Visualization of prion infection in transgenic mice expressing green fluorescent protein-tagged prion protein

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.

Time-tagged ticker tapes for intracellular recordings

Recording transcriptional histories of a cell would enable deeper understanding of cellular developmental trajectories and responses to external perturbations. Here we describe an engineered protein fiber that incorporates diverse fluorescent marks during its growth to store a ticker tape-like history. An embedded HaloTag reporter incorporates user-supplied dyes, leading to colored stripes that map the growth of each individual fiber to wall clock time. A co-expressed eGFP tag driven by a promoter of interest records a history of transcriptional activation. High-resolution multi-spectral imaging on fixed samples reads the cellular histories, and interpolation of eGFP marks relative to HaloTag timestamps provides accurate absolute timing. We demonstrate recordings of doxycycline-induced transcription in HEK cells and cFos promoter activation in cultured neurons, with a single-cell absolute accuracy of 30-40 minutes over a 12-hour recording. The protein-based ticker tape design we present here could be generalized to achieve massively parallel single-cell recordings of diverse physiological modalities.

What is the role of lipids in prion conversion and disease?

The molecular cause of transmissible spongiform encephalopathies (TSEs) involves the conversion of the cellular prion protein (PrP^C) into its pathogenic form, called prion scrapie (PrP^Sc), which is prone to the formation of amorphous and amyloid aggregates found in TSE patients. Although the mechanisms of conversion of PrP^C into PrP^Sc are not entirely understood, two key points are currently accepted: (i) PrP^Sc acts as a seed for the recruitment of native PrP^C, inducing the latter's conversion to PrP^Sc; and (ii) other biomolecules, such as DNA, RNA, or lipids, can act as cofactors, mediating the conversion from PrP^C to PrP^Sc. Interestingly, PrP^C is anchored by a glycosylphosphatidylinositol molecule in the outer cell membrane. Therefore, interactions with lipid membranes or alterations in the membranes themselves have been widely investigated as possible factors for conversion. Alone or in combination with RNA molecules, lipids can induce the formation of PrP in vitro-produced aggregates capable of infecting animal models. Here, we discuss the role of lipids in prion conversion and infectivity, highlighting the structural and cytotoxic aspects of lipid-prion interactions. Strikingly, disorders like Alzheimer's and Parkinson's disease also seem to be caused by changes in protein structure and share pathogenic mechanisms with TSEs. Thus, we posit that comprehending the process of PrP conversion is relevant to understanding critical events involved in a variety of neurodegenerative disorders and will contribute to developing future therapeutic strategies for these devastating conditions.

Generation and characterization of two strains of transgene mice expressing chimeric MiniSOG-MusPrP

BACKGROUND

Although the presences of scrapie associated fibril in the brain tissues is a ultrastructural hallmark for prion diseases, the exact morphological structure of prion during the progression of the disease is still unclear. The host prion protein (PrP) is encoded by PrP gene (PRNP) locating on the chromosome 20 in human and the chromosome 2 in mouse. Recently, a novel correlative light and electron microscopy with Mini Singlet Oxygen Generator (miniSOG) was generated. MiniSOG, a small protein of 106 amino acids, can absorb blue light and emit green fluorescence that is detectable under the fluorescence microscope. MiniSOG can also partially catalyze the polymerization of DAB to form black stained structures in the presence of osmium tetroxide, which is able to be observed under transmission electron microscope.

NEW METHODS

Two kinds of miniSOG-PrP expressing recombinant plasmids were generated. Correlative photooxidation and transmission electron microscope were used to detect these plasmids. The plasmids were microinjected into fertilized FVB/NJ eggs and Tg mice expressing miniSOG-PrP fusion proteins were selected after successive bred withPRNP KO Tg mice.

RESULTS

Those two strains of Tg mice, Tg^SOG23 and Tg^231SOG, developed normally and maintained healthy without detectable abnormality after one-year observation. Western blots and immunohistochemical assays with PrP- and miniSOG-specific antibodies confirmed that the chimeric miniSOG-PrP proteins were expressed in the brain tissues of Tg mice. Digital PCR assays proposed that the copy numbers of the inserted external gene in Tg^SOG23 and Tg^231SOG were 2 and 12, respectively.

COMPARISON WITH EXISTING METHOD(S)

Compared with GFP tag miniSOG is significantly smaller, which makes it easy be operated experimentally and possibly has less influence on the biological function of the labeled protein. Additionally, GFP tag is an ideal marker for immunofluorescent assays, but may not be suitable for ultrastructural assays for prion morphology.

CONCLUSION

Those Tg mice may supply novel and useful experimental animals for further study on the potential morphological structure formation and deposits of prion in the brain tissues during prion infection.

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.

Proteasomal Inhibition Redirects the PrP-Like Shadoo Protein to the Nucleus

The Shadoo protein (Sho) exhibits homology to the hydrophobic region of the cellular isoform of prion protein (PrP^C). As prion-infected brains gradually accumulate infectivity-associated isoforms of prion protein (PrP^Sc), levels of mature endogenous Sho become reduced. To study the regulatory effect of the proteostatic network on Sho expression, we investigated the action of lactacystin, MG132, NH₄Cl, and 3-methyladenine (3-MA) in two cell culture models. In primary mixed neuronal and glial cell cultures (MNGCs) from transgenic mice expressing wild-type Sho from the PrP gene promoter (Tg.Sprn mice), lactacystin- and MG132-mediated inhibition of proteasomal activity shifted the repertoire of Sho species towards unglycosylated forms appearing in the nuclei; conversely, the autophagic modulators NH₄Cl and 3-MA did not affect Sho or PrP^C glycosylation patterns. Mouse N2a neuroblastoma cells expressing Sho under control of a housekeeping gene promoter treated with MG132 or lactacystin also showed increased nuclear localization of unglycosylated Sho. As two proteasomal inhibitors tested in two cell paradigms caused redirection of Sho to nuclei at the expense of processing through the secretory pathway, our findings define a balanced shift in subcellular localization that thereby differs from the decreases in net Sho species seen in prion-infected brains. Our data are indicative of a physiological pathway to access Sho functions in the nucleus under conditions of impaired proteasomal activity. We also infer that these conditions would comprise a context wherein Sho’s N-terminal nucleic acid–binding RGG repeat region is brought into play.

Replication of multiple system atrophy prions in primary astrocyte cultures from transgenic mice expressing human α-synuclein

Glial cytoplasmic inclusions (GCIs) containing aggregated and hyperphosphorylated α-synuclein are the signature neuropathological hallmark of multiple system atrophy (MSA). Native α-synuclein can adopt a prion conformation that self-propagates and spreads throughout the brain ultimately resulting in neurodegeneration. A growing body of evidence argues that, in addition to oligodendrocytes, astrocytes contain α-synuclein inclusions in MSA and other α-synucleinopathies at advanced stages of disease. To study the role of astrocytes in MSA, we added MSA brain homogenate to primary cultures of astrocytes from transgenic (Tg) mouse lines expressing human α-synuclein. Astrocytes from four Tg lines, expressing either wild-type or mutant (A53T or A30P) human α-synuclein, propagated and accumulated α-synuclein prions. Furthermore, we found that MSA-infected astrocytes formed two morphologically distinct α-synuclein inclusions: filamentous and granular. Both types of cytoplasmic inclusions shared several features characteristic of α-synuclein inclusions in synucleinopathies: hyperphosphorylation preceded by aggregation, ubiquitination, thioflavin S–positivity, and co-localization with p62. Our findings demonstrate that human α-synuclein forms distinct inclusion morphologies and propagates within cultured Tg astrocytes exposed to MSA prions, indicating that α-synuclein expression determines the tropism of inclusion formation in certain cells. Thus, our work may prove useful in elucidating the role of astrocytes in the pathogenic mechanisms that feature in neurodegeneration caused by MSA prions.

Flow Cytometric Detection of PrPSc in Neurons and Glial Cells from Prion-Infected Mouse Brains

In prion diseases, an abnormal isoform of prion protein (PrP^Sc) accumulates in neurons, astrocytes, and microglia in the brains of animals affected by prions. Detailed analyses of PrP^Sc-positive neurons and glial cells are required to clarify their pathophysiological roles in the disease. Here, we report a novel method for the detection of PrP^Sc in neurons and glial cells from the brains of prion-infected mice by flow cytometry using PrP^Sc-specific staining with monoclonal antibody (MAb) 132. The combination of PrP^Sc staining and immunolabeling of neural cell markers clearly distinguished neurons, astrocytes, and microglia that were positive for PrP^Sc from those that were PrP^Sc negative. The flow cytometric analysis of PrP^Sc revealed the appearance of PrP^Sc-positive neurons, astrocytes, and microglia at 60 days after intracerebral prion inoculation, suggesting the presence of PrP^Sc in the glial cells, as well as in neurons, from an early stage of infection. Moreover, the kinetic analysis of PrP^Sc revealed a continuous increase in the proportion of PrP^Sc-positive cells for all cell types with disease progression. Finally, we applied this method to isolate neurons, astrocytes, and microglia positive for PrP^Sc from a prion-infected mouse brain by florescence-activated cell sorting. The method described here enables comprehensive analyses specific to PrP^Sc-positive neurons, astrocytes, and microglia that will contribute to the understanding of the pathophysiological roles of neurons and glial cells in PrP^Sc-associated pathogenesis.IMPORTANCE Although formation of PrP^Sc in neurons is associated closely with neurodegeneration in prion diseases, the mechanism of neurodegeneration is not understood completely. On the other hand, recent studies proposed the important roles of glial cells in PrP^Sc-associated pathogenesis, such as the intracerebral spread of PrP^Sc and clearance of PrP^Sc from the brain. Despite the great need for detailed analyses of PrP^Sc-positive neurons and glial cells, methods available for cell type-specific analysis of PrP^Sc have been limited thus far to microscopic observations. Here, we have established a novel high-throughput method for flow cytometric detection of PrP^Sc in cells with more accurate quantitative performance. By applying this method, we succeeded in isolating PrP^Sc-positive cells from the prion-infected mouse brains via fluorescence-activated cell sorting. This allows us to perform further detailed analysis specific to PrP^Sc-positive neurons and glial cells for the clarification of pathological changes in neurons and pathophysiological roles of glial cells.

Misfolding leads the way to unraveling signaling pathways in the pathophysiology of prion diseases

A misfolded version of the prion protein represents an essential component in the pathophysiology of fatal neurodegenerative prion diseases, which affect humans and animals alike. They may be of sporadic origin, acquired through exogenous introduction of infectious misfolded prion protein, or caused by genetic alterations in the prion protein coding gene. We have recently described a novel pathway linking retention of mutant prion protein in the early secretory pathway to activation p38-MAPK and a neurodegenerative phenotype in transgenic mice. Here we review the consequences that mutations in prion protein have on intracellular transport and stress responses focusing on protein quality control. We also discuss the neurotoxic signaling elicited by the accumulation of mutant prion protein in the endoplasmic reticulum and the Golgi apparatus. Improved knowledge about these processes will help us to better understand complex pathogenesis of prion diseases, a prerequisite for therapeutic strategies.

Insights into Mechanisms of Transmission and Pathogenesis from Transgenic Mouse Models of Prion Diseases

Prions represent a new paradigm of protein-mediated information transfer. In the case of mammals, prions are the cause of fatal, transmissible neurodegenerative diseases, sometimes referred to as transmissible spongiform encephalopathies (TSEs), which frequently occur as epidemics. An increasing body of evidence indicates that the canonical mechanism of conformational corruption of cellular prion protein (PrP^C) by the pathogenic isoform (PrP^Sc) that is the basis of prion formation in TSEs is common to a spectrum of proteins associated with various additional human neurodegenerative disorders, including the more common Alzheimer's and Parkinson's diseases. The peerless infectious properties of TSE prions, and the unparalleled tools for their study, therefore enable elucidation of mechanisms of template-mediated conformational propagation that are generally applicable to these related disease states. Many unresolved issues remain including the exact molecular nature of the prion, the detailed cellular and molecular mechanisms of prion propagation, and the means by which prion diseases can be both genetic and infectious. In addition, we know little about the mechanism by which neurons degenerate during prion diseases. Tied to this, the physiological role of the normal form of the prion protein remains unclear and it is uncertain whether or not loss of this function contributes to prion pathogenesis. The factors governing the transmission of prions between species remain unclear, in particular the means by which prion strains and PrP primary structure interact to affect interspecies prion transmission. Despite all these unknowns, advances in our understanding of prions have occurred because of their transmissibility to experimental animals, and the development of transgenic (Tg) mouse models has done much to further our understanding about various aspects of prion biology. In this review, we will focus on advances in our understanding of prion biology that occurred in the past 8 years since our last review of this topic.

Propagation of prions causing synucleinopathies in cultured cells

Increasingly, evidence argues that many neurodegenerative diseases, including progressive supranuclear palsy (PSP), are caused by prions, which are alternatively folded proteins undergoing self-propagation. In earlier studies, PSP prions were detected by infecting human embryonic kidney (HEK) cells expressing a tau fragment [TauRD(LM)] fused to yellow fluorescent protein (YFP). Here, we report on an improved bioassay using selective precipitation of tau prions from human PSP brain homogenates before infection of the HEK cells. Tau prions were measured by counting the number of cells with TauRD(LM)-YFP aggregates using confocal fluorescence microscopy. In parallel studies, we fused α-synuclein to YFP to bioassay α-synuclein prions in the brains of patients who died of multiple system atrophy (MSA). Previously, MSA prion detection required ∼120 d for transmission into transgenic mice, whereas our cultured cell assay needed only 4 d. Variation in MSA prion levels in four different brain regions from three patients provided evidence for three different MSA prion strains. Attempts to demonstrate α-synuclein prions in brain homogenates from Parkinson's disease patients were unsuccessful, identifying an important biological difference between the two synucleinopathies. Partial purification of tau and α-synuclein prions facilitated measuring the levels of these protein pathogens in human brains. Our studies should facilitate investigations of the pathogenesis of both tau and α-synuclein prion disorders as well as help decipher the basic biology of those prions that attack the CNS.

Targeting prion propagation using peptide constructs with signal sequence motifs

Synthetic peptides with sequences derived from the cellular prion protein (PrP(C)) unprocessed N-terminus are able to counteract the propagation of proteinase K resistant prions (PrP(Res), indicating the presence of the prion isoform of the prion protein) in cell cultures (Löfgren et al., 2008). The anti-prion peptides have characteristics like cell penetrating peptides (CPPs) and consist of the prion protein hydrophobic signal sequence followed by a polycationic motif (residues KKRPKP), in mouse PrP(C) corresponding to residues 1-28. Here we analyze the sequence elements required for the anti-prion effect of KKRPKP-conjugates. Neuronal GT1-1 cells were infected with either prion strain RML or 22L. Variable peptide constructs originating from the mPrP1-28 sequence were analyzed for anti-prion effects, measured as disappearance of proteinase K resistant prions (PrP(Res)) in the infected cell cultures. We find that even a 5 amino acid N-terminal shortening of the signal peptide abolishes the anti-prion effect. We show that the signal peptide from PrP(C) can be replaced with the signal peptide from the Neural cell adhesion molecule-1; NCAM11-19, with a retained capacity to reduce PrP(Res) levels. The anti-prion effect is lost if the polycationic N-terminal PrP(C)-motif is conjugated to any conventional CPP, such as TAT48-60, transportan-10 or penetratin. We propose a mechanism by which a signal peptide from a secretory or cell surface protein acts to promote the transport of a prion-binding polycationic PrP(C)-motif to a subcellular location where prion conversion occurs (most likely the Endosome Recycling Compartment), thereby targeting prion propagation.

Translation of the prion protein mRNA is robust in astrocytes but does not amplify during reactive astrocytosis in the mouse brain

Prion diseases induce neurodegeneration in specific brain areas for undetermined reasons. A thorough understanding of the localization of the disease-causing molecule, the prion protein (PrP), could inform on this issue but previous studies have generated conflicting conclusions. One of the more intriguing disagreements is whether PrP is synthesized by astrocytes. We developed a knock-in reporter mouse line in which the coding sequence of the PrP expressing gene (Prnp), was replaced with that for green fluorescent protein (GFP). Native GFP fluorescence intensity varied between and within brain regions. GFP was present in astrocytes but did not increase during reactive gliosis induced by scrapie prion infection. Therefore, reactive gliosis associated with prion diseases does not cause an acceleration of local PrP production. In addition to aiding in Prnp gene activity studies, this reporter mouse line will likely prove useful for analysis of chimeric animals produced by stem cell and tissue transplantation experiments.

Disturbed vesicular trafficking of membrane proteins in prion disease

The pathogenic mechanism of prion diseases remains unknown. We recently reported that prion infection disturbs post-Golgi trafficking of certain types of membrane proteins to the cell surface, resulting in reduced surface expression of membrane proteins and abrogating the signal from the proteins. The surface expression of the membrane proteins was reduced in the brains of mice inoculated with prions, well before abnormal symptoms became evident. Prions or pathogenic prion proteins were mainly detected in endosomal compartments, being particularly abundant in recycling endosomes. Some newly synthesized membrane proteins are delivered to the surface from the Golgi apparatus through recycling endosomes, and some endocytosed membrane proteins are delivered back to the surface through recycling endosomes. These results suggest that prions might cause neuronal dysfunctions and cell loss by disturbing post-Golgi trafficking of membrane proteins via accumulation in recycling endosomes. Interestingly, it was recently shown that delivery of a calcium channel protein to the cell surface was impaired and its function was abrogated in a mouse model of hereditary prion disease. Taken together, these results suggest that impaired delivery of membrane proteins to the cell surface is a common pathogenic event in acquired and hereditary prion diseases.

Alternative fates of newly formed PrPSc upon prion conversion on the plasma membrane

Prion diseases are fatal neurodegenerative diseases characterised by the accumulation of misfolded prion protein (PrP(Sc)) in the brain. They are caused by the templated misfolding of normal cellular protein, PrP(C), by PrP(Sc). We have recently generated a unique cell system in which epitope-tagged PrP(C) competent to produce bona fide PrP(Sc) is expressed in neuroblastoma cells. Using this system we demonstrated that PrP(Sc) forms on the cell surface within minutes of prion exposure. Here, we describe the intracellular trafficking of newly formed PrP(Sc). After formation in GM1-enriched lipid microdomains at the plasma membrane, PrP(Sc) is rapidly internalised to early endosomes containing transferrin and cholera toxin B subunit. Following endocytosis, PrP(Sc) intracellular trafficking diverges: some is recycled to the plasma membrane via Rab11-labelled recycling endosomes; the remaining PrP(Sc) is subject to retromer-mediated retrograde transport to the Golgi. This pathway leads to lysosomal degradation, and we show that this is the dominant PrP(Sc) degradative mechanism in the early stages of prion infection.

p∆TubHA4C, a new versatile vector for constitutive expression in Drosophila

Several vectors for gene expression are available in Drosophila, a hub for genetics and genomics innovation. However, the vectors for ubiquitous expression have a complex structure, including coding exons, that makes in-frame cloning of cDNAs very complicated. In this report we describe a new Drosophila expression vector (p∆TubHA4C) for ubiquitous expression of coding sequences under the control of a minimal 0.9 kb promoter of α1 tubulin (α1t). This plasmid was designed to include optimized multiple cloning sites (polylinker) to provide flexibility in cloning strategies. We also added the option of double labeling the expressed proteins with two C-terminal tags, the viral epitope hemagglutinin and a synthetic tetracysteine (4C) tag that binds small fluorescent compounds. This dual tag allows both in situ and biochemical detection of the desired protein. In particular, the new 4C tag technology combines easy fluorescent labeling with small arsenical compounds in live or fixed cells and tissues, while producing minimal alterations to the tagged protein due to its small size. To demonstrate the potent and ubiquitous expression under the control of the ∆Tub promoter, bacterial lacZ was expressed and monitored in cell culture and transgenic flies. We found that the modified 0.9 kb ΔTub promoter induced similar expression levels to the intact 2.6 kb α1t promoter, supporting the inclusion of all critical regulatory elements in the new and flexible ∆TubHA4C vector.

Spongiform encephalopathy in transgenic mice expressing a point mutation in the β2-α2 loop of the prion protein

Transmissible spongiform encephalopathies are fatal neurodegenerative diseases attributed to misfolding of the cellular prion protein, PrP(C), into a β-sheet-rich, aggregated isoform, PrP(Sc). We previously found that expression of mouse PrP with the two amino acid substitutions S170N and N174T, which result in high structural order of the β2-α2 loop in the NMR structure at pH 4.5 and 20°C, caused transmissible de novo prion disease in transgenic mice. Here we report that expression of mouse PrP with the single-residue substitution D167S, which also results in a structurally well ordered β2-α2 loop at 20°C, elicits spontaneous PrP aggregation in vivo. Transgenic mice expressing PrP(D167S) developed a progressive encephalopathy characterized by abundant PrP plaque formation, spongiform change, and gliosis. These results add to the evidence that the β2-α2 loop has an important role in intermolecular interactions, including that it may be a key determinant of prion protein aggregation.

Rapid cell-surface prion protein conversion revealed using a novel cell system

Prion diseases are fatal neurodegenerative disorders with unique transmissible properties. The infectious and pathological agent is thought to be a misfolded conformer of the prion protein. Little is known about the initial events in prion infection because the infecting prion source has been immunologically indistinguishable from normal cellular prion protein (PrP(C)). Here we develop a unique cell system in which epitope-tagged PrP(C) is expressed in a PrP knockdown (KD) neuroblastoma cell line. The tagged PrP(C), when expressed in our PrP-KD cells, supports prion replication with the production of bona fide epitope-tagged infectious misfolded PrP (PrP(Sc)). Using this epitope-tagged PrP(Sc), we study the earliest events in cellular prion infection and PrP misfolding. We show that prion infection of cells is extremely rapid occurring within 1 min of prion exposure, and we demonstrate that the plasma membrane is the primary site of prion conversion.

Discrimination between alternate membrane protein topologies in living cells using GFP/YFP tagging and pH exchange

Membrane protein function is determined by the relative organization of the protein domains with respect to the membrane. We have experimentally verified the topology of a protein with diverse orientations arising from a single primary sequence (the cellular prion protein, PrP(C)), a novel somatostatin truncated receptor, and the Golgi-associated protein GPBP(91). Tagging with fluorescent proteins (FP) allows location of their expression at the plasma membrane or at endomembranes, but does not inform about their orientation. Exploiting the pH dependency of some FPs, we developed a pH exchange assay in which extracellularly exposed FPs are quenched by application of low pH buffer. We constructed standards to demonstrate and calibrate the assay, and the method was adapted for acidic organelle membrane proteins. This method can serve as a proof of concept, experimentally confirming and/or discriminating in living cells among theoretical topology predictions, providing the proportion of inside/outside orientation for proteins with multiple forms.

Tetracysteine-tagged prion protein allows discrimination between the native and converted forms

The conformational conversion of prion protein (PrP) from a native conformation to the amyloid form is a hallmark of transmissible spongiform encephalopathies. Conversion is usually monitored by fluorescent dyes, which bind generic amyloids and are less suited for living cell imaging. We report a new method for the synthesis of membrane-permeable and membrane-impermeable biarsenical reagents, which are then used to monitor murine PrP (mPrP) misfolding. We introduced tetracysteine (TC) tags into three different positions of mPrP, which folded into a native-like structure. Whereas mPrPs with a TC tag inserted at the N-terminus or C-terminus supported fibril formation, insertion into the helix 2-helix 3 loop inhibited conversion. We devised a quantitative protease-free method to determine the fraction of converted PrP, based on the ability of the fluorescein arsenical helix binder reagent to differentiate between the monomeric and fibrilized form of TC-tagged PrP, and showed that TC-tagged mPrP could be detected on transfected cells, thereby expanding the potential use of this method for the detection and study of conformational diseases.

In vivo generation of neurotoxic prion protein: role for hsp70 in accumulation of misfolded isoforms

Prion diseases are incurable neurodegenerative disorders in which the normal cellular prion protein (PrP^C) converts into a misfolded isoform (PrP^Sc) with unique biochemical and structural properties that correlate with disease. In humans, prion disorders, such as Creutzfeldt-Jakob disease, present typically with a sporadic origin, where unknown mechanisms lead to the spontaneous misfolding and deposition of wild type PrP. To shed light on how wild-type PrP undergoes conformational changes and which are the cellular components involved in this process, we analyzed the dynamics of wild-type PrP from hamster in transgenic flies. In young flies, PrP demonstrates properties of the benign PrP^C; in older flies, PrP misfolds, acquires biochemical and structural properties of PrP^Sc, and induces spongiform degeneration of brain neurons. Aged flies accumulate insoluble PrP that resists high concentrations of denaturing agents and contains PrP^Sc-specific conformational epitopes. In contrast to PrP^Sc from mammals, PrP is proteinase-sensitive in flies. Thus, wild-type PrP rapidly converts in vivo into a neurotoxic, protease-sensitive isoform distinct from prototypical PrP^Sc. Next, we investigated the role of molecular chaperones in PrP misfolding in vivo. Remarkably, Hsp70 prevents the accumulation of PrP^Sc-like conformers and protects against PrP-dependent neurodegeneration. This protective activity involves the direct interaction between Hsp70 and PrP, which may occur in active membrane microdomains such as lipid rafts, where we detected Hsp70. These results highlight the ability of wild-type PrP to spontaneously convert in vivo into a protease-sensitive isoform that is neurotoxic, supporting the idea that protease-resistant PrP^Sc is not required for pathology. Moreover, we identify a new role for Hsp70 in the accumulation of misfolded PrP. Overall, we provide new insight into the mechanisms of spontaneous accumulation of neurotoxic PrP and uncover the potential therapeutic role of Hsp70 in treating these devastating disorders.

Creutzfeldt-Jakob disease is a type of dementia caused by the deposition of the prion protein in the brain. This disorder belongs to a unique class of degenerative diseases that includes mad-cow disease in bovine and scrapie in sheep. An abnormal form of the prion protein is not only responsible for the disease in several mammals, but is also an infectious agent that can transmit the disease within or across species. To shed light on how the prion protein changes from its normal to the disease-causing form, we expressed the prion protein from hamster in transgenic flies. We observed that the prion protein progressively converts to the pathological form and induces neuronal loss in the brain. Thus, the prion protein experiences its typical transition from normal to disease-causing form in flies. This behavior gave us the opportunity to investigate whether other proteins can regulate such transition. We found that the stress-related protein Hsp70 prevents the accumulation of abnormal prion protein and prevents neuronal loss. We also determined that Hsp70 directly interacts with the prion protein in specific membrane domains. Overall, our studies provide new insight into the mechanisms that regulate the accumulation of abnormal prion protein. This discovery could have therapeutic applications in treating these devastating disorders.

Identification of an intracellular site of prion conversion

Prion diseases are fatal, neurodegenerative disorders in humans and animals and are characterized by the accumulation of an abnormally folded isoform of the cellular prion protein (PrP^C), denoted PrP^Sc, which represents the major component of infectious scrapie prions. Characterization of the mechanism of conversion of PrP^C into PrP^Sc and identification of the intracellular site where it occurs are among the most important questions in prion biology. Despite numerous efforts, both of these questions remain unsolved. We have quantitatively analyzed the distribution of PrP^C and PrP^Sc and measured PrP^Sc levels in different infected neuronal cell lines in which protein trafficking has been selectively impaired. Our data exclude roles for both early and late endosomes and identify the endosomal recycling compartment as the likely site of prion conversion. These findings represent a fundamental step towards understanding the cellular mechanism of prion conversion and will allow the development of new therapeutic approaches for prion diseases.

The misfolded form (PrP^Sc or prion) of the naturally occuring prion protein (PrP^C or cellular PrP) is responsible for neurodegenerative diseases such as Creutzfeldt-Jakob disease (CJD), bovine spongiform encephalopathy (BSE) (also known as ‘mad cow disease’) and a new variant of CJD (vCJD), which is thought to be caused by ingestion of cattle-derived foodstuffs contaminated with prions. These diseases are characterized by the accumulation of protein deposits in the central nervous system (CNS). However, unlike other neurodegenerative diseases, prion diseases are infectious and prions are able to propagate in a chain reaction by imposing their malconformed state onto the properly folded cellular proteins. Understanding where the conversion of PrP^C into PrP^Sc occurs in cells has been an unsolved question until now. By analysing the intracellular localization of PrP^C and PrP^Sc and measuring the levels of PrP^Sc produced in infected neuronal cell lines under conditions in which intracellular trafficking of the protein is impaired, we found that prion conversion occurs in the endosomal recycling compartment (ERC) where it transits after being internalized from the cell surface. This study will help to clarify the cellular mechanism of the disease and it opens the way to new therapeutic strategies aimed at the conversion compartment.

Proteasome inhibitors promote the sequestration of PrPSc into aggresomes within the cytosol of prion-infected CAD neuronal cells

Dysfunction of the endoplasmic reticulum associated protein degradation/proteasome system is believed to contribute to the initiation or aggravation of neurodegenerative disorders associated with protein misfolding, and there is some evidence to suggest that proteasome dysfunctions might be implicated in prion disease. This study investigated the effect of proteasome inhibitors on the biogenesis of both the cellular (PrP(C)) and abnormal (PrP(Sc)) forms of prion protein in CAD neuronal cells, a newly introduced prion cell system. In uninfected cells, proteasome impairment altered the intracellular distribution of PrP(C), leading to a strong accumulation in the Golgi apparatus. Moreover, a detergent-insoluble and weakly protease-resistant PrP species of 26 kDa, termed PrP(26K), accumulated in the cells, whether they were prion-infected or not. However, no evidence was found that, in infected cells, this PrP(26K) species converts into the highly proteinase K-resistant PrP(Sc). In the infected cultures, proteasome inhibition caused an increased intracellular aggregation of PrP(Sc) that was deposited into large aggresomes. These findings strengthen the view that, in neuronal cells expressing wild-type PrP(C) from the natural promoter, proteasomal impairment may affect both the process of PrP(C) biosynthesis and the subcellular sites of PrP(Sc) accumulation, despite the fact that these two effects could essentially be disconnected.

Specific biarsenical labeling of cell surface proteins allows fluorescent- and biotin-tagging of amyloid precursor protein and prion proteins

Fluorescent tagging is a powerful tool for imaging proteins in living cells. However, the steric effects imposed by fluorescent tags impair the behavior of many proteins. Here, we report a novel technique, Instant with DTT, EDT, And Low temperature (IDEAL)-labeling, for rapid and specific FlAsH-labeling of tetracysteine-tagged cell surface proteins by using prion protein (PrP) and amyloid precursor protein (APP) as models. In prion-infected cells, FlAsH-labeled tetracysteine-tagged PrP converted from the normal isoform (PrPsen) to the disease-associated isoform (PrPres), suggesting minimal steric effects of the tag. Pulse-chase analysis of PrP and APP by fluorescent gel imaging demonstrated the utility of IDEAL labeling in investigating protein metabolism by identifying an as-yet-unrecognized C-terminal fragment (C3) of PrPsen and by characterizing the kinetics of PrPres and APP metabolism. C3 generation and N-terminal truncation of PrPres were inhibited by the anti-prion compound E64, a cysteine protease inhibitor. Surprisingly, E64 did not inhibit the synthesis of new PrPres, providing insight into the mechanism by which E64 reduces steady-state PrPres levels in prion-infected cells. To expand the versatility of tetracysteine tagging, we created new Alexa Fluor- and biotin-conjugated tetracysteine-binding molecules that were applied to imaging PrP endocytosis and ultrastructural localization. IDEAL-labeling extends the use of biarsenical derivatives to extracellular proteins and beyond microscopic imaging.

Molecular mechanisms of prion pathogenesis

Prion diseases are infectious neurodegenerative diseases occurring in humans and animals with an invariably lethal outcome. One fundamental mechanistic event in prion diseases is the aggregation of aberrantly folded prion protein into large amyloid plaques and fibrous structures associated with neurodegeneration. The cellular prion protein (PrPC) is absolutely required for disease development, and prion knockout mice are not susceptible to prion disease. Prions accumulate not only in the central nervous system but also in lymphoid organs, as shown for new variant and sporadic Creutzfeldt-Jakob patients and for some animals. To date it is largely accepted that prions consist primarily of PrPSc, a misfolded and aggregated beta-sheet-rich isoform of PrPC. However, PrPSc may or may not be completely congruent with the infectious moiety. Here, we discuss the molecular mechanisms leading to neurodegeneration, the role of the immune system in prion pathogenesis, and the existence of prion strains that appear to have different tropisms and biochemical characteristics.

GFP-tagged mutant prion protein forms intra-axonal aggregates in transgenic mice

A nine-octapeptide insertional mutation in the prion protein (PrP) causes a fatal neurodegenerative disorder in both humans and transgenic mice. To determine the precise cellular localization of this mutant PrP (designated PG14), we have generated transgenic mice expressing PG14-EGFP, a fluorescent fusion protein that can be directly visualized in vivo. Tg(PG14-EGFP) mice develop an ataxic neurological illness characterized by astrogliosis, PrP aggregation, and accumulation of a partially protease-resistant form of the mutant PrP. Strikingly, PG14-EGFP forms numerous fluorescent aggregates in the neuropil and white matter of multiple brain regions. These aggregates are particularly prominent along axonal tracts in both brain and peripheral nerve, and similar intracellular deposits are visible along the processes of cultured neurons. Our results reveal intra-axonal aggregates of a mutant PrP, which could contribute to the pathogenesis of familial prion disease by disrupting axonal transport.

Subcellular localization of prion proteins and the 37 kDa/67 kDa laminin receptor fused to fluorescent proteins

The 37 kDa/67 kDa laminin receptor LRP/LR acts as a receptor for both PrPc and PrPSc at the cell surface. Here, we further analyzed the subcellular localization of fluorescent labeled prion protein (PrP) and laminin receptor (LRP/LR) molecules. We show that EGFP-PrP is localized at the cell surface and in a perinuclear compartment, respectively. In contrast, a DsRed-DeltaSP-PrP mutant lacking the signal peptide is almost exclusively found in the nucleus but does not colocalize with heterochromatin. Interestingly, LRP-DsRed efficiently colocalizes with EGFP-PrP in the perinuclear compartment and LRP-ECFP partly colocalizes with DsRed-DeltaSP-PrP in the nucleus, respectively. We conclude that the interactions of PrP and LRP/LR are not restricted to the cell surface but occur also in intracellular compartments suggesting a putative role of LRP/LR in the trafficking of PrP molecules.

Transgenic mouse models of prion diseases

Prions represent a new biological paradigm of protein-mediated information transfer. In mammals, prions are the cause of fatal, transmissible neurodegenerative diseases, often referred to as transmissible spongiform encephalopathies. Many unresolved issues remain, including the exact molecular nature of the prion, the detailed mechanism of prion propagation, and the mechanism by which prion diseases can be both genetic and infectious. In addition, we know little about the mechanism by which neurons degenerate during prion diseases. Tied to this, the physiological function of the normal form of the prion protein remains unclear, and it is uncertain whether loss of this function contributes to prion pathogenesis. The factors governing the transmission of prions between species remain unclear, in particular the means by which prion strains and PrP primary structure interact to affect interspecies prion transmission. Despite all these unknowns, dramatic advances in our understanding of prions have occurred because of their transmissibility to experimental animals and the development of transgenic mouse models has done much to further our understanding about various aspects of prion biology. In this chapter, I review recent advances in our understanding of prion biology that derive from this powerful and informative approach.

Multiple biochemical similarities between infectious and non-infectious aggregates of a prion protein carrying an octapeptide insertion

A nine-octapeptide insertion in the prion protein (PrP) gene is associated with an inherited form of human prion disease. Transgenic (Tg) mice that express the mouse homolog of this mutation (designated PG14) spontaneously accumulate in their brains an insoluble and weakly protease-resistant form of the mutant protein. This form (designated PG14(Spon)) is highly neurotoxic, but is not infectious in animal bioassays. In contrast, when Tg(PG14) mice are inoculated with the Rocky Mountain Laboratory (RML) strain of prions, they accumulate a different form of PG14 PrP (designated PG14(RML)) that is highly protease resistant and infectious in animal transmission experiments. We have been interested in characterizing the molecular properties of PG14(Spon) and PG14(RML), with a view to identifying features that determine two, apparently distinct properties of PrP aggregates: their infectivity and their pathogenicity. In this paper, we have subjected PG14(Spon) and PG14(RML) to a panel of assays commonly used to distinguish infectious PrP (PrP(Sc)) from cellular PrP (PrP(C)), including immobilized metal affinity chromatography, precipitation with sodium phosphotungstate, and immunoprecipitation with PrP(C)- and PrP(Sc)-specific antibodies. Surprisingly, we found that aggregates of PG14(Spon) and PG14(RML) behave identically to each other, and to authentic PrP(Sc), in each of these biochemical assays. PG14(Spon) however, in contrast to PG14(RML) and PrP(Sc), was unable to seed the misfolding of PrP(C) in an in vitro protein misfolding cyclic amplification reaction. Collectively, these results suggest that infectious and non-infectious aggregates of PrP share common structural features accounting for their toxicity, and that self-propagation of PrP involves more subtle molecular differences.

Prion protein expression differences in microglia and astroglia influence scrapie-induced neurodegeneration in the retina and brain of transgenic mice

Activated microglia and astroglia are known to be involved in a variety of neurodegenerative diseases, including prion diseases. In the present experiments, we studied activation of astroglia and microglia after intraocular scrapie infection in transgenic mice expressing prion protein (PrP) in multiple cell types (tg7 mice) or in neurons only (tgNSE mice). In this model, scrapie infection and protease-resistant PrP deposition occurs in the retinas of both strains of mice, but retinal degeneration is observed only in tg7 mice. Our results showed that the retinas of tg7 and tgNSE mice both had astroglial activation with increased chemokine expression during the course of infection. However, only tg7 retinas exhibited strong microglial activation compared to tgNSE retinas, which showed little microglial activation by biochemical or morphological criteria. Therefore, microglial PrP expression might be required for scrapie-induced retinal microglial activation and damage. Furthermore, microglial activation preceded retinal neurodegeneration in tg7 mice, suggesting that activated microglia might contribute to the degenerative process, rather than being a response to the damage. Surprisingly, brain differed from retina in that an altered profile of microglial activation markers was upregulated, and the profiles in the two mouse strains were indistinguishable. Microglial activation in the brain was associated with severe brain vacuolation and neurodegeneration, leading to death. Thus, retinal and brain microglia appeared to differ in their requirements for activation, suggesting that different activation pathways occur in the two tissues.

Insights into prion strains and neurotoxicity

Transmissible spongiform encephalopathies (TSEs) are neurodegenerative diseases that are caused by prions and affect humans and many animal species. It is now widely accepted that the infectious agent that causes TSEs is PrP(Sc), an aggregated moiety of the host-derived membrane glycolipoprotein PrP(C). Although PrP(C) is encoded by the host genome, prions themselves encipher many phenotypic TSE variants, known as prion strains. Prion strains are TSE isolates that, after inoculation into distinct hosts, cause disease with consistent characteristics, such as incubation period, distinct patterns of PrP(Sc) distribution and spongiosis and relative severity of the spongiform changes in the brain. The existence of such strains poses a fascinating challenge to prion research.

N-terminal peptides from unprocessed prion proteins enter cells by macropinocytosis

A peptide derived from the N-terminus of the unprocessed bovine prion protein (bPrPp), incorporating the hydrophobic signal sequence (residues 1-24) and a basic domain (KKRPKP, residues 25-30), internalizes into mammalian cells, even when coupled to a sizeable cargo, and therefore functions as a cell-penetrating peptide (CPP). Confocal microscopy and co-localization studies indicate that the internalization of bPrPp is mainly through macropinocytosis, a fluid-phase endocytosis process, initiated by binding to cell-surface proteoglycans. Electron microscopy studies show internalized bPrPp-DNA-gold complexes residing in endosomal vesicles. bPrPp induces expression of a complexed luciferase-encoding DNA plasmid, demonstrating the peptide's ability to transport the cargo across the endosomal membrane and into the cytosol and nucleus. The novel CPP activity of the unprocessed N-terminal domain of PrP could be important for the retrotranslocation of partly processed PrP and for PrP trafficking inside or between cells, with implications for the infectivity associated with prion diseases.

Intracellular accumulation of a 46 kDa species of mouse prion protein as a result of loss of glycosylation in cultured mammalian cells

Prion diseases are fatal neurodegenerative disorders characterized by the accumulation of an abnormal isoform (PrPSc) of the normal cellular prion protein (PrPC) in the brain. Reportedly, abnormal N-linked glycosylation patterns in PrPC are associated with disease susceptibility; thus, we compared the glycosylation status of normal and several mutant forms of the murine prion protein (MuPrP) in cultured mammalian cells. Substitution of the N-terminal signal sequence of normal MuPrP with a heterologous signal peptide did not alter glycosylation. When expressed without the C-terminal glycophosphatidylinositol anchor signal, the majority of MuPrP remained intracellular and unglycosylated, and a 46 kDa species (p46) of the unglycosylated PrPC was detected on reducing gels. p46 was also observed when wild-type MuPrP was expressed in the presence of tunicamycin or enzymatically deglycosylated in vitro. A rabbit polyclonal anti-serum raised against dimeric MuPrP cross-reacted with p46 and localized the signal within the Golgi apparatus. We propose that the 46 kDa signal is a dimeric form of MuPrP and in the light of recent studies, it can be argued that a relatively stable, non-glycosylated, cytoplasmic PrPC dimer, produced as a result of compromised glycosylation is an intermediate in initiating conversion of PrPC to PrPSc in sporadic transmissible spongiform encephalopathies.

Traffic of prion protein between different compartments on the neuronal surface, and the propagation of prion disease

The key mechanism in prion disease is the conversion of cellular prion protein into an altered, pathogenic conformation, in which cellular mechanisms play a poorly understood role. Both forms of prion protein are lipid-anchored and reside in rafts that appear to protect the native conformation against conversion. Neurons rapidly traffic their cellular prion protein out of its lipid rafts to be endocytosed via coated pits before recycling back to the cell surface. It is argued in this review that understanding the mechanism of this trafficking holds the key to understanding the cellular role in the conformational conversion of prion protein.

The interaction between cytoplasmic prion protein and the hydrophobic lipid core of membrane correlates with neurotoxicity

Prion protein (PrP), normally a cell surface protein, has been detected in the cytosol of a subset of neurons. The appearance of PrP in the cytosol could result from either retro-translocation of misfolded PrP from the endoplasmic reticulum (ER) or impaired import of PrP into the ER. Transgenic mice expressing cytoplasmic PrP (cyPrP) developed neurodegeneration in cerebellar granular neurons, although no detectable pathology was observed in other brain regions. In order to understand why granular neurons in the cerebellum were most susceptible to cyPrP-induced degeneration, we investigated the subcellular localization of cyPrP. Interestingly, we found that cyPrP is membrane-bound. In transfected cells, it binds to the ER and plasma/endocytic vesicular membranes. In transgenic mice, it is associated with synaptic and microsomal membranes. Furthermore, the cerebellar neurodegeneration in transgenic mice correlates with the interaction between cyPrP and the hydrophobic lipid core of the membrane but not with either the aggregation status or the dosage of cyPrP. These results suggest that lipid membrane perturbation could be a cellular mechanism for cyPrP-induced neurotoxicity and explain the seemingly conflicting results concerning cyPrP.

Prion infection of mouse neurospheres

Only a few cell lines have been infected with prions, offering limited genetic diversity and sensitivity to several strains. Here we report that cultured neurospheres expressing cellular prion protein (PrP(C)) can be infected with prions. Neurosphere lines isolated from the brains of mice at embryonic day 13-15 grow as aggregates and contain CNS stem cells. We produced neurosphere cultures from FVB/NCr (FVB) mice, from transgenic (Tg) FVB mice that overexpress mouse PrP-A (Tg4053), and from congenic FVB mice with a targeted null mutation in the PrP gene (Prnp(0/0)) and incubated them with the Rocky Mountain Laboratory prion strain. While monitoring the levels of disease-causing PrP (PrP(Sc)) at each passage, we observed a dramatic rise in PrP(Sc) levels with time in the Tg4053 neurosphere cells, whereas the level of PrP(Sc) decayed to undetectable levels in cell cultures lacking PrP. PrP(Sc) levels in cultures from FVB mice initially declined but then increased with passage. Prions produced in culture were transmissible to mice and produced disease pathology. Intracellular aggregates of PrP(Sc) were present in cells from infected cultures. The susceptibility of neurosphere cultures to prions mirrored that of the mice from which they were derived. Neurosphere lines from Tg4053 mice provide a sensitive in vitro bioassay for mouse prions; neurosphere lines from other Tg mice overexpressing PrP might be used to assay prions from other species, including humans.

GFP-tagged PrP supports compromised prion replication in transgenic mice

The ability of green fluorescent protein (GFP)-prion protein (PrP) fusions to support prion propagation has not been demonstrated. Here, we show that while transgenic mice expressing PrP tagged at its amino terminus with enhanced GFP, referred to as EGFPrP-N, supported prion replication, disease onset was prolonged, the brains of diseased mice did not exhibit typical disease neuropathology and disease-associated EGFPrP-N lacked the full spectrum of biochemical properties normally associated with PrP(Sc). Co-expression of wild-type PrP and EGFPrP-N substantially reduced prion incubation times and resulted in accumulation of protease-resistant EGFPrP(Sc)-N in the brains of transgenic mice as well as chronically infected cultured cells, suggesting that wild-type PrP rescued a compromised amino terminal function in EGFPrP-N. While our results show that EGFPrP(C)-N adopts a conformation necessary for the production of infectious prions, the synergistic interaction of wild-type and EGFPrP-N underscores the importance of the amino terminus in modulating prion pathogenesis.