Molecular Structure of Amyloid Fibrils Controls the Relationship between Fibrillar Size and Toxicity

According to the prevailing view, soluble oligomers or small fibrillar fragments are considered to be the most toxic species in prion diseases. To test this hypothesis, two conformationally different amyloid states were produced from the same highly pure recombinant full-length prion protein (rPrP). The cytotoxic potential of intact fibrils and fibrillar fragments generated by sonication from these two states was tested using cultured cells.For one amyloid state, fibril fragmentation was found to enhance its cytotoxic potential, whereas for another amyloid state formed within the same amino acid sequence, the fragmented fibrils were found to be substantially less toxic than the intact fibrils. Consistent with the previous studies, the toxic effects were more pronounced for cell cultures expressing normal isoform of the prion protein (PrP(C)) at high levels confirming that cytotoxicity was in part PrP(C)-dependent. Silencing of PrP(C) expression by small hairpin RNAs designed to silence expression of human PrP(C) (shRNA-PrP(C)) diminished the deleterious effects of the two amyloid states to a different extent, suggesting that the role of PrP(C)-mediated and PrP(C)-independent mechanisms depends on the structure of the aggregates.This work provides a direct illustration that the relationship between an amyloid's physical dimension and its toxic potential is not unidirectional but is controlled by the molecular structure of prion protein (PrP) molecules within aggregated states. Depending on the structure, a decrease in size of amyloid fibrils can either enhance or abolish their cytotoxic effect. Regardless of the molecular structure or size of PrP aggregates, silencing of PrP(C) expression can be exploited to reduce their deleterious effects

Trans-Dominant Inhibition of Prion Propagation In Vitro Is Not Mediated by an Accessory Cofactor

Previous studies identified prion protein (PrP) mutants which act as dominant negative inhibitors of prion formation through a mechanism hypothesized to require an unidentified species-specific cofactor termed protein X. To study the mechanism of dominant negative inhibition in vitro, we used recombinant PrPC molecules expressed in Chinese hamster ovary cells as substrates in serial protein misfolding cyclic amplification (sPMCA) reactions. Bioassays confirmed that the products of these reactions are infectious. Using this system, we find that: (1) trans-dominant inhibition can be dissociated from conversion activity, (2) dominant-negative inhibition of prion formation can be reconstituted in vitro using only purified substrates, even when wild type (WT) PrPC is pre-incubated with poly(A) RNA and PrPSc template, and (3) Q172R is the only hamster PrP mutant tested that fails to convert into PrPSc and that can dominantly inhibit conversion of WT PrP at sub-stoichiometric levels. These results refute the hypothesis that protein X is required to mediate dominant inhibition of prion propagation, and suggest that PrP molecules compete for binding to a nascent seeding site on newly formed PrPSc molecules, most likely through an epitope containing residue 172

Keratan sulphate in the tumour environment

Keratan sulphate (KS) is a bioactive glycosaminoglycan (GAG) of some complexity composed of the repeat disaccharide D-galactose β1→4 glycosidically linked to N-acetyl glucosamine. During the biosynthesis of KS, a family of glycosyltransferase and sulphotransferase enzymes act sequentially and in a coordinated fashion to add D-galactose (D-Gal) then N-acetyl glucosamine (GlcNAc) to a GlcNAc acceptor residue at the reducing terminus of a nascent KS chain to effect chain elongation. D-Gal and GlcNAc can both undergo sulphation at C6 but this occurs more frequently on GlcNAc than D-Gal. Sulphation along the developing KS chain is not uniform and contains regions of variable length where no sulphation occurs, regions which are monosulphated mainly on GlcNAc and further regions of high sulphation where both of the repeat disaccharides are sulphated. Each of these respective regions in the KS chain can be of variable length leading to KS complexity in terms of chain length and charge localization along the KS chain. Like other GAGs, it is these variably sulphated regions in KS which define its interactive properties with ligands such as growth factors, morphogens and cytokines and which determine the functional properties of tissues containing KS. Further adding to KS complexity is the identification of three different linkage structures in KS to asparagine (N-linked) or to threonine or serine residues (O-linked) in proteoglycan core proteins which has allowed the categorization of KS into three types, namely KS-I (corneal KS, N-linked), KS-II (skeletal KS, O-linked) or KS-III (brain KS, O-linked). KS-I to -III are also subject to variable addition of L-fucose and sialic acid groups. Furthermore, the GlcNAc residues of some members of the mucin-like glycoprotein family can also act as acceptor molecules for the addition of D-Gal and GlcNAc residues which can also be sulphated leading to small low sulphation glycoforms of KS. These differ from the more heavily sulphated KS chains found on proteoglycans. Like other GAGs, KS has evolved molecular recognition and information transfer properties over hundreds of millions of years of vertebrate and invertebrate evolution which equips them with cell mediatory properties in normal cellular processes and in aberrant pathological situations such as in tumourogenesis. Two KS-proteoglycans in particular, podocalyxin and lumican, are cell membrane, intracellular or stromal tissue–associated components with roles in the promotion or regulation of tumour development, mucin-like KS glycoproteins may also contribute to tumourogenesis. A greater understanding of the biology of KS may allow better methodology to be developed to more effectively combat tumourogenic processes

Immunohistochemical Localization and Expression of Fibromodulin in Adult Rat Periodontium and Inflamed Human Gingiva

OBJECTIVE: The aim of this study was to determine the distribution and expression of fibromodulin in adult rat periodontal tissues and inflamed human gingiva. MATERIALS AND METHODS: The distribution of fibromodulin in rat molar periodontium and human gingival tissue was studied by immunohistochemistry. The expression of fibromodulin mRNA from human gingival fibroblasts, periodontal ligament fibroblasts and osteoblasts was studied by reverse transcription-polymerase chain reaction (RT-PCR). For comparative purposes, the distribution and mRNA expression of collagen types I and III, as well as the two small leucine-rich proteoglycans decorin and biglycan were also studied. RESULTS: In the adult rat periodontium, fibromodulin was distributed in the suprabasal gingival epithelium, gingival and periodontal fibroblasts as well as their surrounding extracellular matrices. Strong expression was noted in the palatal gingival tissues and the interfaces of the periodontal ligament with alveolar bone and cementum. In human gingival tissues, staining of fibromodulin was detected in the connective tissue of inflamed gingiva associated with both gingivitis and periodontitis; whereas, weak staining for this molecule was noted in healthy gingival tissues. The expression of mRNA for fibromodulin was strongest in the cultured osteoblasts. Periodontal ligament fibroblasts showed only a weak level of expression for fibromodulin mRNA. CONCLUSIONS: Fibromodulin is differentially expressed throughout the periodontium being primarily associated with collagen type I in non-mineralized sites. In addition fibromodulin showed an upregulation in in-flamed gingival tissue