Prion proteins with insertion mutations have altered N-terminal conformation and increased ligand binding activity and are more susceptible to oxidative attack

Genome editing: An insight into disease resistance, production efficiency, and biomedical applications in livestock

One of the primary concerns for the survival of the human species is the growing demand for food brought on by an increasing global population. New developments in genome-editing technology present promising opportunities for the growth of wholesome and prolific farm animals. Genome editing in large animals is used for a variety of purposes, including biotechnology to improve food production, animal health, and pest management, as well as the development of animal models for fundamental research and biomedicine. Genome editing entails modifying genetic material by removing, adding, or manipulating particular DNA sequences from a particular locus in a way that does not happen naturally. The three primary genome editors are CRISPR/Cas 9, TALENs, and ZFNs. Each of these enzymes is capable of precisely severing nuclear DNA at a predetermined location. One of the most effective inventions is base editing, which enables single base conversions without the requirement for a DNA double-strand break (DSB). As reliable methods for precise genome editing in studies involving animals, cytosine and adenine base editing are now well-established. Effective zygote editing with both cytosine and adenine base editors (ABE) has resulted in the production of animal models. Both base editors produced comparable outcomes for the precise editing of point mutations in somatic cells, advancing the field of gene therapy. This review focused on the principles, methods, recent developments, outstanding applications, the advantages and disadvantages of ZFNs, TALENs, and CRISPR/Cas9 base editors, and prime editing in diverse lab and farm animals. Additionally, we address the methodologies that can be used for gene regulation, base editing, and epigenetic alterations, as well as the significance of genome editing in animal models to better reflect real disease. We also look at methods designed to increase the effectiveness and precision of gene editing tools. Genome editing in large animals is used for a variety of purposes, including biotechnology to improve food production, animal health, and pest management, as well as the development of animal models for fundamental research and biomedicine. This review is an overview of the existing knowledge of the principles, methods, recent developments, outstanding applications, the advantages and disadvantages of zinc finger nucleases (ZFNs), transcription-activator-like endonucleases (TALENs), and clustered regularly interspaced short palindromic repeats associated protein 9 (CRISPR/Cas 9), base editors and prime editing in diverse lab and farm animals, which will offer better and healthier products for the entire human race.

Prion protein conversion at two distinct cellular sites precedes fibrillisation

The self-templating nature of prions plays a central role in prion pathogenesis and is associated with infectivity and transmissibility. Since propagation of proteopathic seeds has now been acknowledged a principal pathogenic process in many types of dementia, more insight into the molecular mechanism of prion replication is vital to delineate specific and common disease pathways. By employing highly discriminatory anti-PrP antibodies and conversion-tolerant PrP chimera, we here report that de novo PrP conversion and formation of fibril-like PrP aggregates are distinct in mechanistic and kinetic terms. De novo PrP conversion occurs within minutes after infection at two subcellular locations, while fibril-like PrP aggregates are formed exclusively at the plasma membrane, hours after infection. Phenotypically distinct pools of abnormal PrP at perinuclear sites and the plasma membrane show differences in N-terminal processing, aggregation state and fibril formation and are linked by exocytic transport via synaptic and large-dense core vesicles.

Characterization of four new monoclonal antibodies against the distal N-terminal region of PrP(c)

Prion diseases are a group of fatal neurodegenerative disorders that affect humans and animals. They are characterized by the accumulation in the central nervous system of a pathological form of the host-encoded prion protein (PrP^C). The prion protein is a membrane glycoprotein that consists of two domains: a globular, structured C-terminus and an unstructured N-terminus. The N-terminal part of the protein is involved in different functions in both health and disease. In the present work we discuss the production and biochemical characterization of a panel of four monoclonal antibodies (mAbs) against the distal N-terminus of PrP^C using a well-established methodology based on the immunization of Prnp^0/0 mice. Additionally, we show their ability to block prion (PrP^Sc) replication at nanomolar concentrations in a cell culture model of prion infection. These mAbs represent a promising tool for prion diagnostics and for studying the physiological role of the N-terminal domain of PrP^C.

Methionine oxidation accelerates the aggregation and enhances the neurotoxicity of the D178N variant of the human prion protein

The D178N mutation of the prion protein (PrP) results in the hereditary prion disease fatal familial insomnia (FFI). Little is known regarding the effects of methionine oxidation on the pathogenesis of D178N-associated FFI. In the present study, we found that the D178N variant was more susceptible to oxidation than wild-type PrP, as indicated by reverse-phase high performance liquid chromatography (RP-HPLC) and mass spectrometry (MS) analysis. Circular dichroism (CD), differential scanning calorimetry (DSC), thioflavin T (ThT) binding assay studies demonstrated that methionine oxidation decreased the structural stability of the D178N variant, and the oxidized D178N variant exhibited a greater propensity to form β-sheet-rich oligomers and aggregates. Moreover, these aggregates of oxidized D178N PrP were more resistant to proteinase K (PK) digestion. Additionally, using fluorescence confocal microscopy, we detected a high degree of aggregation in D178N-transfected Neuro-2a (N2a) cells after treatment with hydrogen peroxide (H2O2). Furthermore, the oxidation and consequent aggregation of the D178N variant induced greater apoptosis of N2a cells, as monitored using flow cytometry. Collectively, these observations suggest that methionine oxidation accelerates the aggregation and enhances the neurotoxicity of the D178N variant, possibly providing direct evidence to link the pathogenesis of D178N-associated FFI with methionine oxidation.

Conformational conversion of prion protein in prion diseases

Prion diseases are a group of infectious fatal neurodegenerative diseases. The conformational conversion of a cellular prion protein (PrP(C)) into an abnormal misfolded isoform (PrP(Sc)) is the key event in prion diseases pathology. Under normal conditions, the high-energy barrier separates PrP(C) from PrP(Sc) isoform. However, pathogenic mutations, modifications as well as some cofactors, such as glycosaminoglycans, nucleic acids, and lipids, could modulate the conformational conversion process. Understanding the mechanism of conformational conversion of prion protein is essential for the biomedical research and the treatment of prion diseases. Particularly, the characterization of cofactors interacting with prion protein might provide new diagnostic and therapeutic strategies.

The N-terminal sequence of prion protein consists an epitope specific to the abnormal isoform of prion protein (PrP(Sc))

The conformation of abnormal prion protein (PrP^Sc) differs from that of cellular prion protein (PrP^C), but the precise characteristics of PrP^Sc remain to be elucidated. To clarify the properties of native PrP^Sc, we attempted to generate novel PrP^Sc-specific monoclonal antibodies (mAbs) by immunizing PrP-deficient mice with intact PrP^Sc purified from bovine spongiform encephalopathy (BSE)-affected mice. The generated mAbs 6A12 and 8D5 selectivity precipitated PrP^Sc from the brains of prion-affected mice, sheep, and cattle, but did not precipitate PrP^C from the brains of healthy animals. In histopathological analysis, mAbs 6A12 and 8D5 strongly reacted with prion-affected mouse brains but not with unaffected mouse brains without antigen retrieval. Epitope analysis revealed that mAbs 8D5 and 6A12 recognized the PrP subregions between amino acids 31–39 and 41–47, respectively. This indicates that a PrP^Sc-specific epitope exists in the N-terminal region of PrP^Sc, and mAbs 6A12 and 8D5 are powerful tools with which to detect native and intact PrP^Sc. We found that the ratio of proteinase K (PK)-sensitive PrP^Sc to PK-resistant PrP^Sc was constant throughout the disease time course.

Cellular aspects of prion replication in vitro

Prion diseases or transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders in mammals that are caused by unconventional agents predominantly composed of aggregated misfolded prion protein (PrP). Prions self-propagate by recruitment of host-encoded PrP into highly ordered β-sheet rich aggregates. Prion strains differ in their clinical, pathological and biochemical characteristics and are likely to be the consequence of distinct abnormal prion protein conformers that stably replicate their alternate states in the host cell. Understanding prion cell biology is fundamental for identifying potential drug targets for disease intervention. The development of permissive cell culture models has greatly enhanced our knowledge on entry, propagation and dissemination of TSE agents. However, despite extensive research, the precise mechanism of prion infection and potential strain effects remain enigmatic. This review summarizes our current knowledge of the cell biology and propagation of prions derived from cell culture experiments. We discuss recent findings on the trafficking of cellular and pathologic PrP, the potential sites of abnormal prion protein synthesis and potential co-factors involved in prion entry and propagation.

Glycosaminoglycans and neuroprotection

Glycosaminoglycans (GAGs) are basic building blocks of the ground substance of the extracellular matrix and present at the cellular level as an important component of the glycocalyx covering the cell membrane. In addition to the general role of GAGs in maintaining the integrity of the cell and extracellular matrix by retaining water, certain GAGs exhibit anticoagulant and neuroprotective properties and serve as cell-surface receptors for various molecules. Although heparin, a highly sulfated GAG, has been used as a drug for more than 70 years due to its anticoagulant attributes, the neuroprotective properties of GAGs came into focus only in recent years. The discovery of some of the roles GAGs play in the pathomechanism of numerous neurodegenerative disorders as well as shedding light on the neuroprotective properties of these compounds in animal studies raised the possibility that GAGs may provide an entirely new avenue in the treatment of neurodegenerative diseases. Indeed, some GAGs were successfully used to improve the cognitive function of patients with various neurodegenerative conditions (Ban et al. (1991, 1992); Conti et al. (1989a, b); Passeri and Cucinotta, (1989); Santini (1989). Although the mechanism by which the GAGs exhibit neuroprotective properties is not entirely clear, there is a general consensus that the major factors of the neuroprotective attributes of GAGs include the impact of GAGs on amyloidogenesis and the regulatory action of GAGs in the apoptotic pathway.

Instability of the octarepeat region of the human prion protein gene

Prion diseases are a family of unique fatal transmissible neurodegenerative diseases that affect humans and many animals. Sporadic Creutzfeldt-Jakob disease (sCJD) is the most common prion disease in humans, accounting for 85–90% of all human prion cases, and exhibits a high degree of diversity in phenotypes. The etiology of sCJD remains to be elucidated. The human prion protein gene has an octapeptide repeat region (octarepeats) that normally contains 5 repeats of 24–27 bp (1 nonapeptide and 4 octapeptide coding sequences). An increase of the octarepeat numbers to six or more or a decrease of the octarepeat number to three is linked to genetic prion diseases with heterogeneous phenotypes in humans. Here we report that the human octarepeat region is prone to either contraction or expansion when subjected to PCR amplification in vitro using Taq or Pwo polymerase and when replicated in wild type E. coli cells. Octarepeat insertion mutants were even less stable, and the mutation rate for the wild type octarepeats was much higher when replicated in DNA mismatch repair-deficient E.coli cells. All observed octarepeat mutants resulting from DNA replication in E.coli were contained in head-to-head plasmid dimers and DNA mfold analysis (http://mfold.rna.albany.edu/?q=mfold/DNA-Folding-Form) indicates that both DNA strands of the octarepeat region would likely form multiple stable hairpin structures, suggesting that the octarepeat sequence may form stable hairpin structures during DNA replication or repair to cause octarepeat instability. These results provide the first evidence supporting a somatic octarepeat mutation-based model for human sCJD etiology: 1) the instability of the octarepeat region leads to accumulation of somatic octarepeat mutations in brain cells during development and aging, 2) this instability is augmented by compromised DNA mismatch repair in aged cells, and 3) eventually some of the octarepeat mutation-containing brain cells start spontaneous de novo prion formation and replication to initiate sCJD.

A multistage pathway for human prion protein aggregation in vitro: from multimeric seeds to β-oligomers and nonfibrillar structures

Aberrant protein aggregation causes numerous neurological diseases including Creutzfeldt-Jakob disease (CJD), but the aggregation mechanisms remain poorly understood. Here, we report AFM results on the formation pathways of β-oligomers and nonfibrillar aggregates from wild-type full-length recombinant human prion protein (WT) and an insertion mutant (10OR) with five additional octapeptide repeats linked to familial CJD. Upon partial denaturing, seeds consisting of 3-4 monomers quickly appeared. Oligomers of ~11-22 monomers then formed through direct interaction of seeds, rather than by subsequent monomer attachment. All larger aggregates formed through association of these β-oligomers. Although both WT and 10OR exhibited identical aggregation mechanisms, the latter oligomerized faster due to lower solubility and, hence, thermodynamic stability. This novel aggregation pathway has implications for prion diseases as well as others caused by protein aggregation.

Macromolecular crowding converts the human recombinant PrPC to the soluble neurotoxic beta-oligomers

Prion diseases are fatal neurodegenerative disorders and are linked with the conversion of the cellular isoform of the prion protein (PrP(C)) into the abnormal beta-sheet-rich isoform. It is widely accepted that the soluble oligomers of beta-PrP are neurotoxic and that they are more pathologically significant. To unravel the molecular mechanism under the conversion process, it is critical to identify the factors that can promote the conversion from PrP(C) to the beta-oligomers. By recording circular dichroism spectra and performing a size-exclusion HPLC assay, we found that the conformation of the recombinant human prion protein (rPrP(C)) was converted from an alpha-helical conformation into beta-sheet oligomers under a macromolecular crowding condition. The soluble beta-oligomers of rPrP were resistant to proteinase K digestion and could bind to the dyes thioflavin T and 8-anilino-1-naphthalene sulfonate. Furthermore, by the 3-(4, 5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium assay, we showed that the soluble beta-oligomers were neurotoxic. These results suggest that macromolecular crowding, which has not been considered before, is a key intracellular factor in the formation of soluble neurotoxic beta-oligomers in prion diseases.

Structure of the flexible amino-terminal domain of prion protein bound to a sulfated glycan

The intrinsically disordered amino-proximal domain of hamster prion protein (PrP) contains four copies of a highly conserved octapeptide sequence, PHGGGWGQ, that is flanked by two polycationic residue clusters. This N-terminal domain mediates the binding of sulfated glycans, which can profoundly influence the conversion of PrP to pathological forms and the progression of prion disease. To investigate the structural consequences of sulfated glycan binding, we performed multidimensional heteronuclear ((1)H, (13)C, (15)N) NMR (nuclear magnetic resonance), circular dichroism (CD), and fluorescence studies on hamster PrP residues 23-106 (PrP 23-106) and fragments thereof when bound to pentosan polysulfate (PPS). While the majority of PrP 23-106 remain disordered upon PPS binding, the octarepeat region adopts a repeating loop-turn structure that we have determined by NMR. The beta-like turns within the repeats are corroborated by CD data demonstrating that these turns are also present, although less pronounced, without PPS. Binding to PPS exposes a hydrophobic surface composed of aligned tryptophan side chains, the spacing and orientation of which are consistent with a self-association or ligand binding site. The unique tryptophan motif was probed by intrinsic tryptophan fluorescence, which displayed enhanced fluorescence of PrP 23-106 when bound to PPS, consistent with the alignment of tryptophan side chains. Chemical-shift mapping identified binding sites on PrP 23-106 for PPS, which include the octarepeat histidine and an N-terminal basic cluster previously linked to sulfated glycan binding. These data may in part explain how sulfated glycans modulate PrP conformational conversions and oligomerizations.

New insights into cellular prion protein (PrPc) functions: the "ying and yang" of a relevant protein

The conversion of cellular prion protein (PrP(c)), a GPI-anchored protein, into a protease-K-resistant and infective form (generally termed PrP(sc)) is mainly responsible for Transmissible Spongiform Encephalopathies (TSEs), characterized by neuronal degeneration and progressive loss of basic brain functions. Although PrP(c) is expressed by a wide range of tissues throughout the body, the complete repertoire of its functions has not been fully determined. Recent studies have confirmed its participation in basic physiological processes such as cell proliferation and the regulation of cellular homeostasis. Other studies indicate that PrP(c) interacts with several molecules to activate signaling cascades with a high number of cellular effects. To determine PrP(c) functions, transgenic mouse models have been generated in the last decade. In particular, mice lacking specific domains of the PrP(c) protein have revealed the contribution of these domains to neurodegenerative processes. A dual role of PrP(c) has been shown, since most authors report protective roles for this protein while others describe pro-apoptotic functions. In this review, we summarize new findings on PrP(c) functions, especially those related to neural degeneration and cell signaling.

Dominant roles of the polybasic proline motif and copper in the PrP23-89-mediated stress protection response

Beta-cleavage of the neurodegenerative disease-associated prion protein (PrP) protects cells from death induced by oxidative insults. The beta-cleavage event produces two fragments, designated N2 and C2. We investigated the role of the N2 fragment (residues 23-89) in cellular stress response, determining mechanisms involved and regions important for this reaction. The N2 fragment differentially modulated the reactive oxygen species (ROS) response induced by serum deprivation, with amelioration when copper bound. Amino acid residues 23-50 alone mediated a ROS reduction response. PrP23-50 ROS reduction was not due to copper binding or direct antioxidant activity, but was instead mediated through proteoglycan binding partners localised in or interacting with cholesterol-rich membrane domains. Furthermore, mutational analyses of both PrP23-50 and N2 showed that their protective capacity requires the sterically constraining double proline motif within the N-terminal polybasic region. Our findings show that N2 is a biologically active fragment that is able to modulate stress-induced intracellular ROS through interaction of its structurally defined N-terminal polybasic region with cell-surface proteoglycans.

Function of PrPC (1-OPRD) in biological activities of gastric cancer cell lines

Approximately 10–15% of the human prion disease is inherited and one of the important genetic mutations occurs in the octapeptide repeat region of prion protein gene. One of the variants, one octapeptide repeat deletion (1-OPRD), existed in several gastric cancer cell lines and its mutation frequency was higher in gastric cancer cases. However, the biological functions of it remain unknown. Wild-type and mutation forms of PrP^C were cloned and transfected into gastric cancer cells. Cell apoptosis, adhesion, invasion, multidrug resistance (MDR) and proliferation were, respectively, investigated. Different expressed genes were screened by gene array and proved by PT-PCR. Further, luciferase report assay was used to explore the transcriptional activation of target genes. Forced overexpression PrP^C (1-OPRD) could promote the gastric cancer cells SGC7901 growth through facilitating G1- to S-phase transition in the cell cycle. PrP^C (1-OPRD) could also inhibit apoptosis, and promote adhesion, invasion and MDR in SGC7901. However, it exhibited no significant difference between wild-type PrP^C (1-OPRD) and PrP^C on apoptosis, invasion or MDR effects. Further experiments indicated that PrP^C (1-OPRD) could trigger the transactivation of cyclinD3 besides cyclinD1 to promote cell transition and proliferation. Overexpression of PrP^C (1-OPRD) might promote the proliferation of gastric cancer cells at least partially through transcriptional activation of cyclinD3 to accelerate the G1-/S-phase transition. The promoting proliferation effect of PrP^C (1-OPRD) was more than that of wild-type PrP^C. However, they showed no difference on apoptosis, adhesion, invasion or MDR effects of gastric cancer cells.

Can copper binding to the prion protein generate a misfolded form of the protein?

The native prion protein (PrP) has a two domain structure, with a globular folded alpha-helical C-terminal domain and a flexible extended N-terminal region. The latter can selectively bind Cu(2+) via four His residues in the octarepeat (OR) region, as well as two sites (His96 and His111) outside this region. In the disease state, the folded C-terminal domain of PrP undergoes a conformational change, forming amorphous aggregates high in beta-sheet content. Cu(2+) bound to the ORs can be redox active and has been shown to induce cleavage within the OR region, a process requiring conserved Trp residues. Using computational modeling, we have observed that electron transfer from Trp residues to copper can be favorable. These models also reveal that an indole-based radical cation or Cu(+) can initiate reactions leading to protein backbone cleavage. We have also demonstrated, by molecular dynamics simulations, that Cu(2+) binding to the His96 and His111 residues in the remaining PrP N-terminal fragment can induce localized beta-sheet structure, allowing us to suggest a potential mechanism for the initiation of beta-sheet misfolding in the C-terminal domain by Cu(2+).

A potential mechanism for Cu2+ reduction, beta-cleavage, and beta-sheet initiation within the N-terminal domain of the prion protein: insights from density functional theory and molecular dynamics calculations

The N-terminal region of the native human prion protein encompasses four highly conserved octarepeats that each contain a single His, Pro, Gln, and Trp residue as well as several Gly residues. At neutral pH these repeats are capable of individually binding copper (Cu(2+)) ions, involving the His side chain and the backbone amide of the Gly residues. In addition, the two His residues at positions 96 and 111 are also capable of binding Cu(2+). At low concentrations of the metal ion or at low pH, one Cu(2+) may be bound by multiple His residues of the four octarepeats. This complex is known to be redox active, while none of the other Cu(2+)-bound complexes are. Using density functional theory and molecular dynamics calculations data demonstrated how this form of the protein could reduce Cu(2+), through a process involving electron transfer from the Trp side chain. The reduced Cu gives rise to reactive oxygen species (ROS), which can lead to beta-cleavage of the prion protein chain at any of the Gly residues around position 90. Protein fragments of lengths similar to those arising from beta-cleavage are predominantly found in both healthy and Creutzfeldt-Jakob disease (CJD)-affected brains. Models of Cu binding to the His96 and His111 residues also indicate that different modes of Cu(2+) binding result in formation of stable beta-hairpin structures in this region of the protein. It is postulated that through interactions with the C-terminal part of the protein these hairpins may initiate misfolding and yield more stable beta-sheet structures that might associate in the same fashion with additional prion proteins.

Ligand binding promotes prion protein aggregation--role of the octapeptide repeats

Aggregation of the normal cellular prion protein, PrP, is important in the pathogenesis of prion disease. PrP binds glycosaminoglycan (GAG) and divalent cations, such as Cu(2+) and Zn(2+). Here, we report our findings that GAG and Cu(2+) promote the aggregation of recombinant human PrP (rPrP). The normal cellular prion protein has five octapeptide repeats. In the presence of either GAG or Cu(2+), mutant rPrPs with eight or ten octapeptide repeats are more aggregation prone, exhibit faster kinetics and form larger aggregates than wild-type PrP. When the GAG-binding motif, KKRPK, is deleted the effect of GAG but not that of Cu(2+) is abolished. By contrast, when the Cu(2+)-binding motif, the octapeptide-repeat region, is deleted, neither GAG nor Cu(2+) is able to promote aggregation. Therefore, the octapeptide-repeat region is critical in the aggregation of rPrP, irrespective of the promoting ligand. Furthermore, aggregation of rPrP in the presence of GAG is blocked with anti-PrP mAbs, whereas none of the tested anti-PrP mAbs block Cu(2+)-promoted aggregation. However, a mAb that is specific for an epitope at the N-terminus enhances aggregation in the presence of either GAG or Cu(2+). Therefore, although binding of either GAG or Cu(2+) promotes the aggregation of rPrP, their aggregation processes are different, suggesting multiple pathways of rPrP aggregation.

PrP mutants with different numbers of octarepeat sequences are more susceptible to the oxidative stress

One of the physiological functions of cellular prion protein (PrP(C)) is believed to work as a cellular resistance to oxidative stress, in which the octarepeats region within PrP plays an important role. However, the detailed mechanism is less clear. In this study, the expressing plasmids of wild-type PrP (PrP-PG5) and various PrP mutants containing 0 (PrP-PG0), 9 (PrP-PG9) and 12 (PrP-PG12) octarepeats were generated and PrP proteins were expressed both in E. coli and in mammalian cells. Protein aggregation and formation of carbonyl groups were clearly seen in the recombinant PrPs expressed from E. coli after treatment of H(2)O(2). MTT and trypan blue staining assays revealed that the cells expressing the mutated PrPs within octarepeats are less viable than the cells expressing wild-type PrP. Statistically significant high levels of intracellular free radicals and low levels of glutathione peroxidase were observed in the cells transfected with plasmids containing deleted or inserted octarepeats. Remarkably more productions of carbonyl groups were detected in the cells expressing PrPs with deleted and inserted octarepeats after exposing to H(2)O(2). Furthermore, cells expressing wild-type PrP showed stronger resistant activity to the challenge of H(2)O(2) at certain extent than the mutated PrPs and mock. These data provided the evidences that the octarepeats number within PrP is critical for maintaining its activity of antioxidation. Loss of its protective function against oxidative stress may be one of the possible pathways for the mutated PrPs to involve in the pathogenesis of familial Creutzfeldt-Jacob diseases.

Binding of recombinant but not endogenous prion protein to DNA causes DNA internalization and expression in mammalian cells

Recombinant prion protein, rPrP, binds DNA. Both the KKRPK motif and the octapeptide repeat region of rPrP are essential for maximal binding. rPrP with pathogenic insertional mutations binds more DNA than wild-type rPrP. DNA promotes the aggregation of rPrP and protects its N terminus from proteinase K digestion. When rPrP is mixed with an expression plasmid and Ca(2+), the rPrP.DNA complex is taken up by mammalian cells leading to gene expression. In the presence of Ca(2+), rPrP by itself is also taken up by cells in a temperature- and pinocytosis-dependent manner. Cells do not take up rPrP(DeltaKKRPK), which lacks the KKRPK motif. Thus, rPrP is the carrier for DNA and the KKRPK motif is essential for its uptake. When mixed with DNA, a pentapeptide KKRPK, but not KKKKK, is sufficient for DNA internalization and expression. In contrast, whereas the normal cellular prion protein, PrP(C), on the cell surface can also internalize DNA, the imported DNA is not expressed. These findings may have relevance to the normal functions of PrP(C) and the pathogenic mechanisms of human prion disease.

Canadian Association of Neurosciences Review: prion protein and prion diseases: the good and the bad

In the 1700's a strange new disease affecting sheep was recognized in Europe. The disease later became known as "Scrapie" and was the first of a family of similar diseases affecting a number of species that are now known as the Transmissible Spongiform Encephalopathies (TSEs). The appearance of a new disease in humans linked to the consumption of meat products from infected cattle has stimulated widespread public concern and scientific interest in the prion protein and related diseases. Nearly 300 years after the first report, these diseases still merit the descriptor "strange". This family of diseases is characterized by a unique profile of histological changes, can be transmitted as inherited or acquired diseases, as well as apparent sporadic spontaneous generation of the disease. These diseases are believed by many, to be caused by a unique protein only infectious agent. The "prion protein" (PrPC), a term first coined by Stanley Prusiner in 1982 is crucial to the development of these diseases, apparently by acting as a substrate for an abnormal disease associated form. However, aside from being critical to the pathogenesis of the disease, the function of PrPC, which is expressed in all mammals, has defied definitive description. Several roles have been proposed on the basis of in vitro studies, however, thus far, in vivo confirmation has not been forthcoming. The biological features of PrPC also seem to be unusual. Numerous mouse models have been generated in an attempt to understand the pathogenesis of these diseases. This review summarizes the current state of histological features, the etiologic agent, the normal metabolism and the function of the prion protein, as well as the limitations of the mouse models.

Prion protein repeat expansion results in increased aggregation and reveals phenotypic variability

Mammalian prion diseases are fatal neurodegenerative disorders dependent on the prion protein PrP. Expansion of the oligopeptide repeats (ORE) found in PrP is associated with inherited prion diseases. Patients with ORE frequently harbor PrP aggregates, but other factors may contribute to pathology, as they often present with unexplained phenotypic variability. We created chimeric yeast-mammalian prion proteins to examine the influence of the PrP ORE on prion properties in yeast. Remarkably, all chimeric proteins maintained prion characteristics. The largest repeat expansion chimera displayed a higher propensity to maintain a self-propagating aggregated state. Strikingly, the repeat expansion conferred increased conformational flexibility, as observed by enhanced phenotypic variation. Furthermore, the repeat expansion chimera displayed an increased rate of prion conversion, but only in the presence of another aggregate, the [RNQ+] prion. We suggest that the PrP ORE increases the conformational flexibility of the prion protein, thereby enhancing the formation of multiple distinct aggregate structures and allowing more frequent prion conversion. Both of these characteristics may contribute to the phenotypic variability associated with PrP repeat expansion diseases.

Prion protein with an octapeptide insertion has impaired neuroprotective activity in transgenic mice

Familial prion diseases are due to dominantly inherited, germline mutations in the PRNP gene that encodes the prion protein (PrP). The cellular mechanism underlying the pathogenic effect of these mutations remains uncertain. To investigate whether pathogenic mutations impair a normal, physiological activity of PrP, we have crossed Tg(PG14) mice, which express PrP with an octapeptide insertion associated with an inherited prion dementia, with Tg(PrPDelta32-134) mice. Tg(PrPDelta32-134) mice, which express an N-terminally truncated form of PrP, spontaneously develop a neurodegenerative phenotype that is stoichiometrically reversed by coexpression of wild-type PrP. We find that, at equivalent expression levels, PG14 PrP is significantly less efficient than wild-type PrP in suppressing the development of clinical symptoms and neuropathology in Tg(PrPDelta32-134) mice. Thus, our results suggest that some features of the neurological illness associated with inherited PrP mutations may be attributable to a loss of PrP neuroprotective function. This mechanism stands in contrast to the toxic gain-of-function mechanisms that are usually invoked to explain the pathogenesis of dominantly inherited neurodegenerative disorders.

Bcl‐2 overexpression delays caspase‐3 activation and rescues cerebellar degeneration in prion‐deficient mice that overexpress amino‐terminally truncated prion

Prnp knockout mice that overexpress an amino-truncated form of PrPc (deltaPrP) are ataxic and display cerebellar cell loss and premature death. Studies on the molecular and intracellular events that trigger cell death in these mutants may contribute to elucidate the functions of PrPc and to the design of treatments for prion disease. Here we examined the effects of Bcl-2 overexpression in neurons on the development of the neurological syndrome and cerebellar pathology of deltaPrP. We show that deltaPrP overexpression activates the stress-associated kinases ERK1-2 in reactive astroglia, p38 and the phosphorylation of p53, which leads to the death of cerebellar neurons in mutant mice. We found that the expression of deltaPrP in cell lines expressing very low levels of PrPc strongly induces the activation of apoptotic pathways, thereby leading to caspase-3 activation and cell death, which can be prevented by coexpressing Bcl-2. Finally, we corroborate in vivo that neuronal-directed Bcl-2 overexpression in deltaPrP mice (deltaPrP Bcl-2) markedly reduces caspase-3 activation, glial activation, and neuronal cell death in cerebellum by improving locomotor deficits and life expectancy.

Human prion proteins with pathogenic mutations share common conformational changes resulting in enhanced binding to glycosaminoglycans

Mutation in the prion gene PRNP accounts for 10-15% of human prion diseases. However, little is known about the mechanisms by which mutant prion proteins (PrPs) cause disease. Here we investigated the effects of 10 different pathogenic mutations on the conformation and ligand-binding activity of recombinant human PrP (rPrP). We found that mutant rPrPs react more strongly with N terminus-specific antibodies, indicative of a more exposed N terminus. The N terminus of PrP contains a glycosaminoglycan (GAG)-binding motif. Binding of GAG is important in prion disease. Accordingly, all mutant rPrPs bind more GAG, and GAG promotes the aggregation of mutant rPrPs more efficiently than wild-type recombinant normal cellular PrP (rPrP(C)). Furthermore, point mutations in PRNP also cause conformational changes in the region between residues 109 and 136, resulting in the exposure of a second, normally buried, GAG-binding motif. Importantly, brain-derived PrP from transgenic mice, which express a pathogenic mutant with nine extra octapeptide repeats, also binds more strongly to GAG than wild-type PrP(C). Thus, several rPrPs with distinct pathogenic mutations have common conformational changes, which enhance binding to GAG. These changes may contribute to the pathogenesis of inherited prion diseases.

Aggregation of prion protein with insertion mutations is proportional to the number of inserts

Mutation in the prion gene, PRNP, accounts for approx. 10-15% of human prion diseases. However, little is known about the mechanisms by which a mutant prion protein (PrP) causes disease. We compared the biochemical properties of a wild-type human prion protein, rPrP(C) (recombinant wild-type PrP), which has five octapeptide-repeats, with two recombinant human prion proteins with insertion mutations, one with three more octapeptide repeats, rPrP(8OR), and the other with five more octapeptide repeats, rPrP(10OR). We found that the insertion mutant proteins are more prone to aggregate, and the degree and kinetics of aggregation are proportional to the number of inserts. The octapeptide-repeat and alpha-helix 1 regions are important in aggregate formation, because aggregation is inhibited with monoclonal antibodies that are specific for epitopes in these regions. We also showed that a small amount of mutant protein could enhance the formation of mixed aggregates that are composed of mutant protein and wild-type rPrP(C). Accordingly, rPrP(10OR) is also more efficient in promoting the aggregation of rPrP(C) than rPrP(8OR). These findings provide a biochemical explanation for the clinical observations that the severity of the disease in patients with insertion mutations is proportional to the number of inserts, and thus have implications for the pathogenesis of inherited human prion disease.

Cellular prion protein promotes proliferation and G1/S transition of human gastric cancer cells SGC7901 and AGS

The function of cellular prion protein (PrP(C)), the essential protein for the pathogenesis and transmission of prion diseases, is still largely unknown. The putative roles of PrP(C) are thought to be related to cell signaling, survival, and differentiation. In a previous study, we showed that PrP(C) was overexpressed in gastric cancer tissues. In the present report, we show that ectopic expression of PrP(C) could promote tumorigenesis, proliferation, and G1/S transition in gastric cancer cells. Furthermore, CyclinD1, a protein related to cell cycle, was shown to be significantly up-regulated by PrP(C) at both mRNA and protein levels. PI3K/Akt pathway mediated above PrP(C) signal since PrP(C) increased the expression of phosphorylated Akt, and the specific inhibitor of Akt, LY294002, could markedly suppress growth of SGC7901 and transactivation of CyclinD1 induced by PrP(C). Octapeptide repeat region played a vital role in this function, as deletion of this region abolished or reduced these effects. Collectively, this study demonstrates that overexpression of PrP(C) might promote the tumorigenesis and proliferation of gastric cancer cells at least partially through activation of PI3K/Akt pathway and subsequent transcriptional activation of CyclinD1 to regulate the G1/S phase transition, in which octapeptide repeat region might be an indispensable region.

Conformation-sensitive immunoassays improve the serodiagnosis of solitary cysticercus granuloma in Indian patients

Neurocysticercosis (NCC), infection of the central nervous system with larva of Taenia solium, presents in over 60% of patients in India as a solitary cysticercus granuloma (SCG). The low cyst number in these patients frequently results in an insignificant humoral response. Consequently, serological tests for patients with SCG must consider the detection of low antibody levels. Lentil lectin-specific T. solium glycoproteins of molecular weights 50, 38, 24, 18, 14 and 13 kDa are specific antigens for cysticercus antibodies in serological tests for NCC, however they do not detect antibodies in 40% of patients with SCG. To improve this rate of detection, the conformations of these protein antigens were altered to unmask additional epitopes available for antibody binding. Secondary structures of the proteins induced by reduction of disulfide bonds led to the loss of conformational epitopes necessary for cysticercus antibody recognition. Urea-induced tertiary conformations of the antigenic proteins led to the detection of antibodies in 46% of 60 patients with SCG who were serologically negative on immunoblots when the antigens were used in quaternary conformation. Conformation-sensitive immunoassays show potential for serodiagnosis of patients with SCG.

Prions and their partners in crime

Prions, the infectious agents of transmissible spongiform encephalopathies (TSEs), have defied full characterization for decades. The dogma has been that prions lack nucleic acids and are composed of a pathological, self-inducing form of the host's prion protein (PrP). Recent progress in propagating TSE infectivity in cell-free systems has effectively ruled out the involvement of foreign nucleic acids. However, host-derived nucleic acids or other non-PrP molecules seem to be crucial. Interactions between TSE-associated PrP and its normal counterpart are also pathologically important, so the physiological functions of normal PrP and how they might be corrupted by TSE infections have been the subject of recent research.

Cell expression of a four extra octarepeat mutated PrPCmodifies cell structure and cell cycle regulation

RK13 cell lines generated to express bovine PrP(C) with a four extra octarepeat insertional mutation (Bo-10ORPrP(C)) show partially insoluble PrP(C) and lower rates of cell growth when compared to either the same cells expressing wild type Bo-6ORPrP(C) or the original RK13 cell line. The expression of Bo-10ORPrP(C) in cell cultures was also associated with changes in cell size and reorganization of the actin cytoskeleton. This last process was reversed by Clostridium difficile toxin-B, a specific inhibitor of small GTPase proteins. Further, in clones expressing Bo-10ORPrP(C), increased proportions of cells at cell cycle stage G2/M were observed. Proteasome inhibitors caused a further expansion of G2/M-stage cells that was more marked in cell lines expressing Bo-10ORPrP(C) than those expressing Bo-6ORPrP(C), while this effect was minimal or null in the original RK13 cell line. Hence, the presence of Bo-10ORPrP(C) in RK13 cells promotes cell cycle arrest at G2/M, and the effect is amplified by proteasome inhibition. These findings suggest a role for PrP(C) in cell morphology and cell cycle regulation, and open new avenues for understanding the mechanisms underlying PrP mutation-associated diseases.