Binding of neural cell adhesion molecules (N-CAMs) to the cellular prion protein

The brain interactome of a permissive prion replication substrate

Bank voles are susceptible to prion strains from many different species, yet the molecular mechanisms underlying the ability of bank vole prion protein (BVPrP) to function as a universal prion acceptor remain unclear. Potential differences in molecular environments and protein interaction networks on the cell surface of brain cells may contribute to BVPrP's unusual behavior. To test this hypothesis, we generated knock-in mice that express physiological levels of BVPrP (M109 isoform) and employed mass spectrometry to compare the interactomes of mouse (Mo) PrP and BVPrP following mild in vivo crosslinking of brain tissue. Substantial overlap was observed between the top interactors for BVPrP and MoPrP, with established PrP-interactors such as neural cell adhesion molecules, subunits of Na+/K+-ATPases, and contactin-1 being equally present in the two interactomes. We conclude that the molecular environments of BVPrP and MoPrP in the brains of mice are very similar. This suggests that the unorthodox properties of BVPrP are unlikely to be mediated by differential interactions with other proteins.

Comparing Prion Proteins Across Species: Is Zebrafish a Useful Model?

Despite the considerable body of research dedicated to the field of neurodegeneration, the gap in knowledge on the prion protein and its intricate involvement in brain diseases remains substantial. However, in the past decades, many steps forward have been taken toward a better understanding of the molecular mechanisms underlying both the physiological role of the prion protein and the misfolding event converting it into its pathological counterpart, the prion. This review aims to provide an overview of the main findings regarding this protein, highlighting the advantages of many different animal models that share a conserved amino acid sequence and/or structure with the human prion protein. A particular focus will be given to the species Danio rerio, a compelling research organism for the investigation of prion biology, thanks to its conserved orthologs, ease of genetic manipulation, and cost-effectiveness of high-throughput experimentation. We will explore its potential in filling some of the gaps on physiological and pathological aspects of the prion protein, with the aim of directing the future development of therapeutic interventions.

Roles of prion proteins in mammalian development

Prion protein (PrP) is highly conserved and is expressed in most tissues in a developmental stage-specific manner. Glycosylated cellular prion protein (PrPC) is found in most cells and subcellular areas as a physiological regulating molecule. On the other hand, the amyloid form of PrPC, scrapie PrP (PrPSC), causes transmissible pathogenesis in the central nervous system and induces degeneration of the nervous system. Although many amyloids are reversible and critical in determining the fate, differentiation, and physiological functions of cells, thus far, PrPSC originating from PrPC is not. Although many studies have focused on disorders involving PrPC and the deletion mammalian models for PrPC have no severe phenotype, it has been suggested that PrPC has a role in normal development. It is conserved and expressed from gametes to adult somatic cells. In addition, severe developmental phenotypes appear in PrP null zebrafish embryos and in various mammalian cell model systems. In addition, it has been well established that PrPC is strongly involved in the stemness and differentiation of embryonic stem cells and progenitors. Thus far, many studies on PrPC have focused mostly on disease-associated conditions with physiological roles as a complex platform but not on development. The known roles of PrPC depend on the interacting molecules through its flexible tail and domains. PrPC interacts with membrane, and various intracellular and extracellular molecules. In addition, PrPC and amyloid can stimulate signaling pathways differentially. In this review, we summarize the function of prion protein and discuss its role in development.

Slow Misfolding of a Molten Globule form of a Mutant Prion Protein Variant into a β-rich Dimer

Misfolding of the prion protein is linked to multiple neurodegenerative diseases. A better understanding of the process requires the identification and structural characterization of intermediate conformations via which misfolding proceeds. In this study, three conserved aromatic residues (Tyr168, Phe174, and Tyr217) located in the C-terminal domain of mouse PrP (wt moPrP) were mutated to Ala. The resultant mutant protein, 3A moPrP, is shown to adopt a molten globule (MG)-like native conformation. Hydrogen-deuterium exchange studies coupled with mass spectrometry revealed that for 3A moPrP, the free energy gap between the MG-like native conformation and misfolding-prone partially unfolded forms is reduced. Consequently, 3A moPrP misfolds in native conditions even in the absence of salt, unlike wt moPrP, which requires the addition of salt to misfold. 3A moPrP misfolds to a β-rich dimer in the absence of salt, which can rapidly form an oligomer upon the addition of salt. In the presence of salt, 3A moPrP misfolds to a β-rich oligomer about a thousand-fold faster than wt moPrP. Importantly, the misfolded structure of the dimer is similar to that of the salt-induced oligomer. Misfolding to oligomer seems to be induced at the level of the dimeric unit by monomer-monomer association, and the oligomer grows by accretion of misfolded dimeric units. Additionally, it is shown that the conserved aromatic residues collectively stabilize not only monomeric protein, but also the structural core of the β-rich oligomers. Finally, it is also shown that 3A moPrP misfolds much faster to amyloid-fibrils than does the wt protein.

Unfolding Mechanism and Fibril Formation Propensity of Human Prion Protein in the Presence of Molecular Crowding Agents

The pathological process of prion diseases implicates that the normal physiological cellular prion protein (PrPC) converts into misfolded abnormal scrapie prion (PrPSc) through post-translational modifications that increase β-sheet conformation. We recently demonstrated that HuPrP(90-231) thermal unfolding is partially irreversible and characterized by an intermediate state (β-PrPI), which has been revealed to be involved in the initial stages of PrPC fibrillation, with a seeding activity comparable to that of human infectious prions. In this study, we report the thermal unfolding characterization, in cell-mimicking conditions, of the truncated (HuPrP(90-231)) and full-length (HuPrP(23-231)) human prion protein by means of CD and NMR spectroscopy, revealing that HuPrP(90-231) thermal unfolding is characterized by two successive transitions, as in buffer solution. The amyloidogenic propensity of HuPrP(90-231) under crowded conditions has also been investigated. Our findings show that although the prion intermediate, structurally very similar to β-PrPI, forms at a lower temperature compared to when it is dissolved in buffer solution, in cell-mimicking conditions, the formation of prion fibrils requires a longer incubation time, outlining how molecular crowding influences both the equilibrium states of PrP and its kinetic pathways of folding and aggregation.

PrP meets alpha-synuclein: Molecular mechanisms and implications for disease

The discovery of prions has challenged dogmas and has revolutionized our understanding of protein-misfolding diseases. The concept of self-propagation via protein conformational changes, originally discovered for the prion protein (PrP), also applies to other proteins that exhibit similar behavior, such as alpha-synuclein (aSyn), a central player in Parkinson's disease and in other synucleinopathies. aSyn pathology appears to spread from one cell to another during disease progression, and involves the misfolding and aggregation of aSyn. How the transfer of aSyn between cells occurs is still being studied, but one important hypothesis involves receptor-mediated transport. Interestingly, recent studies indicate that the cellular prion protein (PrPC) may play a crucial role in this process. PrPC has been shown to act as a receptor/sensor for protein aggregates in different neurodegenerative disorders, including Alzheimer's disease and amyotrophic lateral sclerosis. Here, we provide a comprehensive overview of the current state of knowledge regarding the interaction between aSyn and PrPC and discuss its role in synucleinopathies. We examine the properties of PrP and aSyn, including their structure, function, and aggregation. Additionally, we discuss the current understanding of PrPC's role as a receptor/sensor for aSyn aggregates and identify remaining unanswered questions in this area of research. Ultimately, we posit that exploring the interaction between aSyn and PrPC may offer potential treatment options for synucleinopathies.

In Vitro and In Vivo Evidence towards Fibronectin's Protective Effects against Prion Infection

A distinctive signature of the prion diseases is the accumulation of the pathogenic isoform of the prion protein, PrPSc, in the central nervous system of prion-affected humans and animals. PrPSc is also found in peripheral tissues, raising concerns about the potential transmission of pathogenic prions through human food supplies and posing a significant risk to public health. Although muscle tissues are considered to contain levels of low prion infectivity, it has been shown that myotubes in culture efficiently propagate PrPSc. Given the high consumption of muscle tissue, it is important to understand what factors could influence the establishment of a prion infection in muscle tissue. Here we used in vitro myotube cultures, differentiated from the C2C12 myoblast cell line (dC2C12), to identify factors affecting prion replication. A range of experimental conditions revealed that PrPSc is tightly associated with proteins found in the systemic extracellular matrix, mostly fibronectin (FN). The interaction of PrPSc with FN decreased prion infectivity, as determined by standard scrapie cell assay. Interestingly, the prion-resistant reserve cells in dC2C12 cultures displayed a FN-rich extracellular matrix while the prion-susceptible myotubes expressed FN at a low level. In agreement with the in vitro results, immunohistopathological analyses of tissues from sheep infected with natural scrapie demonstrated a prion susceptibility phenotype linked to an extracellular matrix with undetectable levels of FN. Conversely, PrPSc deposits were not observed in tissues expressing FN. These data indicate that extracellular FN may act as a natural barrier against prion replication and that the extracellular matrix composition may be a crucial feature determining prion tropism in different tissues.

Mutations of evolutionarily conserved aromatic residues suggest that misfolding of the mouse prion protein may commence in multiple ways

The misfolding of the mammalian prion protein from its α-helix rich cellular isoform to its β-sheet rich infectious isoform is associated with several neurodegenerative diseases. The determination of the structural mechanism by which misfolding commences, still remains an unsolved problem. In the current study, native-state hydrogen exchange coupled with mass spectrometry has revealed that the N state of the mouse prion protein (moPrP) at pH 4 is in dynamic equilibrium with multiple partially unfolded forms (PUFs) capable of initiating misfolding. Mutation of three evolutionarily conserved aromatic residues, Tyr168, Phe174, and Tyr217 present at the interface of the β2-α2 loop and the C-terminal end of α3 in the structured C-terminal domain of moPrP significantly destabilize the native state (N) of the protein. They also reduce the free energy differences between the N state and two PUFs identified as PUF1 and PUF2**. It is shown that PUF2** in which the β2-α2 loop and the C-terminal end of α3 are disordered, has the same stability as the previously identified PUF2*, but to have a very different structure. Misfolding can commence from both PUF1 and PUF2**, as it can from PUF2*. Hence, misfolding can commence and proceed in multiple ways from structurally distinct precursor conformations. The increased extents to which PUF1 and PUF2** are populated at equilibrium in the case of the mutant variants, greatly accelerate their misfolding. The results suggest that the three aromatic residues may have been evolutionarily selected to impede the misfolding of moPrP.

The Multifaceted Functions of Prion Protein (PrPC) in Cancer

The cellular prion protein (PrPC) is a glycoprotein anchored to the cell surface by glycosylphosphatidylinositol (GPI). PrPC is expressed both in the brain and in peripheral tissues. Investigations on PrPC's functions revealed its direct involvement in neurodegenerative and prion diseases, as well as in various physiological processes such as anti-oxidative functions, copper homeostasis, trans-membrane signaling, and cell adhesion. Recent findings have revealed the ectopic expression of PrPC in various cancers including gastric, melanoma, breast, colorectal, pancreatic, as well as rare cancers, where PrPC promotes cellular migration and invasion, tumor growth, and metastasis. Through its downstream signaling, PrPC has also been reported to be involved in resistance to chemotherapy and tumor cell apoptosis. This review summarizes the variance of expression of PrPC in different types of cancers and discusses its roles in their development and progression, as well as its use as a potential target to treat such cancers.

Prions induce an early Arc response and a subsequent reduction in mGluR5 in the hippocampus

Synapse dysfunction and loss are central features of neurodegenerative diseases, caused in part by the accumulation of protein oligomers. Amyloid-β, tau, prion, and α-synuclein oligomers bind to the cellular prion protein (PrPC), resulting in the activation of macromolecular complexes and signaling at the post-synapse, yet the early signaling events are unclear. Here we sought to determine the early transcript and protein alterations in the hippocampus during the pre-clinical stages of prion disease. We used a transcriptomic approach focused on the early-stage, prion-infected hippocampus of male wild-type mice, and identify immediate early genes, including the synaptic activity response gene, Arc/Arg3.1, as significantly upregulated. In a longitudinal study of male, prion-infected mice, Arc/Arg-3.1 protein was increased early (40% of the incubation period), and by mid-disease (pre-clinical), phosphorylated AMPA receptors (pGluA1-S845) were increased and metabotropic glutamate receptors (mGluR5 dimers) were markedly reduced in the hippocampus. Notably, sporadic Creutzfeldt-Jakob disease (sCJD) post-mortem cortical samples also showed low levels of mGluR5 dimers. Together, these findings suggest that prions trigger an early Arc response, followed by an increase in phosphorylated GluA1 and a reduction in mGluR5 receptors.

Mechanisms of prion-induced toxicity

Prion diseases are devastating neurodegenerative diseases caused by the structural conversion of the normally benign prion protein (PrP^C) to an infectious, disease-associated, conformer, PrP^Sc. After decades of intense research, much is known about the self-templated prion conversion process, a phenomenon which is now understood to be operative in other more common neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. In this review, we provide the current state of knowledge concerning a relatively poorly understood aspect of prion diseases: mechanisms of neurotoxicity. We provide an overview of proposed functions of PrP^C and its interactions with other extracellular proteins in the central nervous system, in vivo and in vitro models used to delineate signaling events downstream of prion propagation, the application of omics technologies, and the emerging appreciation of the role played by non-neuronal cell types in pathogenesis.

The prion protein and its ligands: Insights into structure-function relationships

The prion protein is a multifunctional protein that exists in at least two different folding states. It is subject to diverse proteolytic processing steps that lead to prion protein fragments some of which are membrane-bound whereas others are soluble. A multitude of ligands bind to the prion protein and besides proteinaceous binding partners, interaction with metal ions and nucleic acids occurs. Although of great importance, information on structural and functional consequences of prion protein binding to its partners is limited. Here, we will reflect on the structure-function relationship of the prion protein and its binding partners considering the different folding states and prion protein fragments.

Extracellular vesicles with diagnostic and therapeutic potential for prion diseases

Prion diseases (PrD) or transmissible spongiform encephalopathies (TSE) are invariably fatal and pathogenic neurodegenerative disorders caused by the self-propagated misfolding of cellular prion protein (PrP^C) to the neurotoxic pathogenic form (PrP^TSE) via a yet undefined but profoundly complex mechanism. Despite several decades of research on PrD, the basic understanding of where and how PrP^C is transformed to the misfolded, aggregation-prone and pathogenic PrP^TSE remains elusive. The primary clinical hallmarks of PrD include vacuolation-associated spongiform changes and PrP^TSE accumulation in neural tissue together with astrogliosis. The difficulty in unravelling the disease mechanisms has been related to the rare occurrence and long incubation period (over decades) followed by a very short clinical phase (few months). Additional challenge in unravelling the disease is implicated to the unique nature of the agent, its complexity and strain diversity, resulting in the heterogeneity of the clinical manifestations and potentially diverse disease mechanisms. Recent advances in tissue isolation and processing techniques have identified novel means of intercellular communication through extracellular vesicles (EVs) that contribute to PrP^TSE transmission in PrD. This review will comprehensively discuss PrP^TSE transmission and neurotoxicity, focusing on the role of EVs in disease progression, biomarker discovery and potential therapeutic agents for the treatment of PrD.

Prion protein with a mutant N-terminal octarepeat region undergoes cobalamin-dependent assembly into high-molecular weight complexes

The cellular prion protein (PrP^C) has a C-terminal globular domain and a disordered N-terminal region encompassing five octarepeats (ORs). Encounters between Cu(II) ions and four OR sites produce interchangeable binding geometries; however, the significance of Cu(II) binding to ORs in different combinations is unclear. To understand the impact of specific binding geometries, OR variants were designed that interact with multiple or single Cu(II) ions in specific locked coordinations. Unexpectedly, we found that one mutant produced detergent-insoluble, protease-resistant species in cells in the absence of exposure to the infectious prion protein isoform, scrapie-associated prion protein (PrP^Sc). Formation of these assemblies, visible as puncta, was reversible and dependent upon medium formulation. Cobalamin (Cbl), a dietary cofactor containing a corrin ring that coordinates a Co³⁺ ion, was identified as a key medium component, and its effect was validated by reconstitution experiments. Although we failed to find evidence that Cbl interacts with Cu-binding OR regions, we instead noted interactions of Cbl with the PrP^C C-terminal domain. We found that some interactions occurred at a binding site of planar tetrapyrrole compounds on the isolated globular domain, but others did not, and N-terminal sequences additionally had a marked effect on their presence and position. Our studies define a conditional effect of Cbl wherein a mutant OR region can act in cis to destabilize a globular domain with a wild type sequence. The unexpected intersection between the properties of PrP^Sc's disordered region, Cbl, and conformational remodeling events may have implications for understanding sporadic prion disease that does not involve exposure to PrP^Sc.

Anchorless risk or released benefit? An updated view on the ADAM10-mediated shedding of the prion protein

The prion protein (PrP) is a broadly expressed glycoprotein linked with a multitude of (suggested) biological and pathological implications. Some of these roles seem to be due to constitutively generated proteolytic fragments of the protein. Among them is a soluble PrP form, which is released from the surface of neurons and other cell types by action of the metalloprotease ADAM10 in a process termed 'shedding'. The latter aspect is the focus of this review, which aims to provide a comprehensive overview on (i) the relevance of proteolytic processing in regulating cellular PrP functions, (ii) currently described involvement of shed PrP in neurodegenerative diseases (including prion diseases and Alzheimer's disease), (iii) shed PrP's expected roles in intercellular communication in many more (patho)physiological conditions (such as stroke, cancer or immune responses), (iv) and the need for improved research tools in respective (future) studies. Deeper mechanistic insight into roles played by PrP shedding and its resulting fragment may pave the way for improved diagnostics and future therapeutic approaches in diseases of the brain and beyond.

PrP C as a Transducer of Physiological and Pathological Signals

After the discovery of prion phenomenon, the physiological role of the cellular prion protein (PrP ^C ) remained elusive. In the past decades, molecular and cellular analysis has shed some light regarding interactions and functions of PrP ^C in health and disease. PrP ^C , which is located mainly at the plasma membrane of neuronal cells attached by a glycosylphosphatidylinositol (GPI) anchor, can act as a receptor or transducer from external signaling. Although the precise role of PrP ^C remains elusive, a variety of functions have been proposed for this protein, namely, neuronal excitability and viability. Although many issues must be solved to clearly define the role of PrP ^C , its connection to the central nervous system (CNS) and to several misfolding-associated diseases makes PrP ^C an interesting pharmacological target. In a physiological context, several reports have proposed that PrP ^C modulates synaptic transmission, interacting with various proteins, namely, ion pumps, channels, and metabotropic receptors. PrP ^C has also been implicated in the pathophysiological cell signaling induced by β-amyloid peptide that leads to synaptic dysfunction in the context of Alzheimer's disease (AD), as a mediator of Aβ-induced cell toxicity. Additionally, it has been implicated in other proteinopathies as well. In this review, we aimed to analyze the role of PrP ^C as a transducer of physiological and pathological signaling.

The cellular prion protein interacts with and promotes the activity of Na,K-ATPases

The prion protein (PrP) is best known for its ability to cause fatal neurodegenerative diseases in humans and animals. Here, we revisited its molecular environment in the brain using a well-developed affinity-capture mass spectrometry workflow that offers robust relative quantitation. The analysis confirmed many previously reported interactions. It also pointed toward a profound enrichment of Na,K-ATPases (NKAs) in proximity to cellular PrP (PrP^C). Follow-on work validated the interaction, demonstrated partial co-localization of the ATP1A1 and PrP^C, and revealed that cells exposed to cardiac glycoside (CG) inhibitors of NKAs exhibit correlated changes to the steady-state levels of both proteins. Moreover, the presence of PrP^C was observed to promote the ion uptake activity of NKAs in a human co-culture paradigm of differentiated neurons and glia cells, and in mouse neuroblastoma cells. Consistent with this finding, changes in the expression of 5’-nucleotidase that manifest in wild-type cells in response to CG exposure can also be observed in untreated PrP^C-deficient cells. Finally, the endoproteolytic cleavage of the glial fibrillary acidic protein, a hallmark of late-stage prion disease, can also be induced by CGs, raising the prospect that a loss of NKA activity may contribute to the pathobiology of prion diseases.

Altered cellular localisation and expression, together with unconventional protein trafficking, of prion protein, PrPC, in type 1 diabetes

AIMS/HYPOTHESIS

Normal cellular prion protein (PrP^C) is a conserved mammalian glycoprotein found on the outer plasma membrane leaflet through a glycophosphatidylinositol anchor. 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. The misfolded pathogenic isoform PrP^Sc (the scrapie form of PrP) is a causative agent of neurodegenerative prion diseases. The aim of this study is to evaluate PrP^C localisation, expression and trafficking in pancreases from organ donors with and without type 1 diabetes and to infer PrP^C function through studies on interacting protein partners.

METHODS

In order to evaluate localisation and trafficking of PrP^C in the human pancreas, 12 non-diabetic, 12 type 1 diabetic and 12 autoantibody-positive organ donor tissue samples were analysed using immunofluorescence analysis. Furthermore, total RNA was isolated from 29 non-diabetic, 29 type 1 diabetic and 24 autoantibody-positive donors to estimate PrP^C expression in the human pancreas. Additionally, we performed PrP^C-specific immunoblot analysis on total pancreatic protein from non-diabetic and type 1 diabetic organ donors to test whether changes in PrP^C mRNA levels leads to a concomitant increase in PrP^C protein levels in human pancreases.

RESULTS

In non-diabetic and type 1 diabetic pancreases (the latter displaying both insulin-positive [INS(+)] and -negative [INS(-)] islets), we found PrP^C in islets co-registering with beta cells in all INS(+) islets and, strikingly, unexpected activation of PrP^C in alpha cells within diabetic INS(-) islets. We found PrP^C localised to the plasma membrane and endoplasmic reticulum (ER) but not the Golgi, defining two cellular pools and an unconventional protein trafficking mechanism bypassing the Golgi. We demonstrate PrP^C co-registration with established protein partners, neural cell adhesion molecule 1 (NCAM1) and stress-inducible phosphoprotein 1 (STI1; encoded by STIP1) on the plasma membrane and ER, respectively, linking PrP^C function with cyto-protection, signalling, differentiation and morphogenesis. We demonstrate that both PRNP (encoding PrP^C) and STIP1 gene expression are dramatically altered in type 1 diabetic and autoantibody-positive pancreases.

CONCLUSIONS/INTERPRETATION

As the first study to address PrP^C expression in non-diabetic and type 1 diabetic human pancreas, we provide new insights for PrP^C in the pathogenesis of type 1 diabetes. We evaluated the cell-type specific expression of PrP^C in the human pancreas and discovered possible connections with potential interacting proteins that we speculate might address mechanisms relevant to the role of PrP^C in the human pancreas.

Integration of Mass Spectrometry Data for Structural Biology

Mass spectrometry (MS) is increasingly being used to probe the structure and dynamics of proteins and the complexes they form with other macromolecules. There are now several specialized MS methods, each with unique sample preparation, data acquisition, and data processing protocols. Collectively, these methods are referred to as structural MS and include cross-linking, hydrogen-deuterium exchange, hydroxyl radical footprinting, native, ion mobility, and top-down MS. Each of these provides a unique type of structural information, ranging from composition and stoichiometry through to residue level proximity and solvent accessibility. Structural MS has proved particularly beneficial in studying protein classes for which analysis by classic structural biology techniques proves challenging such as glycosylated or intrinsically disordered proteins. To capture the structural details for a particular system, especially larger multiprotein complexes, more than one structural MS method with other structural and biophysical techniques is often required. Key to integrating these diverse data are computational strategies and software solutions to facilitate this process. We provide a background to the structural MS methods and briefly summarize other structural methods and how these are combined with MS. We then describe current state of the art approaches for the integration of structural MS data for structural biology. We quantify how often these methods are used together and provide examples where such combinations have been fruitful. To illustrate the power of integrative approaches, we discuss progress in solving the structures of the proteasome and the nuclear pore complex. We also discuss how information from structural MS, particularly pertaining to protein dynamics, is not currently utilized in integrative workflows and how such information can provide a more accurate picture of the systems studied. We conclude by discussing new developments in the MS and computational fields that will further enable in-cell structural studies.

The amyloid-inhibiting NCAM-PrP peptide targets Aβ peptide aggregation in membrane-mimetic environments

Substantial research efforts have gone into elucidating the role of protein misfolding and self-assembly in the onset and progression of Alzheimer's disease (AD). Aggregation of the Amyloid-β (Aβ) peptide into insoluble fibrils is closely associated with AD. Here, we use biophysical techniques to study a peptide-based approach to target Aβ amyloid aggregation. A peptide construct, NCAM-PrP, consists of a largely hydrophobic signal sequence linked to a positively charged hexapeptide. The NCAM-PrP peptide inhibits Aβ amyloid formation by forming aggregates which are unavailable for further amyloid aggregation. In a membrane-mimetic environment, Aβ and NCAM-PrP form specific heterooligomeric complexes, which are of lower aggregation states compared to Aβ homooligomers. The Aβ:NCAM-PrP interaction appears to take place on different aggregation states depending on the absence or presence of a membrane-mimicking environment. These insights can be useful for the development of potential future therapeutic strategies targeting Aβ at several aggregation states.

The IDIP framework for assessing protein function and its application to the prion protein

The quest to determine the function of a protein can represent a profound challenge. Although this task is the mandate of countless research groups, a general framework for how it can be approached is conspicuously lacking. Moreover, even expectations for when the function of a protein can be considered to be 'known' are not well defined. In this review, we begin by introducing concepts pertinent to the challenge of protein function assignments. We then propose a framework for inferring a protein's function from four data categories: 'inheritance', 'distribution', 'interactions' and 'phenotypes' (IDIP). We document that the functions of proteins emerge at the intersection of inferences drawn from these data categories and emphasise the benefit of considering them in an evolutionary context. We then apply this approach to the cellular prion protein (PrP^C ), well known for its central role in prion diseases, whose function continues to be considered elusive by many investigators. We document that available data converge on the conclusion that the function of the prion protein is to control a critical post-translational modification of the neural cell adhesion molecule in the context of epithelial-to-mesenchymal transition and related plasticity programmes. Finally, we argue that this proposed function of PrP^C has already passed the test of time and is concordant with the IDIP framework in a way that other functions considered for this protein fail to achieve. We anticipate that the IDIP framework and the concepts analysed herein will aid the investigation of other proteins whose primary functional assignments have thus far been intractable.

Zinc transporters and their functional integration in mammalian cells

Zinc is a ubiquitous biological metal in all living organisms. The spatiotemporal zinc dynamics in cells provide crucial cellular signaling opportunities, but also challenges for intracellular zinc homeostasis with broad disease implications. Zinc transporters play a central role in regulating cellular zinc balance and subcellular zinc distributions. The discoveries of two complementary families of mammalian zinc transporters (ZnTs and ZIPs) in the mid-1990s spurred much speculation on their metal selectivity and cellular functions. After two decades of research, we have arrived at a biochemical description of zinc transport. However, in vitro functions are fundamentally different from those in living cells, where mammalian zinc transporters are directed to specific subcellular locations, engaged in dedicated macromolecular machineries, and connected with diverse cellular processes. Hence, the molecular functions of individual zinc transporters are reshaped and deeply integrated in cells to promote the utilization of zinc chemistry to perform enzymatic reactions, tune cellular responsiveness to pathophysiologic signals, and safeguard cellular homeostasis. At present, the underlying mechanisms driving the functional integration of mammalian zinc transporters are largely unknown. This knowledge gap has motivated a shift of the research focus from in vitro studies of purified zinc transporters to in cell studies of mammalian zinc transporters in the context of their subcellular locations and protein interactions. In this review, we will outline how knowledge of zinc transporters has been accumulated from in-test-tube to in-cell studies, highlighting new insights and paradigm shifts in our understanding of the molecular and cellular basis of mammalian zinc transporter functions.

The Role of Cellular Prion Protein in Promoting Stemness and Differentiation in Cancer

Simple Summary

Aside from its well-established role in prion disorders, in the last decades the significance of cellular prion protein (PrP^C) expression in human cancers has attracted great attention. An extensive body of work provided evidence that PrP^C contributes to tumorigenesis by regulating tumor growth, differentiation, and resistance to conventional therapies. In particular, PrP^C over-expression has been related to the acquisition of a malignant phenotype of cancer stem cells (CSCs) in a variety of solid tumors, encompassing pancreatic ductal adenocarcinoma, osteosarcoma, breast, gastric, and colorectal cancers, and primary brain tumors as well. According to consensus, increased levels of PrP^C endow CSCs with self-renewal, proliferative, migratory, and invasive capacities, along with increased resistance to anti-cancer agents. In addition, increasing evidence demonstrates that PrPc also participates in multi-protein complexes to modulate the oncogenic properties of CSCs, thus sustaining tumorigenesis. Therefore, strategies aimed at targeting PrP^C and/or PrP^C-organized complexes could be a promising approach for anti-cancer therapy.

Abstract

Cellular prion protein (PrP^C) is seminal to modulate a variety of baseline cell functions to grant homeostasis. The classic role of such a protein was defined as a chaperone-like molecule being able to rescue cell survival. Nonetheless, PrP^C also represents the precursor of the deleterious misfolded variant known as scrapie prion protein (PrP^Sc). This variant is detrimental in a variety of prion disorders. This multi-faceted role of PrP is greatly increased by recent findings showing how PrP^C in its folded conformation may foster tumor progression by acting at multiple levels. The present review focuses on such a cancer-promoting effect. The manuscript analyzes recent findings on the occurrence of PrP^C in various cancers and discusses the multiple effects, which sustain cancer progression. Within this frame, the effects of PrP^C on stemness and differentiation are discussed. A special emphasis is provided on the spreading of PrP^C and the epigenetic effects, which are induced in neighboring cells to activate cancer-related genes. These detrimental effects are further discussed in relation to the aberrancy of its physiological and beneficial role on cell homeostasis. A specific paragraph is dedicated to the role of PrP^C beyond its effects in the biology of cancer to represent a potential biomarker in the follow up of patients following surgical resection.

Cellular Prion Protein (PrPc): Putative Interacting Partners and Consequences of the Interaction

Cellular prion protein (PrPc) is a small glycosylphosphatidylinositol (GPI) anchored protein most abundantly found in the outer leaflet of the plasma membrane (PM) in the central nervous system (CNS). PrPc misfolding causes neurodegenerative prion diseases in the CNS. PrPc interacts with a wide range of protein partners because of the intrinsically disordered nature of the protein’s N-terminus. Numerous studies have attempted to decipher the physiological role of the prion protein by searching for proteins which interact with PrPc. Biochemical characteristics and biological functions both appear to be affected by interacting protein partners. The key challenge in identifying a potential interacting partner is to demonstrate that binding to a specific ligand is necessary for cellular physiological function or malfunction. In this review, we have summarized the intracellular and extracellular interacting partners of PrPc and potential consequences of their binding. We also briefly describe prion disease-related mutations at the end of this review.

Prion Protein at the Leading Edge: Its Role in Cell Motility

Cell motility is a central process involved in fundamental biological phenomena during embryonic development, wound healing, immune surveillance, and cancer spreading. Cell movement is complex and dynamic and requires the coordinated activity of cytoskeletal, membrane, adhesion and extracellular proteins. Cellular prion protein (PrPC) has been implicated in distinct aspects of cell motility, including axonal growth, transendothelial migration, epithelial-mesenchymal transition, formation of lamellipodia, and tumor migration and invasion. The preferential location of PrPC on cell membrane favors its function as a pivotal molecule in cell motile phenotype, being able to serve as a scaffold protein for extracellular matrix proteins, cell surface receptors, and cytoskeletal multiprotein complexes to modulate their activities in cellular movement. Evidence points to PrPC mediating interactions of multiple key elements of cell motility at the intra- and extracellular levels, such as integrins and matrix proteins, also regulating cell adhesion molecule stability and cell adhesion cytoskeleton dynamics. Understanding the molecular mechanisms that govern cell motility is critical for tissue homeostasis, since uncontrolled cell movement results in pathological conditions such as developmental diseases and tumor dissemination. In this review, we discuss the relevant contribution of PrPC in several aspects of cell motility, unveiling new insights into both PrPC function and mechanism in a multifaceted manner either in physiological or pathological contexts.

Combating Proteins with Proteins: Engineering Cell-Penetrating Peptide Antagonists of Amyloid-β Aggregation and Associated Neurotoxicity

A central event that underlies the etiology of Alzheimer's disease (AD) is the self-assembly of the amyloid-β (Aβ) peptide into aggregates termed amyloids. Increasing evidence implicates soluble prefibrillar Aβ oligomers in the neurodegeneration and synaptic dysfunction in AD. Recently we introduced a new class of highly promising antagonists of Aβ amyloidogenesis: designed cell-penetrating peptides (CPPs). These CPPs combine the attractive intrinsic properties of peptides (high target specificity and selectivity, biocompatibility, biodegradability, and ease and low cost of production) with potent therapeutic effects (inhibition of Aβ oligomerization, fiber formation, and neurotoxicity) and highly efficient delivery (to target cells and subcellular organelles).

The Quest for Cellular Prion Protein Functions in the Aged and Neurodegenerating Brain

Cellular (also termed ‘natural’) prion protein has been extensively studied for many years for its pathogenic role in prionopathies after misfolding. However, neuroprotective properties of the protein have been demonstrated under various scenarios. In this line, the involvement of the cellular prion protein in neurodegenerative diseases other than prionopathies continues to be widely debated by the scientific community. In fact, studies on knock-out mice show a vast range of physiological functions for the protein that can be supported by its ability as a cell surface scaffold protein. In this review, we first summarize the most commonly described roles of cellular prion protein in neuroprotection, including antioxidant and antiapoptotic activities and modulation of glutamate receptors. Second, in light of recently described interaction between cellular prion protein and some amyloid misfolded proteins, we will also discuss the molecular mechanisms potentially involved in protection against neurodegeneration in pathologies such as Alzheimer’s, Parkinson’s, and Huntington’s diseases.

RhoA/ROCK Regulates Prion Pathogenesis by Controlling Connexin 43 Activity

Scrapie infection, which converts cellular prion protein (PrP^C) into the pathological and infectious isoform (PrP^Sc), leads to neuronal cell death, glial cell activation and PrP^Sc accumulation. Previous studies reported that PrP^C regulates RhoA/Rho-associated kinase (ROCK) signaling and that connexin 43 (Cx43) expression is upregulated in in vitro and in vivo prion-infected models. However, whether there is a link between RhoA/ROCK and Cx43 in prion disease pathogenesis is uncertain. Here, we investigated the role of RhoA/ROCK signaling and Cx43 in prion diseases using in vitro and in vivo models. Scrapie infection induced RhoA activation, accompanied by increased phosphorylation of LIM kinase 1/2 (LIMK1/2) at Thr508/Thr505 and cofilin at Ser3 and reduced phosphorylation of RhoA at Ser188 in hippocampal neuronal cells and brains of mice. Scrapie infection-induced RhoA activation also resulted in PrP^Sc accumulation followed by a reduction in the interaction between RhoA and p190RhoGAP (a GTPase-activating protein). Interestingly, scrapie infection significantly enhanced the interaction between RhoA and Cx43. Moreover, RhoA and Cx43 colocalization was more visible in both the membrane and cytoplasm of scrapie-infected hippocampal neuronal cells than in controls. Finally, RhoA and ROCK inhibition reduced PrP^Sc accumulation and the RhoA/Cx43 interaction, leading to decreased Cx43 hemichannel activity in scrapie-infected hippocampal neuronal cells. These findings suggest that RhoA/ROCK regulates Cx43 activity, which may have an important role in the pathogenesis of prion disease.

Changes in cellular prion protein expression, processing and localisation during differentiation of the neuronal cell line CAD 5

BACKGROUND INFORMATION

Cellular prion protein (PrP^C ) is infamous for its role in prion diseases. The physiological function of PrP^C remains enigmatic, but several studies point to its involvement in cell differentiation processes. To test this possibility, we monitored PrP^C changes during the differentiation of prion-susceptible CAD 5 cells, and then we analysed the effect of PrP^C ablation on the differentiation process.

RESULTS

Neuronal CAD 5 cells differentiate within 5 days of serum withdrawal, with the majority of the cells developing long neurites. This process is accompanied by an up to sixfold increase in PrP^C expression and enhanced N-terminal β-cleavage of the protein, which suggests a role for the PrP^C in the differentiation process. Moreover, the majority of PrP^C in differentiated cells is inside the cell, and a large proportion of the protein does not associate with membrane lipid rafts. In contrast, PrP^C in proliferating cells is found mostly on the cytoplasmic membrane and is predominantly associated with lipid rafts. To determine the importance of PrP^C in cell differentiation, a CAD 5 PrP^-/- cell line with ablated PrP^C expression was created using the CRISPR/Cas9 system. We observed no considerable difference in morphology, proliferation rate or expression of molecular markers between CAD 5 and CAD 5 PrP^-/- cells during the differentiation initiated by serum withdrawal.

CONCLUSIONS

PrP^C characteristics, such as cell localisation, level of expression and posttranslational modifications, change during CAD 5 cell differentiation, but PrP^C ablation does not change the course of the differentiation process.

SIGNIFICANCE

Ablation of PrP^C expression does not affect CAD 5 cell differentiation, although we observed many intriguing changes in PrP^C features during the process. Our study does not support the concept that PrP^C is important for neuronal cell differentiation, at least in simple in vitro conditions.

Emerging Role of Cellular Prion Protein in the Maintenance and Expansion of Glioma Stem Cells

Cellular prion protein (PrP^C) is a membrane-anchored glycoprotein representing the physiological counterpart of PrP scrapie (PrP^Sc), which plays a pathogenetic role in prion diseases. Relatively little information is however available about physiological role of PrP^C. Although PrP^C ablation in mice does not induce lethal phenotypes, impairment of neuronal and bone marrow plasticity was reported in embryos and adult animals. In neurons, PrP^C stimulates neurite growth, prevents oxidative stress-dependent cell death, and favors antiapoptotic signaling. However, PrP^C activity is not restricted to post-mitotic neurons, but promotes cell proliferation and migration during embryogenesis and tissue regeneration in adult. PrP^C acts as scaffold to stabilize the binding between different membrane receptors, growth factors, and basement proteins, contributing to tumorigenesis. Indeed, ablation of PrP^C expression reduces cancer cell proliferation and migration and restores cell sensitivity to chemotherapy. Conversely, PrP^C overexpression in cancer stem cells (CSCs) from different tumors, including gliomas—the most malignant brain tumors—is predictive for poor prognosis, and correlates with relapses. The mechanisms of the PrP^C role in tumorigenesis and its molecular partners in this activity are the topic of the present review, with a particular focus on PrP^C contribution to glioma CSCs multipotency, invasiveness, and tumorigenicity.

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.

Prion Protein in Glioblastoma Multiforme

The cellular prion protein (PrPc) is an evolutionarily conserved cell surface protein encoded by the PRNP gene. PrPc is ubiquitously expressed within nearly all mammalian cells, though most abundantly within the CNS. Besides being implicated in the pathogenesis and transmission of prion diseases, recent studies have demonstrated that PrPc contributes to tumorigenesis by regulating tumor growth, differentiation, and resistance to conventional therapies. In particular, PrPc over-expression has been related to the acquisition of a malignant phenotype of cancer stem cells (CSCs) in a variety of solid tumors, encompassing pancreatic ductal adenocarcinoma (PDAC), osteosarcoma, breast cancer, gastric cancer, and primary brain tumors, mostly glioblastoma multiforme (GBM). Thus, PrPc is emerging as a key in maintaining glioblastoma cancer stem cells’ (GSCs) phenotype, thereby strongly affecting GBM infiltration and relapse. In fact, PrPc contributes to GSCs niche’s maintenance by modulating GSCs’ stem cell-like properties while restraining them from differentiation. This is the first review that discusses the role of PrPc in GBM. The manuscript focuses on how PrPc may act on GSCs to modify their expression and translational profile while making the micro-environment surrounding the GSCs niche more favorable to GBM growth and infiltration.

Pro-Inflammatory S100A9 Protein Aggregation Promoted by NCAM1 Peptide Constructs

Amyloid cascade and neuroinflammation are hallmarks of neurodegenerative diseases, and pro-inflammatory S100A9 protein is central to both of them. Here, we have shown that NCAM1 peptide constructs carrying polycationic sequences derived from Aβ peptide (KKLVFF) and PrP protein (KKRPKP) significantly promote the S100A9 amyloid self-assembly in a concentration-dependent manner by making transient interactions with individual S100A9 molecules, perturbing its native structure and acting as catalysts. Since the individual molecule misfolding is a rate-limiting step in S100A9 amyloid aggregation, the effects of the NCAM1 construct on the native S100A9 are so critical for its amyloid self-assembly. S100A9 rapid self-assembly into large aggregated clumps may prevent its amyloid tissue propagation, and by modulating S100A9 aggregation as a part of the amyloid cascade, the whole process may be effectively tuned.

Altered Domain Structure of the Prion Protein Caused by Cu2+ Binding and Functionally Relevant Mutations: Analysis by Cross-Linking, MS/MS, and NMR

The cellular isoform of the prion protein (PrP^C) serves as precursor to the infectious isoform (PrP^Sc), and as a cell-surface receptor, which binds misfolded protein oligomers as well as physiological ligands such as Cu²⁺ ions. PrP^C consists of two domains: a flexible N-terminal domain and a structured C-terminal domain. Both the physiological and pathological functions of PrP depend on intramolecular interactions between these two domains, but the specific amino acid residues involved have proven challenging to define. Here, we employ a combination of chemical cross-linking, mass spectrometry, NMR, molecular dynamics simulations, and functional assays to identify residue-level contacts between the N- and C-terminal domains of PrP^C. We also determine how these interdomain contacts are altered by binding of Cu²⁺ ions and by functionally relevant mutations. Our results provide a structural basis for interpreting both the normal and toxic activities of PrP.

Cellular and Molecular Mechanisms of Prion Disease

Prion diseases are rapidly progressive, incurable neurodegenerative disorders caused by misfolded, aggregated proteins known as prions, which are uniquely infectious. Remarkably, these infectious proteins have been responsible for widespread disease epidemics, including kuru in humans, bovine spongiform encephalopathy in cattle, and chronic wasting disease in cervids, the latter of which has spread across North America and recently appeared in Norway and Finland. The hallmark histopathological features include widespread spongiform encephalopathy, neuronal loss, gliosis, and deposits of variably sized aggregated prion protein, ranging from small, soluble oligomers to long, thin, unbranched fibrils, depending on the disease. Here, we explore recent advances in prion disease research, from the function of the cellular prion protein to the dysfunction triggering neurotoxicity, as well as mechanisms underlying prion spread between cells. We also highlight key findings that have revealed new therapeutic targets and consider unanswered questions for future research.

Structural Determinants of the Prion Protein N-Terminus and Its Adducts with Copper Ions

The N-terminus of the prion protein is a large intrinsically disordered region encompassing approximately 125 amino acids. In this paper, we review its structural and functional properties, with a particular emphasis on its binding to copper ions. The latter is exploited by the region’s conformational flexibility to yield a variety of biological functions. Disease-linked mutations and proteolytic processing of the protein can impact its copper-binding properties, with important structural and functional implications, both in health and disease progression.

A Bioluminescent Cell Assay to Quantify Prion Protein Dimerization

The prion protein (PrP) is a cell surface protein that in disease misfolds and becomes infectious causing Creutzfeldt-Jakob disease in humans, scrapie in sheep, and chronic wasting disease in deer and elk. Little is known regarding the dimerization of PrP and its role in disease. We developed a bioluminescent prion assay (BPA) to quantify PrP dimerization by bimolecular complementation of split Gaussia luciferase (GLuc) halves that are each fused to PrP. Fusion constructs between PrP and N- and C-terminal GLuc halves were expressed on the surface of RK13 cells (RK13-DC cells) and dimerized to yield a bioluminescent signal that was decreased in the presence of eight different antibodies to PrP. Dimerization of PrP was independent of divalent cations and was induced under stress. Challenge of RK13-DC cells with seven different prion strains did not lead to detectable infection but was measurable by bioluminescence. Finally, we used BPA to screen a compound library for compounds inhibiting PrP dimerization. One of the most potent compounds to inhibit PrP dimerization was JTC-801, which also inhibited prion replication in RML-infected ScN2a and SMB cells with an EC₅₀ of 370 nM and 220 nM, respectively. We show here that BPA is a versatile tool to study prion biology and to identify anti-prion compounds.

Prions activate a p38 MAPK synaptotoxic signaling pathway

Synaptic degeneration is one of the earliest pathological correlates of prion disease, and it is a major determinant of the progression of clinical symptoms. However, the cellular and molecular mechanisms underlying prion synaptotoxicity are poorly understood. Previously, we described an experimental system in which treatment of cultured hippocampal neurons with purified PrP^Sc, the infectious form of the prion protein, induces rapid retraction of dendritic spines, an effect that is entirely dependent on expression of endogenous PrP^C by the target neurons. Here, we use this system to dissect pharmacologically the underlying cellular and molecular mechanisms. We show that PrP^Sc initiates a stepwise synaptotoxic signaling cascade that includes activation of NMDA receptors, calcium influx, stimulation of p38 MAPK and several downstream kinases, and collapse of the actin cytoskeleton within dendritic spines. Synaptic degeneration is restricted to excitatory synapses, spares presynaptic structures, and results in decrements in functional synaptic transmission. Pharmacological inhibition of any one of the steps in the signaling cascade, as well as expression of a dominant-negative form of p38 MAPK, block PrP^Sc-induced spine degeneration. Moreover, p38 MAPK inhibitors actually reverse the degenerative process after it has already begun. We also show that, while PrP^C mediates the synaptotoxic effects of both PrP^Sc and the Alzheimer’s Aβ peptide in this system, the two species activate distinct signaling pathways. Taken together, our results provide powerful insights into the biology of prion neurotoxicity, they identify new, druggable therapeutic targets, and they allow comparison of prion synaptotoxic pathways with those involved in other neurodegenerative diseases.

Prion diseases are a group of fatal neurodegenerative disorders that includes Creutzfeldt-Jakob disease and kuru in humans, and bovine spongiform encephalopathy in cattle. The infectious agent, or prion, that transmits these diseases is a naked protein molecule, the prion protein (PrP), which is an altered form of a normal, cellular protein. Although a great deal is known about how prions propagate themselves and transmit infection, the process by which they actually cause neurons to degenerate has remained mysterious. Here, we have used a specialized neuronal culture system to dissect the cellular and molecular mechanisms by which prions damage synapses, the structures that connect nerve cells and that play a crucial role in learning, memory, and neurological disease. Our results define a stepwise molecular pathway underlying prion synaptic toxicity, which involves activation of glutamate neurotransmitter receptors, influx of calcium ions into the neuron, and stimulation of specific mitogen-activated protein kinases, which attach phosphate groups to proteins to regulate their activity. We demonstrate that specific drugs, as well as a dominant-negative kinase mutant, block these steps and thereby prevent the synaptic degeneration produced by prions. Our results provide new insights into the pathogenesis of prion diseases, they uncover new drug targets for treating these diseases, and they allow us to compare prion diseases to other, more common neurodegenerative disorders like Alzheimer’s disease.

Prion gene paralogs are dispensable for early zebrafish development and have nonadditive roles in seizure susceptibility

Normally folded prion protein (PrP^C) and its functions in healthy brains remain underappreciated compared with the intense study of its misfolded forms ("prions," PrP^Sc) during the pathobiology of prion diseases. This impedes the development of therapeutic strategies in Alzheimer's and prion diseases. Disrupting the zebrafish homologs of PrP^C has provided novel insights; however, mutagenesis of the zebrafish paralog prp2 did not recapitulate previous dramatic developmental phenotypes, suggesting redundancy with the prp1 paralog. Here, we generated zebrafish prp1 loss-of-function mutant alleles and dual prp1^-/-;prp2^-/- mutants. Zebrafish prp1^-/- and dual prp1^-/-;prp2^-/- mutants resemble mammalian Prnp knockouts insofar as they lack overt phenotypes, which surprisingly contrasts with reports of severe developmental phenotypes when either prp1 or prp2 is knocked down acutely. Previous studies suggest that PrP^C participates in neural cell development/adhesion, including in zebrafish where loss of prp2 affects adhesion and deposition patterns of lateral line neuromasts. In contrast with the expectation that prp1's functions would be redundant to prp2, they appear to have opposing functions in lateral line neurodevelopment. Similarly, loss of prp1 blunted the seizure susceptibility phenotypes observed in prp2 mutants, contrasting the expected exacerbation of phenotypes if these prion gene paralogs were serving redundant roles. In summary, prion mutant fish lack the overt phenotypes previously predicted, and instead they have subtle phenotypes similar to mammals. No evidence was found for functional redundancy in the zebrafish prion gene paralogs, and the phenotypes observed when each gene is disrupted individually are consistent with ancient functions of prion proteins in neurodevelopment and modulation of neural activity.

The prion protein is embedded in a molecular environment that modulates transforming growth factor β and integrin signaling

At times, it can be difficult to discern if a lack of overlap in reported interactions for a protein-of-interest reflects differences in methodology or biology. In such instances, systematic analyses of protein-protein networks across diverse paradigms can provide valuable insights. Here, we interrogated the interactome of the prion protein (PrP), best known for its central role in prion diseases, in four mouse cell lines. Analyses made use of identical affinity capture and sample processing workflows. Negative controls were generated from PrP knockout lines of the respective cell models, and the relative levels of peptides were quantified using isobaric labels. The study uncovered 26 proteins that reside in proximity to PrP. All of these proteins are predicted to have access to the outer face of the plasma membrane, and approximately half of them were not reported to interact with PrP before. Strikingly, although several proteins exhibited profound co-enrichment with PrP in a given model, except for the neural cell adhesion molecule 1, no protein was highly enriched in all PrP-specific interactomes. However, Gene Ontology analyses revealed a shared association of the majority of PrP candidate interactors with cellular events at the intersection of transforming growth factor β and integrin signaling.

Structural and mechanistic aspects influencing the ADAM10-mediated shedding of the prion protein

Background

Proteolytic processing of the prion protein (PrPC) by endogenous proteases generates bioactive membrane-bound and soluble fragments which may help to explain the pleiotropic roles of this protein in the nervous system and in brain diseases. Shedding of almost full-length PrPC into the extracellular space by the metalloprotease ADAM10 is of peculiar relevance since soluble PrP stimulates axonal outgrowth and is protective in neurodegenerative conditions such as Alzheimer’s and prion disease. However, molecular determinates and mechanisms regulating the shedding of PrP are entirely unknown.

Methods

We produced an antibody recognizing the neo-epitope of shed PrP generated by ADAM10 in biological samples and used it to study structural and mechanistic aspects affecting the shedding. For this, we investigated genetically modified cellular and murine models by biochemical and morphological approaches.

Results

We show that the novel antibody specifically detects shed PrP in cell culture supernatants and murine brain. We demonstrate that ADAM10 is the exclusive sheddase of PrPC in the nervous system and reveal that the glycosylation state and type of membrane-anchorage of PrPC severely affect its shedding. Furthermore, we provide evidence that PrP shedding can be modulated by pharmacological inhibition and stimulation and present data suggesting that shedding is a relevant part of a compensatory network ensuring PrPC homeostasis of the cell.

Conclusions

With the new antibody, our study introduces a new tool to reliably investigate PrP-shedding. In addition, this study provides novel and important insight into the regulation of this cleavage event, which is likely to be relevant for diagnostic and therapeutic approaches even beyond neurodegeneration.

The function of the cellular prion protein in health and disease

The essential role of the cellular prion protein (PrP^C) in prion disorders such as Creutzfeldt-Jakob disease is well documented. Moreover, evidence is accumulating that PrP^C may act as a receptor for protein aggregates and transduce neurotoxic signals in more common neurodegenerative disorders, such as Alzheimer's disease. Although the pathological roles of PrP^C have been thoroughly characterized, a general consensus on its physiological function within the brain has not yet been established. Knockout studies in various organisms, ranging from zebrafish to mice, have implicated PrP^C in a diverse range of nervous system-related activities that include a key role in the maintenance of peripheral nerve myelination as well as a general ability to protect against neurotoxic stimuli. Thus, the function of PrP^C may be multifaceted, with different cell types taking advantage of unique aspects of its biology. Deciphering the cellular function(s) of PrP^C and the consequences of its absence is not simply an academic curiosity, since lowering PrP^C levels in the brain is predicted to be a powerful therapeutic strategy for the treatment of prion disease. In this review, we outline the various approaches that have been employed in an effort to uncover the physiological and pathological functions of PrP^C. While these studies have revealed important clues about the biology of the prion protein, the precise reason for PrP^C's existence remains enigmatic.

An inter-domain regulatory mechanism controls toxic activities of PrPC

The normal function of PrP^C, the cellular prion protein, has remained mysterious since its first description over 30 years ago. Amazingly, although complete deletion of the gene encoding PrP^C has little phenotypic consequence, expression in transgenic mice of PrP molecules carrying certain internal deletions produces dramatic neurodegenerative phenotypes. In our recent paper, ¹ we have demonstrated that the flexible, N-terminal domain of PrP^C possesses toxic effector functions, which are regulated by a docking interaction with the structured, C-terminal domain. Disruption of this inter-domain interaction, for example by deletions of the hinge region or by binding of antibodies to the C-terminal domain, results in abnormal ionic currents and degeneration of dendritic spines in cultured neuronal cells. This mechanism may contribute to the neurotoxicity of PrP^Sc and possibly other protein aggregates, and could play a role in the physiological activity of PrP^C. These results also provide a warning about the potential toxic side effects of PrP-directed antibody therapies for prion and Alzheimer's diseases.

Prion protein is required for tumor necrosis factor α (TNFα)-triggered nuclear factor κB (NF-κB) signaling and cytokine production

The expression of normal cellular prion protein (PrP) is required for the pathogenesis of prion diseases. However, the physiological functions of PrP remain ambiguous. Here, we identified PrP as being critical for tumor necrosis factor (TNF) α-triggered signaling in a human melanoma cell line, M2, and a pancreatic ductal cell adenocarcinoma cell line, BxPC-3. In M2 cells, TNFα up-regulates the expression of p-IκB-kinase α/β (p-IKKα/β), p-p65, and p-JNK, but down-regulates the IκBα protein, all of which are downstream signaling intermediates in the TNF receptor signaling cascade. When PRNP is deleted in M2 cells, the effects of TNFα are no longer detectable. More importantly, p-p65 and p-JNK responses are restored when PRNP is reintroduced into the PRNP null cells. TNFα also activates NF-κB and increases TNFα production in wild-type M2 cells, but not in PrP-null M2 cells. Similar results are obtained in the BxPC-3 cells. Moreover, TNFα activation of NF-κB requires ubiquitination of receptor-interacting serine/threonine kinase 1 (RIP1) and TNF receptor-associated factor 2 (TRAF2). TNFα treatment increases the binding between PrP and the deubiquitinase tumor suppressor cylindromatosis (CYLD), in these treated cells, binding of CYLD to RIP1 and TRAF2 is reduced. We conclude that PrP traps CYLD, preventing it from binding and deubiquitinating RIP1 and TRAF2. Our findings reveal that PrP enhances the responses to TNFα, promoting proinflammatory cytokine production, which may contribute to inflammation and tumorigenesis.

Role of Prion protein in premature senescence of human fibroblasts

Prion protein (PrP) is essentially known for its capacity to induce neurodegenerative prion diseases in mammals caused by a conformational change in its normal cellular isoform (PrP^C) into an infectious and disease-associated misfolded form, called scrapie isoform (PrP^Sc). Although its sequence is highly conserved, less information is available on its physiological role under normal conditions. However, increasing evidence supports a role for PrP^C in the cellular response to oxidative stress. In the present study, a new link between PrP and senescence is highlighted. The role of PrP in premature senescence induced by copper was investigated. WI-38 human fibroblasts were incubated with copper sulfate (CuSO₄) to trigger premature senescence. This induced an increase of PrP mRNA level, an increase of protein abundance of the normal form of PrP and a nuclear localization of the protein. Knockdown of PrP expression using specific small interfering RNA (siRNA) gave rise to appearance of several biomarkers of senescence as a senescent morphology, an increase of senescence associated β-galactosidase activity and a decrease of the cellular proliferative potential. Overall these data suggest that PrP protects cells against premature senescence induced by copper.

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.

Functions of the Prion Protein

Although initially disregarded compared to prion pathogenesis, the functions exerted by the cellular prion protein PrP^C have gained much interest over the past two decades. Research aiming at unraveling PrP^C functions started to intensify when it became appreciated that it would give clues as to how it is subverted in the context of prion infection and, more recently, in the context of Alzheimer's disease. It must now be admitted that PrP^C is implicated in an incredible variety of biological processes, including neuronal homeostasis, stem cell fate, protection against stress, or cell adhesion. It appears that these diverse roles can all be fulfilled through the involvement of PrP^C in cell signaling events. Our aim here is to provide an overview of our current understanding of PrP^C functions from the animal to the molecular scale and to highlight some of the remaining gaps that should be addressed in future research.