PrP^C monomer is an important objective. PrP^C may be shed from the cell surface to generate soluble derivatives. Herein, we studied a recombinant derivative of PrP^C (soluble cellular prion protein, S-PrP) that corresponds closely in sequence to a soluble form of PrP^C shed from the cell surface by proteases in the A Disintegrin And Metalloprotease (ADAM) family. S-PrP activated cell-signaling in PC12 and N2a cells. TrkA was transactivated by Src family kinases and extracellular signal-regulated kinase 1/2 was activated downstream of Trk receptors. These cell-signaling events were dependent on the N-methyl-d-aspartate receptor (NMDA-R) and low-density lipoprotein receptor-related protein-1 (LRP1), which functioned as a cell-signaling receptor system in lipid rafts. Membrane-anchored PrP^C and neural cell adhesion molecule were not required for S-PrP-initiated cell-signaling. S-PrP promoted PC12 cell neurite outgrowth. This response required the NMDA-R, LRP1, Src family kinases, and Trk receptors. In Schwann cells, S-PrP interacted with the LRP1/NMDA-R system to activate extracellular signal-regulated kinase 1/2 and promote cell migration. The effects of S-PrP on PC12 cell neurite outgrowth and Schwann cell migration were similar to those caused by other proteins that engage the LRP1/NMDA-R system, including activated α₂-macroglobulin and tissue-type plasminogen activator. Collectively, these results demonstrate that shed forms of PrP^C may exhibit important biological activities in the central nervous system and the peripheral nervous system by serving as ligands for the LRP1/NMDA-R system.

Incomplete glycosylation during prion infection unmasks a prion protein epitope that facilitates prion detection and strain discrimination

The causative factors underlying conformational conversion of cellular prion protein (PrP^C) into its infectious counterpart (PrP^Sc) during prion infection remain undetermined, in part because of a lack of monoclonal antibodies (mAbs) that can distinguish these conformational isoforms. Here we show that the anti-PrP mAb PRC7 recognizes an epitope that is shielded from detection when glycans are attached to Asn-196. We observed that whereas PrP^C is predisposed to full glycosylation and is therefore refractory to PRC7 detection, prion infection leads to diminished PrP^Sc glycosylation at Asn-196, resulting in an unshielded PRC7 epitope that is amenable to mAb recognition upon renaturation. Detection of PRC7-reactive PrP^Sc in experimental and natural infections with various mouse-adapted scrapie strains and with prions causing deer and elk chronic wasting disease and transmissible mink encephalopathy uncovered that incomplete PrP^Sc glycosylation is a consistent feature of prion pathogenesis. We also show that interrogating the conformational properties of the PRC7 epitope affords a direct means of distinguishing different prion strains. Because the specificity of our approach for prion detection and strain discrimination relies on the extent to which N-linked glycosylation shields or unshields PrP epitopes from antibody recognition, it dispenses with the requirement for additional standard manipulations to distinguish PrP^Sc from PrP^C, including evaluation of protease resistance. Our findings not only highlight an innovative and facile strategy for prion detection and strain differentiation, but are also consistent with a mechanism of prion replication in which structural instability of incompletely glycosylated PrP contributes to the conformational conversion of PrP^C to PrP^Sc.

Prion protein post-translational modifications modulate heparan sulfate binding and limit aggregate size in prion disease

Many aggregation-prone proteins linked to neurodegenerative disease are post-translationally modified during their biogenesis. In vivo pathogenesis studies have suggested that the presence of post-translational modifications can shift the aggregate assembly pathway and profoundly alter the disease phenotype. In prion disease, the N-linked glycans and GPI-anchor on the prion protein (PrP) impair fibril assembly. However, the relevance of the two glycans to aggregate structure and disease progression remains unclear. Here we show that prion-infected knockin mice expressing an additional PrP glycan (tri-glycosylated PrP) develop new plaque-like deposits on neuronal cell membranes, along the subarachnoid space, and periventricularly, suggestive of high prion mobility and transit through the interstitial fluid. These plaque-like deposits were largely non-congophilic and composed of full length, uncleaved PrP, indicating retention of the glycophosphatidylinositol (GPI) anchor. Prion aggregates sedimented in low density fractions following ultracentrifugation, consistent with oligomers, and bound low levels of heparan sulfate (HS) similar to other predominantly GPI-anchored prions. Collectively, these results suggest that highly glycosylated PrP primarily converts as a GPI-anchored glycoform, with low involvement of HS co-factors, limiting PrP assembly mainly to oligomers. Since PrP^C is highly glycosylated, these findings may explain the high frequency of diffuse, synaptic, and plaque-like deposits in the brain as well as the rapid conversion commonly observed in human and animal prion disease.

Region-Specific Sialylation Pattern of Prion Strains Provides Novel Insight into Prion Neurotropism

Mammalian prions are unconventional infectious agents that invade and replicate in an organism by recruiting a normal form of a prion protein (PrP^C) and converting it into misfolded, disease-associated state referred to as PrP^Sc. PrP^C is posttranslationally modified with two N-linked glycans. Prion strains replicate by selecting substrates from a large pool of PrP^C sialoglycoforms expressed by a host. Brain regions have different vulnerability to prion infection, however, molecular mechanisms underlying selective vulnerability is not well understood. Toward addressing this question, the current study looked into a possibility that sialylation of PrP^Sc might be involved in defining selective vulnerability of brain regions. The current work found that in 22L -infected animals, PrP^Sc is indeed sialylated in a region dependent manner. PrP^Sc in hippocampus and cortex was more sialylated than PrP^Sc from thalamus and stem. Similar trends were also observed in brain materials from RML- and ME7-infected animals. The current study established that PrP^Sc sialylation status is indeed region-specific. Together with previous studies demonstrating that low sialylation status accelerates prion replication, this work suggests that high vulnerability of certain brain region to prion infection could be attributed to their low sialylation status.

Combining molecular dynamics simulations and experimental analyses in protein misfolding

The fold of a protein determines its function and its misfolding can result in loss-of-function defects. In addition, for certain proteins their misfolding can lead to gain-of-function toxicities resulting in protein misfolding diseases such as Alzheimer's, Parkinson's, or the prion diseases. In all of these diseases one or more proteins misfold and aggregate into disease-specific assemblies, often in the form of fibrillar amyloid deposits. Most, if not all, protein misfolding diseases share a fundamental molecular mechanism that governs the misfolding and subsequent aggregation. A wide variety of experimental methods have contributed to our knowledge about misfolded protein aggregates, some of which are briefly described in this review. The misfolding mechanism itself is difficult to investigate, as the necessary timescale and resolution of the misfolding events often lie outside of the observable parameter space. Molecular dynamics simulations fill this gap by virtue of their intrinsic, molecular perspective and the step-by-step iterative process that forms the basis of the simulations. This review focuses on molecular dynamics simulations and how they combine with experimental analyses to provide detailed insights into protein misfolding and the ensuing diseases.

Atomic insights into the effects of pathological mutants through the disruption of hydrophobic core in the prion protein

Destabilization of prion protein induces a conformational change from normal prion protein (PrP^C) to abnormal prion protein (PrP^SC). Hydrophobic interaction is the main driving force for protein folding, and critically affects the stability and solvability. To examine the importance of the hydrophobic core in the PrP, we chose six amino acids (V176, V180, T183, V210, I215, and Y218) that make up the hydrophobic core at the middle of the H2-H3 bundle. A few pathological mutants of these amino acids have been reported, such as V176G, V180I, T183A, V210I, I215V, and Y218N. We focused on how these pathologic mutations affect the hydrophobic core and thermostability of PrP. For this, we ran a temperature-based replica-exchange molecular dynamics (T-REMD) simulation, with a cumulative simulation time of 28 μs, for extensive ensemble sampling. From the T-REMD ensemble, we calculated the protein folding free energy difference between wild-type and mutant PrP using the thermodynamic integration (TI) method. Our results showed that pathological mutants V176G, T183A, I215V, and Y218N decrease the PrP stability. At the atomic level, we examined the change in pair-wise hydrophobic interactions from valine-valine to valine-isoleucine (and vice versa), which is induced by mutation V180I, V210I (I215V) at the 180^th-210^th (176^th-215^th) pair. Finally, we investigated the importance of the π-stacking between Y218 and F175.

Prion protein-Semisynthetic prion protein (PrP) variants with posttranslational modifications

Deciphering the pathophysiologic events in prion diseases is challenging, and the role of posttranslational modifications (PTMs) such as glypidation and glycosylation remains elusive due to the lack of homogeneous protein preparations. So far, experimental studies have been limited in directly analyzing the earliest events of the conformational change of cellular prion protein (PrP^C ) into scrapie prion protein (PrP^Sc ) that further propagates PrP^C misfolding and aggregation at the cellular membrane, the initial site of prion infection, and PrP misfolding, by a lack of suitably modified PrP variants. PTMs of PrP, especially attachment of the glycosylphosphatidylinositol (GPI) anchor, have been shown to be crucially involved in the PrP^Sc formation. To this end, semisynthesis offers a unique possibility to understand PrP behavior invitro and invivo as it provides access to defined site-selectively modified PrP variants. This approach relies on the production and chemoselective linkage of peptide segments, amenable to chemical modifications, with recombinantly produced protein segments. In this article, advances in understanding PrP conversion using semisynthesis as a tool to obtain homogeneous posttranslationally modified PrP will be discussed.

Revealing the Biological Attributes of N-Glycan Isomers in Breast Cancer Brain Metastasis Using Porous Graphitic Carbon (PGC) Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)

Breast cancer is a leading cancer in women and is considered to be the second-most common metastatic cancer following lung cancer. An estimated 10-16% of breast cancer patients are suffering from brain metastasis, and the diagnostic cases of breast cancer brain metastasis are increasing. Nevertheless, the mechanisms behind this process are still unclear. Aberrant glycosylation has been proved to be related to many diseases and cancer metastasis. However, studies of N-glycan isomer function in breast cancer brain metastasis are limited. In this study, the expressions of N-glycan isomers derived from five breast cancer cell lines and one brain cancer cell line were investigated and compared to a brain-seeking cell line, 231BR, to acquire a better understanding of the role glycan isomers play in breast cancer brain metastasis. The high temperature nanoPGC-LC-MS/MS achieved an efficient isomeric separation and permitted the identification and quantitation of 144 isomers from 50 N-glycan compositions. There were significant expression alterations of these glycan isomers among the different breast cancer cell lines. The increase of total glycan abundance and sialylation level were observed to be associated with breast cancer invasion. With regard to individual isomers, the greatest number of sialylated isomers was observed along with significant expression alterations in 231BR, suggesting a relationship between glycan sialylation and breast cancer brain metastasis. Furthermore, the increase of the α2,6-sialylation level in 231BR likely contributes to the passage of breast cancer cells through the blood-brain barrier, thus facilitating breast cancer brain metastasis. Meanwhile, the upregulation of highly sialylated glycan isomers with α2,6-linked sialic acids were found to be associated with breast cancer metastasis. This investigation of glycan isomer expressions, especially the unique isomeric expression in brain-seeking cell line 231BR, provides new information toward understanding the potential roles glycan isomers play during breast cancer metastasis and more clues for a deeper insight of this bioprocess.

Impact of N-glycosylation site variants during human PrP aggregation and fibril nucleation

Misfolding and aggregation of the human prion protein (PrP) cause neurodegenerative transmissible spongiform encephalopathies such as Creutzfeldt-Jakob disease. Mature native PrP is composed of 209 residues and is folded into a C-terminal globular domain (residues 125-209) comprising a small two-stranded β-sheet and three α-helices. The N-terminal domain (residues 23-124) is intrinsically disordered. Expression of truncated PrP (residues 90-231) is sufficient to cause prion disease and residues 90/100-231 is comprising the amyloid-like fibril core of misfolded infectious PrP. During PrP fibril formation under native conditions in vitro, the disordered N-terminal domain slows down fibril formation likely due to a mechanism of initial aggregation forming morphologically disordered aggregates. The morphological disordered aggregate is a transient phase. Nucleation of fibrils occurs from this initial aggregate. The aggregate phase is largely circumvented by seeding with preformed PrP fibrils. In vivo PrP is N-glycosylated at positions Asn181 and Asn197. Little is known about the importance of these positions and their glycans for PrP stability, aggregation and fibril formation. We have in this study taken a step towards that goal by mutating residues 181 and 197 for cysteines to study the positional impact on these processes. We have further by organic synthetic chemistry and chemical modification generated synthetic glycosylations in these positions. Our data shows that residue 181 when mutated to a cysteine is a key residue for self-chaperoning, rendering a trap in the initial aggregate preventing conformational changes towards amyloid fibril formation. Position 197 is less involved in the aggregate trapping and is more geared towards β-sheet structure conversion within amyloid fibrils. As expected, synthetic glycosylated 197 is less affected towards fibril formation compared to glycosylated 181. Our data are rather compatible with the parallel in-register intermolecular β-sheet model structure of the PrP90-231 fibril and sheds light on the misfolding transitions of PrP in vitro. We hypothesize that glycosylation of position 181 is a key site for prion strain differentiation in vivo.

Analysis of the Glycosylation Profile of Disease-Associated Water-Soluble Prion Protein Using Lectins

The disease-associated water-soluble form of hamster prion protein (ws-PrP^Sc) has recently been found to be less stable than classical PrP^Sc. Since the stability of PrP to degradation correlates with its glycosylation level, the aim of this study was to investigate whether there are differences between the glycosylation of ws-PrP^Sc and classical PrP^Sc of hamster which might account for the ws-PrP^Sc minor stability compared with that of the classical PrP^Sc. Thus, ws-PrP and classical PrP were captured from noninfected or scrapie-infected hamster brain homogenate [high-speed supernatant (S^HS) and high-speed pellet (P^HS)] and blood plasma by anti-PrP antibodies (3F4 and 6H4) and subjected to screening for glycans by lectins under denaturing or nondenaturing procedures in a sandwich lectin-ELISA. Glycans have been found in minor quantities and differently exposed on ws-PrP^Sc from S^HS and plasma compared with classical PrP^Sc from P^HS. These differences have been shown to be potentially responsible for the instability of ws-PrP^Sc. Treatment of infected blood with GdnHCl significantly (P<0.01) increased the detection of ws-PrP^Sc in ELISA, reflecting an increase in its stability, and showed efficacy in removing high-abundance proteins in silver-stained gels. This increase in ws-PrP^Sc stability is due to an interaction of GdnHCl not only with high-abundance proteins but also with the ws-PrP^Sc glycosylation with particular regard to the mannose sugar. Analysis of lectins immunoreactivity toward total proteins from plasma collected before and at different time points after infection revealed that mannose might exert a stabilizing effect toward all of hamster blood glycoproteins, regardless of scrapie infection. Since low levels of ws-PrP^Sc/soluble-infectivity have been estimated both in blood and brain of hamster, this glycosylation-related instability may have negatively influenced the propensity of ws-PrP^C to convert to ws-PrP^Sc both in blood and the brain. Therefore, PrP^C glycosylation characteristics may provide a tool for the determination risk of prion transmissibility.

Prion Strain-Specific Structure and Pathology: A View from the Perspective of Glycobiology

Prion diseases display multiple disease phenotypes characterized by diverse clinical symptoms, different brain regions affected by the disease, distinct cell tropism and diverse PrP^Sc deposition patterns. The diversity of disease phenotypes within the same host is attributed to the ability of PrP^C to acquire multiple, alternative, conformationally distinct, self-replicating PrP^Sc states referred to as prion strains or subtypes. Structural diversity of PrP^Sc strains has been well documented, yet the question of how different PrP^Sc structures elicit multiple disease phenotypes remains poorly understood. The current article reviews emerging evidence suggesting that carbohydrates in the form of sialylated N-linked glycans, which are a constitutive part of PrP^Sc, are important players in defining strain-specific structures and disease phenotypes. This article introduces a new hypothesis, according to which individual strain-specific PrP^Sc structures govern selection of PrP^C sialoglycoforms that form strain-specific patterns of carbohydrate epitopes on PrP^Sc surface and contribute to defining the disease phenotype and outcomes.

The cellular and pathologic prion protein

The cellular prion protein, PrP^C, is a small, cell surface glycoprotein with a function that is currently somewhat ill defined. It is also the key molecule involved in the family of neurodegenerative disorders called transmissible spongiform encephalopathies, which are also known as prion diseases. The misfolding of PrP^C to a conformationally altered isoform, designated PrP^TSE, is the main molecular process involved in pathogenesis and appears to precede many other pathologic and clinical manifestations of disease, including neuronal loss, astrogliosis, and cognitive loss. PrP^TSE is also believed to be the major component of the infectious "prion," the agent responsible for disease transmission, and preparations of this protein can cause prion disease when inoculated into a naïve host. Thus, understanding the biochemical and biophysical properties of both PrP^C and PrP^TSE, and ultimately the mechanisms of their interconversion, is critical if we are to understand prion disease biology. Although entire books could be devoted to research pertaining to the protein, herein we briefly review the state of knowledge of prion biochemistry, including consideration of prion protein structure, function, misfolding, and dysfunction.

Glycosylation Significantly Inhibits the Aggregation of Human Prion Protein and Decreases Its Cytotoxicity

Prion diseases are primarily caused by the misfolding of prion proteins in humans, cattle, sheep, and cervid species. The effects of glycosylation on prion protein (PrP) structure and function have not been thoroughly elucidated to date. In this study, we attempt to elucidate the effects of glycosylation on the aggregation and toxicity of human PrP. As revealed by immunocytochemical staining, wild-type PrP and its monoglycosylated mutants N181D, N197D, and T199N/N181D/N197D are primarily attached to the plasma membrane. In contrast, PrP F198S, a pathological mutant with an altered residue within the glycosylation site, and an unglycosylated PrP mutant, N181D/N197D, primarily exist in the cytoplasm. In the pathological mutant V180I, there is an equal mix of membranous and cytoplasmic PrP, indicating that N-linked glycosylation deficiency impairs the correct localization of human PrP at the plasma membrane. As shown by immunoblotting and flow cytometry, human PrP located in the cytoplasm displays considerably greater PK resistance and aggregation ability and is associated with considerably higher cellular ROS levels than PrP located on the plasma membrane. Furthermore, glycosylation deficiency enhances human PrP cytotoxicity induced by MG132 or the toxic prion peptide PrP 106-126. Therefore, we propose that glycosylation acts as a necessary cofactor in determining PrP localization on the plasma membrane and that it significantly inhibits the aggregation of human PrP and decreases its cytotoxicity.

Food Forensics: Using Mass Spectrometry To Detect Foodborne Protein Contaminants, as Exemplified by Shiga Toxin Variants and Prion Strains

Food forensicists need a variety of tools to detect the many possible food contaminants. As a result of its analytical flexibility, mass spectrometry is one of those tools. Use of the multiple reaction monitoring (MRM) method expands its use to quantitation as well as detection of infectious proteins (prions) and protein toxins, such as Shiga toxins. The sample processing steps inactivate prions and Shiga toxins; the proteins are digested with proteases to yield peptides suitable for MRM-based analysis. Prions are detected by their distinct physicochemical properties and differential covalent modification. Shiga toxin analysis is based on detecting peptides derived from the five identical binding B subunits comprising the toxin. ¹⁵N-labeled internal standards are prepared from cloned proteins. These examples illustrate the power of MRM, in that the same instrument can be used to safely detect and quantitate protein toxins, prions, and small molecules that might contaminate our food.

Inflammatory response of microglia to prions is controlled by sialylation of PrPSc

Neuroinflammation is recognized as one of the obligatory pathogenic features of neurodegenerative diseases including Alzheimer’s, Parkinson’s or prion diseases. In prion diseases, space and time correlations between deposition of disease-associated, pathogenic form of the prion protein or PrP^Sc and microglial-mediated neuroinflammation has been established. Yet, it remains unclear whether activation of microglia is triggered directly by a contact with PrP^Sc, and what molecular features of PrP^Sc microglia sense and respond to that drive microglia to inflammatory states. The current study asked the questions whether PrP^Sc can directly trigger activation of microglia and whether the degree of microglia response depends on the nature of terminal carbohydrate groups on the surface of PrP^Sc particles. PrP^Sc was purified from brains of mice infected with mouse-adapted prion strain 22L or neuroblastoma N2a cells stably infected with 22L. BV2 microglial cells or primary microglia were cultured in the presence of purified 22L. We found that exposure of BV2 cells or primary microglia to purified PrP^Sc triggered proinflammatory responses characterized by an increase in the levels of TNFα, IL6, nitric oxide (NO) and expression of inducible Nitric Oxide Synthase (iNOS). Very similar patterns of inflammatory response were induced by PrP^Sc purified from mouse brains and neuroblastoma cells arguing that microglia response is independent of the source of PrP^Sc. To test whether the microglial response is mediated by carbohydrate epitopes on PrP^Sc surface, the levels of sialylation of PrP^Sc N-linked glycans was altered by treatment of purified PrP^Sc with neuraminidase. Partial cleavage of sialic acid residues was found to boost the inflammatory response of microglia to PrP^Sc. Moreover, transient degradation of Iκβα observed upon treatment with partially desialylated PrP^Sc suggests that canonical NFκB activation pathway is involved in inflammatory response. The current study is the first to demonstrate that PrP^Sc can directly trigger inflammatory response in microglia. In addition, this work provides direct evidence that the chemical nature of the carbohydrate groups on PrP^Sc surface is important for microglial activation.

Recent advances in mass spectrometry-based glycoproteomics

Protein glycosylation plays fundamental roles in many biological processes as one of the most common, and the most complex, posttranslational modification. Alterations in glycosylation profile are now known to be associated with many diseases. As a result, the discovery and detailed characterization of glycoprotein disease biomarkers is a primary interest of biomedical research. Advances in mass spectrometry (MS)-based glycoproteomics and glycomics are increasingly enabling qualitative and quantitative approaches for site-specific structural analysis of protein glycosylation. While the complexity presented by glycan heterogeneity and the wide dynamic range of clinically relevant samples like plasma, serum, cerebrospinal fluid, and tissue make comprehensive analyses of the glycoproteome a challenging task, the ongoing efforts into the development of glycoprotein enrichment, enzymatic digestion, and separation strategies combined with novel quantitative MS methodologies have greatly improved analytical sensitivity, specificity, and throughput. This review summarizes current MS-based glycoproteomics approaches and highlights recent advances in its application to cancer biomarker and neurodegenerative disease research.

Analysis of Covalent Modifications of Amyloidogenic Proteins Using Two-Dimensional Electrophoresis: Prion Protein and Its Sialylation

A number of proteins associated with neurodegenerative disease undergo several types of posttranslational modifications. They include N-linked glycosylation of the prion protein and amyloid precursor protein, phosphorylation of tau and α-synuclein. Posttranslational modifications alter physical properties of proteins including their net and surface charges, affecting their processing, life-time and propensity to acquire misfolded, disease-associated states. As such, analysis of posttranslational modifications is important for understanding the mechanisms of pathogenesis. Recent studies documented that sialylation of the disease-associated form of the prion protein or PrP^Sc controls the fate of prions in an organism and outcomes of prion infection. For assessing sialylation status of PrP^Sc, we developed a reliable protocol that involves two-dimensional electrophoresis followed by Western blot (2D). The current chapter describes the procedure for the analysis of sialylation status of PrP^Sc from various sources including central nervous system, secondary lymphoid organs, cultured cells, or PrP^Sc produced in Protein Misfolding Cyclic Amplification.

Biological and biochemical characterization of M2B cells: Classical BSE prion is conserved in transgenic mice overexpressing bovine prion protein gene

M2B cells with persistent classical bovine spongiform encephalopathy (C-BSE) have been established previously. In this study, we performed strain characterization of the M2B cell line in bovine PrP^C overexpressing mice (Tg 1896). Mice intracranially inoculated with M2B cells and C-BSE survived for 451 ± 7 and 465 ± 31 d post inoculation, respectively. Although biochemical properties, including deglycosylation and conformational stability, differed between M2B cells and C-BSE, inoculation with M2B cell lysate and C-BSE resulted in comparable phenotypes. Comparable vacuolation scores and PrP^Sc depositions were observed in the brain of Tg 1896 inoculated with both M2B cell lysate and C-BSE. Our results show that biochemical and biological characteristics of M2B cells and C-BSE are classifiable in the same strain.

Analyses of N-linked glycans of PrPSc revealed predominantly 2,6-linked sialic acid residues

Mammalian prions (PrP^Sc ) consist of misfolded, conformationally altered, self-replicating states of the sialoglycoprotein called prion protein or PrP^C . Recent studies revealed that the sialylation status of PrP^Sc plays a major role in evading innate immunity and infecting a host. Establishing the type of linkage by which sialic acid residues are attached to galactose is important, as it helps to identify the sialyltransferases responsible for sialylating PrP^C and outline strategies for manipulating the sialyation status of PrP^Sc . Using enzymatic treatment with sialidases and lectin blots, this study demonstrated that in N-linked glycans of PrP^Sc , the sialic acid residues are predominantly alpha 2,6-linked. High percentages of alpha 2,6-linked sialic acids were observed in PrP^Sc of three prion strains 22L, RML, and ME7, as well as PrP^Sc from brain, spleen, or N2a cells cultured in vitro. Moreover, the variation in the percentage of alpha 2,3- versus 2,6-linked sialic acid was found to be relatively minor between brain-, spleen-, or cell-derived PrP^Sc , suggesting that the type of linkage is independent of tissue type. Based on the current results, we propose that sialyltransferases of St6Gal family, which is responsible for attaching sialic acids via alpha 2,6-linkages to N-linked glycans, controls sialylation of PrP^C and PrP^Sc .

Characterization of isomeric glycan structures by LC-MS/MS

The characterization of glycosylation is critical for obtaining a comprehensive view of the regulation and functions of glycoproteins of interest. Due to the complex nature of oligosaccharides, stemming from variable compositions and linkages, and ion suppression effects, the chromatographic separation of glycans, including isomeric structures, is necessary for exhaustive characterization by MS. This review introduces the fundamental principles underlying the techniques in LC utilized by modern day glycomics researchers. Recent advances in porous graphitized carbon, reverse phase, ion exchange, and hydrophilic interaction LC utilized in conjunction with MS, for the characterization of protein glycosylation, are described with an emphasis on methods capable of resolving isomeric glycan structures.

Biosafety of Prions

Prions are the infectious agents that cause devastating and untreatable disorders known as Transmissible Spongiform Encephalopathies (TSEs). The pathologic events and the infectious nature of these transmissible agents are not completely understood yet. Due to the difficulties in inactivating prions, working with them requires specific recommendations and precautions. Moreover, with the advent of innovative technologies, such as the Protein Misfolding Cyclic Amplification (PMCA) and the Real Time Quaking-Induced Conversion (RT-QuIC), prions could be amplified in vitro and the infectious features of the amplified products need to be carefully assessed.

Semisynthetic prion protein (PrP) variants carrying glycan mimics at position 181 and 197 do not form fibrils

Semisynthesis and characterization of homogeneously mono- and di-PEGylated full length PrP variants to study the impact of PEGylation (as N-glycan mimics) on protein folding and aggregation.

The prion protein (PrP) is an N-glycosylated protein attached to the outer leaflet of eukaryotic cell membranes via a glycosylphosphatidylinositol (GPI) anchor. Different prion strains have distinct glycosylation patterns and the extent of glycosylation of potentially pathogenic misfolded prion protein (PrP^Sc) has a major impact on several prion-related diseases (transmissible spongiform encephalopathies, TSEs). Based on these findings it is hypothesized that posttranslational modifications (PTMs) of PrP influence conversion of cellular prion protein (PrP^C) into PrP^Sc and, as such, modified PrP variants are critical tools needed to investigate the impact of PTMs on the pathogenesis of TSEs. Here we report a semisynthetic approach to generate PrP variants modified with monodisperse polyethyleneglycol (PEG) units as mimics of N-glycans. Incorporating PEG at glycosylation sites 181 and 197 in PrP induced only small changes to the secondary structure when compared to unmodified, wildtype PrP. More importantly, in vitro aggregation was abrogated for all PEGylated PrP variants under conditions at which wildtype PrP aggregated. Furthermore, the addition of PEGylated PrP as low as 10 mol% to wildtype PrP completely blocked aggregation. A similar effect was observed for synthetic PEGylated PrP segments comprising amino acids 179–231 alone if these were added to wildtype PrP in aggregation assays. This behavior raises the question if large N-glycans interfere with aggregation in vivo and if PEGylated PrP peptides could serve as potential therapeutics.