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

      Katorcha, E.; Baskakov, I.V. (Blackwell Publishing Ltd, 2017)
      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.
    • From posttranslational modifications to disease phenotype: A substrate selection hypothesis in neurodegenerative diseases

      Baskakov, Ilia V. (MDPI AG, 2021-01-18)
      A number of neurodegenerative diseases including prion diseases, tauopathies and synu-cleinopathies exhibit multiple clinical phenotypes. A diversity of clinical phenotypes has been attributed to the ability of amyloidogenic proteins associated with a particular disease to acquire multiple, conformationally distinct, self-replicating states referred to as strains. Structural diversity of strains formed by tau, α-synuclein or prion proteins has been well documented. However, the question how different strains formed by the same protein elicit different clinical phenotypes remains poorly understood. The current article reviews emerging evidence suggesting that posttranslational modifications are important players in defining strain-specific structures and disease phenotypes. This article put forward a new hypothesis referred to as substrate selection hypothesis, according to which individual strains selectively recruit protein isoforms with a subset of posttranslational modifications that fit into strain-specific structures. Moreover, it is proposed that as a result of selective recruitment, strain-specific patterns of posttranslational modifications are formed, giving rise to unique disease phenotypes. Future studies should define whether cell-, region-and age-specific differences in metabolism of posttranslational modifications play a causative role in dictating strain identity and structural diversity of strains of sporadic origin. © 2021 by the author.
    • Limited understanding of the functional diversity of N-linked glycans as a major gap of prion biology

      Baskakov, I.V. (Taylor and Francis Inc., 2017)
      Among a broad range of hypotheses on the molecular nature of transmissible spongiform encephalopathy or scrapie agents discussed in 1960s was a hypothesis of self-replicating polysaccharides. While the studies of the past 40 years provided unambiguous proof that this is not the case, emerging evidence suggests that carbohydrates in the form of sialylated N-linked glycans, which are a constitutive part of mammalian prions or PrPSc, are essential in determining prion fate in an organism. The current extra-view article discusses recent advancements on the role of N-linked glycans and specifically their sialylation status in controlling prion fate. In addition, this manuscript introduces a new concept on the important role of strain-specific functional carbohydrate epitopes on the PrPSc surface as main determinants of strain-specific biologic features. According to this concept, individual strain-specific folding patterns of PrPSc govern selection of PrPC sialoglycoforms expressed by a host that can be accommodated within particular PrPSc structures. Strain-specific patterns of functional carbohydrate epitopes formed by N-linked glycans on PrPSc surfaces define strain-specific biologic features. As a constitutive part of PrPSc, the individual strain-specific patterns of carbohydrate epitopes propagate faithfully within a given host as long as individual strain-specific PrPSc structures are maintained, ensuring inheritance of strain-specific biologic features.
    • Prion strain-specific structure and pathology: A view from the perspective of glycobiology

      Baskakov, I.V.; Katorcha, E.; Makarava, N. (MDPI AG, 2018)
      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. � 2018 by the authors. Licensee MDPI, Basel, Switzerland.
    • Region-specific sialylation pattern of prion strains provides novel insight into prion neurotropism

      Makarava, N.; Chang, J.C.-Y.; Baskakov, I.V. (MDPI AG, 2020)
      Mammalian prions are unconventional infectious agents that invade and replicate in an organism by recruiting a normal form of a prion protein (PrPC) and converting it into misfolded, disease-associated state referred to as PrPSc. PrPC is posttranslationally modified with two N-linked glycans. Prion strains replicate by selecting substrates from a large pool of PrPC 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 PrPSc might be involved in defining selective vulnerability of brain regions. The current work found that in 22L-infected animals, PrPSc is indeed sialylated in a region dependent manner. PrPSc in hippocampus and cortex was more sialylated than PrPSc from thalamus and stem. Similar trends were also observed in brain materials from RML-and ME7-infected animals. The current study established that PrPSc 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. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.