PrP Knockout Cells Expressing Transmembrane PrP Resist Prion Infection

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.

Potential Therapeutic Use of Stem Cells for Prion Diseases

Prion diseases are neurodegenerative disorders that are progressive, incurable, and deadly. The prion consists of PrPSc, the misfolded pathogenic isoform of the cellular prion protein (PrPC). PrPC is involved in a variety of physiological functions, including cellular proliferation, adhesion, differentiation, and neural development. Prion protein is expressed on the membrane surface of a variety of stem cells (SCs), where it plays an important role in the pluripotency and self-renewal matrix, as well as in SC differentiation. SCs have been found to multiply the pathogenic form of the prion protein, implying their potential as an in vitro model for prion diseases. Furthermore, due to their capability to self-renew, differentiate, immunomodulate, and regenerate tissue, SCs are prospective cell treatments in many neurodegenerative conditions, including prion diseases. Regenerative medicine has become a new revolution in disease treatment in recent years, particularly with the introduction of SC therapy. Here, we review the data demonstrating prion diseases' biology and molecular mechanism. SC biology, therapeutic potential, and its role in understanding prion disease mechanisms are highlighted. Moreover, we summarize preclinical studies that use SCs in prion diseases.

(Patho)Physiology of Glycosylphosphatidylinositol-Anchored Proteins I: Localization at Plasma Membranes and Extracellular Compartments

Glycosylphosphatidylinositol (GPI)-anchored proteins (APs) are anchored at the outer leaflet of plasma membranes (PMs) of all eukaryotic organisms studied so far by covalent linkage to a highly conserved glycolipid rather than a transmembrane domain. Since their first description, experimental data have been accumulating for the capability of GPI-APs to be released from PMs into the surrounding milieu. It became evident that this release results in distinct arrangements of GPI-APs which are compatible with the aqueous milieu upon loss of their GPI anchor by (proteolytic or lipolytic) cleavage or in the course of shielding of the full-length GPI anchor by incorporation into extracellular vesicles, lipoprotein-like particles and (lyso)phospholipid- and cholesterol-harboring micelle-like complexes or by association with GPI-binding proteins or/and other full-length GPI-APs. In mammalian organisms, the (patho)physiological roles of the released GPI-APs in the extracellular environment, such as blood and tissue cells, depend on the molecular mechanisms of their release as well as the cell types and tissues involved, and are controlled by their removal from circulation. This is accomplished by endocytic uptake by liver cells and/or degradation by GPI-specific phospholipase D in order to bypass potential unwanted effects of the released GPI-APs or their transfer from the releasing donor to acceptor cells (which will be reviewed in a forthcoming manuscript).

Genetically engineered cellular models of prion propagation

For over three decades, cultured cells have been a useful tool for dissecting the molecular details of prion replication and the identification of candidate therapeutics for prion disease. A major issue limiting the translatability of these studies has been the inability to reliably propagate disease-relevant, non-mouse strains of prions in cells relevant to prion pathogenesis. In recent years, fueled by advances in gene editing technology, it has become possible to propagate prions from hamsters, cervids, and sheep in immortalized cell lines originating from the central nervous system. In particular, the use of CRISPR-Cas9-mediated gene editing to generate versions of prion-permissive cell lines that lack endogenous PrP expression has provided a blank canvas upon which re-expression of PrP leads to species-matched susceptibility to prion infection. When coupled with the ability to propagate prions in cells or organoids derived from stem cells, these next-generation cellular models should provide an ideal paradigm for identifying small molecules and other biological therapeutics capable of interfering with prion replication in animal and human prion disorders. In this review, we summarize recent advances that have widened the spectrum of prion strains that can be propagated in cultured cells and cutting-edge tissue-based models.

Propagation and Dissemination Strategies of Transmissible Spongiform Encephalopathy Agents in Mammalian Cells

Transmissible spongiform encephalopathies or prion disorders are fatal infectious diseases that cause characteristic spongiform degeneration in the central nervous system. The causative agent, the so-called prion, is an unconventional infectious agent that propagates by converting the host-encoded cellular prion protein PrP into ordered protein aggregates with infectious properties. Prions are devoid of coding nucleic acid and thus rely on the host cell machinery for propagation. While it is now established that, in addition to PrP, other cellular factors or processes determine the susceptibility of cell lines to prion infection, exact factors and cellular processes remain broadly obscure. Still, cellular models have uncovered important aspects of prion propagation and revealed intercellular dissemination strategies shared with other intracellular pathogens. Here, we summarize what we learned about the processes of prion invasion, intracellular replication and subsequent dissemination from ex vivo cell models.

The aminoglycoside G418 hinders de novo prion infection in cultured cells

The study of prions and the discovery of candidate therapeutics for prion disease have been facilitated by the ability of prions to replicate in cultured cells. Paradigms in which prion proteins from different species are expressed in cells with low or no expression of endogenous prion protein (PrP) have expanded the range of prion strains that can be propagated. In these systems, cells stably expressing a PrP of interest are typically generated via coexpression of a selectable marker and treatment with an antibiotic. Here, we report the unexpected discovery that the aminoglycoside G418 (Geneticin) interferes with the ability of stably transfected cultured cells to become infected with prions. In G418-resistant lines of N2a or CAD5 cells, the presence of G418 reduced levels of protease-resistant PrP following challenge with the RML or 22L strains of mouse prions. G418 also interfered with the infection of cells expressing hamster PrP with the 263K strain of hamster prions. Interestingly, G418 had minimal to no effect on protease-resistant PrP levels in cells with established prion infection, arguing that G418 selectively interferes with de novo prion infection. As G418 treatment had no discernible effect on cellular PrP levels or its localization, this suggests that G418 may specifically target prion assemblies or processes involved in the earliest stages of prion infection.

Volatile Anesthetic Sevoflurane Precursor 1,1,1,3,3,3-Hexafluoro-2-Propanol (HFIP) Exerts an Anti-Prion Activity in Prion-Infected Culture Cells

Prion disease is a neurodegenerative disorder with progressive neurologic symptoms and accelerated cognitive decline. The causative protein of prion disease is the prion protein (PrP), and structural transition of PrP from the normal helix rich form (PrP^C) to the abnormal β-sheet rich form (PrP^Sc) occurs in prion disease. While so far numerous therapeutic agents for prion diseases have been developed, none of them are still useful. A fluorinated alcohol, hexafluoro isopropanol (HFIP), is a precursor to the inhalational anesthetic sevoflurane and its metabolites. HFIP is also known as a robust α-helix inducer and is widely used as a solvent for highly aggregated peptides. Here we show that the α-helix-inducing activity of HFIP caused the conformational transformation of the fibrous structure of PrP into amorphous aggregates in vitro. HFIP added to the ScN2a cell medium, which continuously expresses PrP^Sc, reduced PrP^Sc protease resistance after 24-h incubation. It was also clarified that ScN2a cells are more susceptible to HFIP than any of the cells being compared. Based on these findings, HFIP is expected to develop as a therapeutic agent for prion disease.

From Cell Culture to Organoids-Model Systems for Investigating Prion Strain Characteristics

Prion diseases are the hallmark protein folding neurodegenerative disease. Their transmissible nature has allowed for the development of many different cellular models of disease where prion propagation and sometimes pathology can be induced. This review examines the range of simple cell cultures to more complex neurospheres, organoid, and organotypic slice cultures that have been used to study prion disease pathogenesis and to test therapeutics. We highlight the advantages and disadvantages of each system, giving special consideration to the importance of strains when choosing a model and when interpreting results, as not all systems propagate all strains, and in some cases, the technique used, or treatment applied, can alter the very strain properties being studied.

A New Take on Prion Protein Dynamics in Cellular Trafficking

The mobility of cellular prion protein (PrP^C) in specific cell membrane domains and among distinct cell compartments dictates its molecular interactions and directs its cell function. PrP^C works in concert with several partners to organize signaling platforms implicated in various cellular processes. The scaffold property of PrP^C is able to gather a molecular repertoire to create heterogeneous membrane domains that favor endocytic events. Dynamic trafficking of PrP^C through multiple pathways, in a well-orchestrated mechanism of intra and extracellular vesicular transport, defines its functional plasticity, and also assists the conversion and spreading of its infectious isoform associated with neurodegenerative diseases. In this review, we highlight how PrP^C traffics across intra- and extracellular compartments and the consequences of this dynamic transport in governing cell functions and contributing to prion disease pathogenesis.

Membrane insertion and intercellular transfer of glycosylphosphatidylinositol-anchored proteins: potential therapeutic applications

Anchorage of a subset of cell surface proteins in eukaryotic cells is mediated by a glycosylphosphatidylinositol (GPI) moiety covalently attached to the carboxy-terminus of the protein moiety. Experimental evidence for the potential of GPI-anchored proteins (GPI-AP) of being released from cells into the extracellular environment has been accumulating, which involves either the loss or retention of the GPI anchor. Release of GPI-AP from donor cells may occur spontaneously or in response to endogenous or environmental signals. The experimental evidence for direct insertion of exogenous GPI-AP equipped with the complete anchor structure into the outer plasma membrane bilayer leaflets of acceptor cells is reviewed as well as the potential underlying molecular mechanisms. Furthermore, promiscuous transfer of certain GPI-AP between plasma membranes of different cells in vivo under certain (patho)physiological conditions has been reported. Engineering of target cell surfaces using chimeric GPI-AP with complete GPI anchor may be useful for therapeutic applications.

Cellular models for discovering prion disease therapeutics: Progress and challenges

Prions, which cause fatal neurodegenerative disorders such as Creutzfeldt-Jakob disease, are misfolded and infectious protein aggregates. Currently, there are no treatments available to halt or even delay the progression of prion disease in the brain. The infectious nature of prions has resulted in animal paradigms that accurately recapitulate all aspects of prion disease, and these have proven to be instrumental for testing the efficacy of candidate therapeutics. Nonetheless, infection of cultured cells with prions provides a much more powerful system for identifying molecules capable of interfering with prion propagation. Certain lines of cultured cells can be chronically infected with various types of mouse prions, and these models have been used to unearth candidate anti-prion drugs that are at least partially efficacious when administered to prion-infected rodents. However, these studies have also revealed that not all types of prions are equal, and that drugs active against mouse prions are not necessarily effective against prions from other species. Despite some recent progress, the number of cellular models available for studying non-mouse prions remains limited. In particular, human prions have proven to be particularly challenging to propagate in cultured cells, which has severely hindered the discovery of drugs for Creutzfeldt-Jakob disease. In this review, we summarize the cellular models that are presently available for discovering and testing drugs capable of blocking the propagation of prions and highlight challenges that remain on the path towards developing therapies for prion disease.

GPI-anchor signal sequence influences PrPC sorting, shedding and signalling, and impacts on different pathomechanistic aspects of prion disease in mice

The cellular prion protein (PrP^C) is a cell surface glycoprotein attached to the membrane by a glycosylphosphatidylinositol (GPI)-anchor and plays a critical role in transmissible, neurodegenerative and fatal prion diseases. Alterations in membrane attachment influence PrP^C-associated signaling, and the development of prion disease, yet our knowledge of the role of the GPI-anchor in localization, processing, and function of PrP^Cin vivo is limited We exchanged the PrP^C GPI-anchor signal sequence of for that of Thy-1 (PrP^CGPIThy-1) in cells and mice. We show that this modifies the GPI-anchor composition, which then lacks sialic acid, and that PrP^CGPIThy-1 is preferentially localized in axons and is less prone to proteolytic shedding when compared to PrP^C. Interestingly, after prion infection, mice expressing PrP^CGPIThy-1 show a significant delay to terminal disease, a decrease of microglia/astrocyte activation, and altered MAPK signaling when compared to wild-type mice. Our results are the first to demonstrate in vivo, that the GPI-anchor signal sequence plays a fundamental role in the GPI-anchor composition, dictating the subcellular localization of a given protein and, in the case of PrP^C, influencing the development of prion disease.

The prion protein (PrP^C) is a glycoprotein attached to the neuronal surface via a GPI-anchor. When misfolded to PrP^Sc, it leads to fatal neurodegenerative diseases which propagates from host to host. PrP^Sc is the principal component of the infectious agent of prion diseases, the “prion”. Misfolding occurs at the plasma membrane, and when PrP^C lacks the GPI-anchor, neuropathology and incubation time of prion disease are strongly modified. Moreover, the composition of the PrP^C GPI-anchor impacts on the conversion process. To study the role of the GPI-anchor in the pathophysiology of prion diseases in vivo, we have generated transgenic mice where the PrP^C GPI-signal sequence (GPI-SS) is replaced for the one of Thy-1, a neuronal protein with a distinct GPI-anchor and membrane localization. We found that the resulting protein, PrP^CGPIThy-1, shows a different GPI-anchor composition, increased axonal localization, and reduced enzymatic shedding. After prion infection, disease progression is significantly delayed, and the neuropathology and cellular signaling are changed.

The present work demonstrates that the GPI-SS per se determines the GPI-anchor composition and localization of a given protein and it stresses the importance of PrP^C membrane anchorage in prion disease.

Cell biology of prion infection

The development of multiple cell culture models of prion infection over the last two decades has led to a significant increase in our understanding of how prions infect cells. In particular, new techniques to distinguish exogenous from endogenous prions have allowed us for the first time to look in depth at the earliest stages of prion infection through to the establishment of persistent infection. These studies have shown that prions can infect multiple cell types, both neuronal and nonneuronal. Once in contact with the cell, they are rapidly taken up via multiple endocytic pathways. After uptake, the initial replication of prions occurs almost immediately on the plasma membrane and within multiple endocytic compartments. Following this acute stage of prion replication, persistent prion infection may or may not be established. Establishment of a persistent prion infection in cells appears to depend upon the achievement of a delicate balance between the rate of prion replication and degradation, the rate of cell division, and the efficiency of prion spread from cell to cell. Overall, cell culture models have shown that prion infection of the cell is a complex and variable process which can involve multiple cellular pathways and compartments even within a single cell.

Organization of GPI-anchored proteins at the cell surface and its physiopathological relevance

Glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) are a class of proteins attached to the extracellular leaflet of the plasma membrane via a post-translational modification, the glycolipid anchor. The presence of both glycolipid anchor and protein portion confers them unique features. GPI-APs are expressed in all eukaryotes, from fungi to plants and animals. They display very diverse functions ranging from enzymatic activity, signaling, cell adhesion, cell wall metabolism, neuritogenesis, and immune response. Likewise other plasma membrane proteins, the spatio-temporal organization of GPI-APs is critical for their biological activities in physiological conditions. In this review, we will summarize the latest findings on plasma membrane organization of GPI-APs and the mechanism of its regulation in different cell types. We will also examine the involvement of specific GPI-APs namely the prion protein PrP^C, the Folate Receptor alpha and the urokinase plasminogen activator receptor in human diseases focusing on neurodegenerative diseases and cancer.

Exosomes and their role in the intercellular trafficking of normal and disease associated prion proteins

Over the past decade, small extracellular vesicles called exosomes have been observed to harbour protein and genetic cargo that can assist in health and also cause disease. Many groups are extensively investigating the mechanisms involved that regulate the trafficking and packaging of exosomal contents and how these processes may be deregulated in disease. Prion diseases are transmissible neurodegenerative disorders and are characterized by the presence of detectable misfolded prion proteins. The disease associated form of the prion protein can be found in exosomes and its transmissible properties have provided a reliable experimental read out that can be used to understand how exosomes and their cargo are involved in cell-cell communication and in the spread of prion diseases. This review reports on the current understanding of how exosomes are involved in the intercellular spread of infectious prions. Furthermore, we discuss how these principles are leading future investigations in developing new exosome based diagnostic tools and therapeutic drugs that could be applied to other neurodegenerative diseases.

Cell Biology of Prion Protein

Cellular prion protein (PrP^C) is a mammalian glycoprotein which is usually found anchored to the plasma membrane via a glycosylphosphatidylinositol (GPI) anchor. The precise function of PrP^C remains elusive but may depend upon its cellular localization. PrP^C misfolds to a pathogenic isoform PrP^Sc, the causative agent of neurodegenerative prion diseases. Nonetheless some forms of prion disease develop in the apparent absence of infectious PrP^Sc, suggesting that molecular species of PrP distinct from PrP^Sc may represent the primary neurotoxic culprits. Indeed, in some inherited cases of human prion disease, the predominant form of PrP detectable in the brain is not PrP^Sc but rather ^CtmPrP, a transmembrane form of the protein. The relationship between the neurodegeneration occurring in prion diseases involving PrP^Sc and that associated with ^CtmPrP remains unclear. However, the different membrane topology of the PrP mutants, as well as the presence of the GPI anchor, could influence both the function and the intracellular localization and trafficking of the protein, all being potentially very important in the pathophysiological mechanism that ultimately causes the disease. Here, we review the latest findings on the fundamental aspects of prions biology, from the PrP^C biosynthesis, function, and structure up to its intracellular traffic and analyze the possible roles of the different topological isoforms of the protein, as well as the GPI anchor, in the pathogenesis of the disease.

Endogenous Brain Lipids Inhibit Prion Amyloid Formation In Vitro

The normal cellular prion protein (PrP^C) resides in detergent-resistant outer membrane lipid rafts in which conversion to the pathogenic misfolded form is believed to occur. Once misfolding occurs, the pathogenic isoform polymerizes into highly stable amyloid fibrils. In vitro assays have demonstrated an intimate association between prion conversion and lipids, specifically phosphatidylethanolamine, which is a critical cofactor in the formation of synthetic infectious prions. In the current work, we demonstrate an alternative inhibitory function of lipids in the prion conversion process as assessed in vitro by real-time quaking-induced conversion (RT-QuIC). Using an alcohol-based extraction technique, we removed the lipid content from chronic wasting disease (CWD)-infected white-tailed deer brain homogenates and found that lipid extraction enabled RT-QuIC detection of CWD prions in a 2-log₁₀-greater concentration of brain sample. Conversely, addition of brain-derived lipid extracts to CWD prion brain or lymph node samples inhibited amyloid formation in a dose-dependent manner. Subsequent lipid analysis demonstrated that this inhibitory function was restricted to the polar lipid fraction in brain. We further investigated three phospholipids commonly found in lipid membranes, phosphatidylethanolamine, phosphatidylcholine, and phosphatidylinositol, and found all three similarly inhibited RT-QuIC. These results demonstrating polar-lipid, and specifically phospholipid, inhibition of prion-seeded amyloid formation highlight the diverse roles lipid constituents may play in the prion conversion process.IMPORTANCE Prion conversion is likely influenced by lipid interactions, given the location of normal prion protein (PrP^C) in lipid rafts and lipid cofactors generating infectious prions in in vitro models. Here, we use real-time quaking-induced conversion (RT-QuIC) to demonstrate that endogenous brain polar lipids can inhibit prion-seeded amyloid formation, suggesting that prion conversion is guided by an environment of proconversion and anticonversion lipids. These experiments also highlight the applicability of RT-QuIC to identify potential therapeutic inhibitors of prion conversion.

Dual role of cellular prion protein in normal host and Alzheimer's disease

Using PrPC-knockout cell lines, it has been shown that the inhibition of apoptosis through STI1 is mediated by PrPC-dependent SOD activation. Antioxidant PrPC may contribute to suppression of inflammasome activation. PrPC is functionally involved in copper metabolism, signal transduction, neuroprotection, and cell maturation. Recently several reports have shown that PrPC participates in trans-membrane signaling processes associated with hematopoietic stem cell replication and neuronal differentiation. In another role, PrPC also tends to function as a neurotoxic protein. Aβ oligomer, which is associated with neurodegeneration in Alzheimer's disease (AD), has also been reported to act as a ligand of PrPC. However, the physiological role of PrPC as an Aβ42-binding protein is not clear. Actually, PrPC is critical in Aβ42-mediated autophagy in neurons. PrPC shows a beneficial role in lipid rafts to promote autophagy. Further search for PrPC-interaction molecules using Prnp-/- mice and various types of Prnp-/- cell lines under various conditions may elucidate other important PrPC important functions.