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The main risk of emergence of prion diseases in humans is associated with a cross-species transmission of prions of zoonotic origin. Prion transmission between species is regulated by a species barrier. Successful cross-species transmission is often accompanied by strain adaptation and result in stable changes of strain-specific disease phenotype. Amino acid sequences of host PrPC and donor PrPSc as well as strain-specific structure of PrPSc are believed to be the main factors that control species barrier and strain adaptation. Yet, despite our knowledge of the primary structures of mammalian prions, predicting the fate of prion strain adaptation is very difficult if possible at all. The current study asked the question whether changes in cofactor environment affect the fate of prions adaptation. To address this question, hamster strain 263K was propagated under normal or RNA-depleted conditions using serial Protein Misfolding Cyclic Amplification (PMCA) conducted first in mouse and then hamster substrates. We found that 263K propagated under normal conditions in mouse and then hamster substrates induced the disease phenotype similar to the original 263K. Surprisingly, 263K that propagated first in RNA-depleted mouse substrate and then normal hamster substrate produced a new disease phenotype upon serial transmission. Moreover, 263K that propagated in RNA-depleted mouse and then RNA-depleted hamster substrates failed to induce clinical diseases for three serial passages despite a gradual increase of PrPSc in animals. To summarize, depletion of RNA in prion replication reactions changed the rate of strain adaptation and the disease phenotype upon subsequent serial passaging of PMCA-derived materials in animals. The current studies suggest that replication environment plays an important role in determining the fate of prion strain adaptation. [ABSTRACT FROM AUTHOR]

Mammalian prions (PrPSc) consist of misfolded, conformationally altered, self-replicating states of the sialoglycoprotein called prion protein or PrPC. Recent studies revealed that the sialylation status of PrPSc 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 PrPC and outline strategies for manipulating the sialyation status of PrPSc. Using enzymatic treatment with sialidases and lectin blots, this study demonstrated that in N-linked glycans of PrPSc, the sialic acid residues are predominantly alpha 2,6-linked. High percentages of alpha 2,6-linked sialic acids were observed in PrPSc of three prion strains 22L, RML, and ME7, as well as PrPSc 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 PrPSc, 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 PrPC and PrPSc. [ABSTRACT FROM AUTHOR]

Aggregation of misfolded proteins or peptides is a common feature of neurodegenerative diseases including Alzheimer’s, Parkinson’s, Huntington’s, prion and other diseases. Recent years have witnessed a growing number of reports of overlap in neuropathological features that were once thought to be unique to only one neurodegenerative disorder. However, the origin for the overlap remains unclear. One possibility is that diseases with mixed brain pathologies might arise from cross-seeding of one amyloidogenic protein by aggregated states of unrelated proteins. In the current study we examined whether prion replication can be induced by cross-seeding by α-synuclein or Aβ peptide. We found that α-synuclein aggregates formed in cultured cells or in vitro display cross-seeding activity and trigger misfolding of the prion protein (PrPC) in serial Protein Misfolding Cyclic Amplification reactions, producing self-replicating PrP states characterized by a short C-terminal proteinase K (PK)-resistant region referred to as PrPres. Non-fibrillar α-synuclein or fibrillar Aβ failed to cross-seed misfolding of PrPC. Remarkably, PrPres triggered by aggregated α-synuclein in vitro propagated in animals and, upon serial transmission, produced PrPSc and clinical prion disease characterized by spongiosis and astrocytic gliosis. The current study demonstrates that aggregated α-synuclein is potent in cross-seeding of prion protein misfolding and aggregation in vitro, producing self-replicating states that can lead to transmissible prion diseases upon serial passaging in wild type animals. In summary, the current work documents direct cross-seeding between unrelated amyloidogenic proteins associated with different neurodegenerative diseases. This study suggests that early interaction between unrelated amyloidogenic proteins might underlie the etiology of mixed neurodegenerative proteinopathies. [ABSTRACT FROM AUTHOR]

Mammalian prion or PrPSc is a proteinaceous infectious agent that consists of a misfolded, self-replicating state of a sialoglycoprotein called the prion protein, or PrPC. Sialylation of the prion protein N-linked glycans was discovered more than 30 years ago, yet the role of sialylation in prion pathogenesis remains poorly understood. Recent years have witnessed extraordinary growth in interest in sialylation and established a critical role for sialic acids in host invasion and host-pathogen interactions. This review article summarizes current knowledge on the role of sialylation of the prion protein in prion diseases. First, we discuss the correlation between sialylation of PrPSc glycans and prion infectivity and describe the factors that control sialylation of PrPSc. Second, we explain how glycan sialylation contributes to the prion replication barrier, defines strain-specific glycoform ratios, and imposes constraints for PrPSc structure. Third, several topics, including a possible role for sialylation in animal-to-human prion transmission, prion lymphotropism, toxicity, strain interference, and normal function of PrPC, are critically reviewed. Finally, a metabolic hypothesis on the role of sialylation in the etiology of sporadic prion diseases is proposed. [ABSTRACT FROM AUTHOR]

Mammalian prion or PrPSc is a proteinaceous infectious agent that consists of a misfolded, self-replicating state of a sialoglycoprotein called the prion protein, or PrPC. Sialylation of the prion protein N-linked glycans was discovered more than 30 years ago, yet the role of sialylation in prion pathogenesis remains poorly understood. Recent years have witnessed extraordinary growth in interest in sialylation and established a critical role for sialic acids in host invasion and host-pathogen interactions. This review article summarizes current knowledge on the role of sialylation of the prion protein in prion diseases. First, we discuss the correlation between sialylation of PrPSc glycans and prion infectivity and describe the factors that control sialylation of PrPSc. Second, we explain how glycan sialylation contributes to the prion replication barrier, defines strain-specific glycoform ratios, and imposes constraints for PrPSc structure. Third, several topics, including a possible role for sialylation in animal-to-human prion transmission, prion lymphotropism, toxicity, strain interference, and normal function of PrPC, are critically reviewed. Finally, a metabolic hypothesis on the role of sialylation in the etiology of sporadic prion diseases is proposed. [ABSTRACT FROM AUTHOR]

Sialylated glycans on the surface of mammalian cells act as part of a "self-associated molecular pattern," helping the immune system to recognize "self" from "altered self" or "nonself." To escape the host immune system, some bacterial pathogens have evolved biosynthetic pathways for host-like sialic acids, whereas others recruited host sialic acids for decorating their surfaces. Prions lack nucleic acids and are not conventional pathogens. Nevertheless, prions might use a similar strategy for invading and colonizing the lymphoreticular system. Here we show that the sialylation status of the infectious, disease-associated state of the prion protein (PrPSc) changes with colonization of secondary lymphoid organs (SLOs). As a result, spleen-derived PrPSc is more sialylated than brain-derived PrPSc. Enhanced sialylation of PrPSc is recapitulated in vitro by incubating brain-derived PrPSc with primary splenocytes or cultured macrophage RAW 264.7 cells. General inhibitors of sialyltranserases (STs), the enzymes that transfer sialic acid residues onto terminal positions of glycans, suppressed extrasialylation of PrPSc. A fluorescently labeled precursor of sialic acid revealed ST activity associated with RAW macrophages. This study illustrates that, upon colonization of SLOs, the sialylation status of prions changes by host STs. We propose that this mechanism is responsible for camouflaging prions in SLOs and has broad implications. [ABSTRACT FROM AUTHOR]

The central molecular event underlying prion diseases involves conformational change of the cellular form of the prion protein (PrPC), which is a sialoglycoprotein, into the disease-associated, transmissible form denoted PrPSc. Recent studies revealed a correlation between the sialylation status of PrPSc and incubation time to disease and introduced a new hypothesis that progression of prion diseases could be controlled or reversed by altering the sialylation level of PrPC. Of the four known mammalian sialidases, the enzymes that cleave off sialic acid residues, only NEU1, NEU3 and NEU4 are expressed in the brain. To test whether cellular sialidases control the steady-state sialylation level of PrPC and to identify the putative sialidase responsible for desialylating PrPC, we analyzed brain-derived PrPC from knockout mice deficient in Neu1, Neu3, Neu4, or from Neu3/Neu4 double knockouts. Surprisingly, no differences in the sialylation of PrPC or its proteolytic product C1 were noticed in any of the knockout mice tested as compared to the age-matched controls. However, significantly higher amounts of the C1 fragment relative to full-length PrPC were detected in the brains of Neu1 knockout mice as compared to WT mice or to the other knockout mice. Additional experiments revealed that in neuroblastoma cell line the sialylation pattern of C1 could be changed by an inhibitor of sialylatransferases. In summary, this study suggests that targeting cellular sialidases is apparently not the correct strategy for altering the sialylation levels of PrPC, whereas modulating the activity of sialylatransferases might offer a more promising approach. Our findings also suggest that catabolism of PrPC involves its α-cleavage followed by desialylation of the resulting C1 fragments by NEU1 and consequent fast degradation of the desialylated products. [ABSTRACT FROM AUTHOR]

The central event underlying prion diseases involves conformational change of the cellular form of the prion protein (PrPC) into the disease-associated, transmissible form (PrPSc). PrPC is a sialoglycoprotein that contains two conserved N-glycosylation sites. Among the key parameters that control prion replication identified over the years are amino acid sequence of host PrPC and the strain-specific structure of PrPSc. The current work highlights the previously unappreciated role of sialylation of PrPC glycans in prion pathogenesis, including its role in controlling prion replication rate, infectivity, cross-species barrier and PrPSc glycoform ratio. The current study demonstrates that undersialylated PrPC is selected during prion amplification in Protein Misfolding Cyclic Amplification (PMCAb) at the expense of oversialylated PrPC. As a result, PMCAb-derived PrPSc was less sialylated than brain-derived PrPSc. A decrease in PrPSc sialylation correlated with a drop in infectivity of PMCAb-derived material. Nevertheless, enzymatic de-sialylation of PrPC using sialidase was found to increase the rate of PrPSc amplification in PMCAb from 10- to 10,000-fold in a strain-dependent manner. Moreover, de-sialylation of PrPC reduced or eliminated a species barrier of for prion amplification in PMCAb. These results suggest that the negative charge of sialic acid controls the energy barrier of homologous and heterologous prion replication. Surprisingly, the sialylation status of PrPC was also found to control PrPSc glycoform ratio. A decrease in PrPC sialylation levels resulted in a higher percentage of the diglycosylated glycoform in PrPSc. 2D analysis of charge distribution revealed that the sialylation status of brain-derived PrPC differed from that of spleen-derived PrPC. Knocking out lysosomal sialidase Neu1 did not change the sialylation status of brain-derived PrPC, suggesting that Neu1 is not responsible for desialylation of PrPC. The current work highlights previously unappreciated role of PrPC sialylation in prion diseases and opens multiple new research directions, including development of new therapeutic approaches. [ABSTRACT FROM AUTHOR]

Prion diseases display multiple disease phenotypes characterized by diverse clinical symptoms, different brain regions affected by the disease, distinct cell tropism and diverse PrPSc deposition patterns. The diversity of disease phenotypes within the same host is attributed to the ability of PrPC to acquire multiple, alternative, conformationally distinct, self-replicating PrPSc states referred to as prion strains or subtypes. Structural diversity of PrPSc strains has been well documented, yet the question of how different PrPSc 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 PrPSc, are important players in defining strain-specific structures and disease phenotypes. This article introduces a new hypothesis, according to which individual strain-specific PrPSc structures govern selection of PrPC sialoglycoforms that form strain-specific patterns of carbohydrate epitopes on PrPSc surface and contribute to defining the disease phenotype and outcomes. [ABSTRACT FROM AUTHOR]

Prion or PrPSc is a proteinaceous infectious agent that consists of a misfolded and aggregated form of a sialoglycoprotein called prion protein or PrPC. PrPC has two sialylated N-linked carbohydrates. In PrPSc, the glycans are directed outward, with the terminal sialic acid residues creating a negative charge on the surface of prion particles. The current study proposes a new hypothesis that electrostatic repulsion between sialic residues creates structural constraints that control prion replication and PrPSc glycoform ratio. In support of this hypothesis, here we show that diglycosylated PrPC molecules that have more sialic groups per molecule than monoglycosylated PrPC were preferentially excluded from conversion. However, when partially desialylated PrPC was used as a substrate, recruitment of three glycoforms into PrPSc was found to be proportional to their respective populations in the substrate. In addition, hypersialylated molecules were also excluded from conversion in the strains with the strongest structural constraints, a strategy that helped reduce electrostatic repulsion. Moreover, as predicted by the hypothesis, partial desialylation of PrPC significantly increased the replication rate. This study illustrates that sialylation of N-linked glycans creates a prion replication barrier that controls replication rate and glycoform ratios and has broad implications. [ABSTRACT FROM AUTHOR]