Different tau fibril types reduce prion level in chronically and de novo infected cells.

Neurodegenerative diseases are often characterized by the codeposition of different amyloidogenic proteins, normally defining distinct proteinopathies. An example is represented by prion diseases, where the classical deposition of the aberrant conformational isoform of the prion protein (PrPSc) can be associated with tau insoluble species, which are usually involved in another class of diseases called tauopathies. How this copresence of amyloidogenic proteins can influence the progression of prion diseases is still a matter of debate. Recently, the cellular form of the prion protein, PrPC, has been investigated as a possible receptor of amyloidogenic proteins, since its binding activity with Aß, tau, and α-synuclein has been reported, and it has been linked to several neurotoxic behaviors exerted by these proteins. We have previously shown that the treatment of chronically prion-infected cells with tau K18 fibrils reduced PrPSc levels. In this work, we further explored this mechanism by using another tau construct that includes the sequence that forms the core of Alzheimer's disease tau filaments in vivo to obtain a distinct fibril type. Despite a difference of six amino acids, these two constructs form fibrils characterized by distinct biochemical and biological features. However, their effects on PrPSc reduction were comparable and probably based on the binding to PrPC at the plasma membrane, inhibiting the pathological conversion event. Our results suggest PrPC as receptor for different types of tau fibrils and point out a role of tau amyloid fibrils in preventing the pathological PrPC to PrPSc conformational change.

An optimized Western blot assay provides a comprehensive assessment of the physiological endoproteolytic processing of the prion protein.

The prion protein (PrPC) is subjected to several conserved endoproteolytic events producing bioactive fragments that are of increasing interest for their physiological functions and their implication in the pathogenesis of prion diseases and other neurodegenerative diseases. However, systematic and comprehensive investigations on the full spectrum of PrPC proteoforms have been hampered by the lack of methods able to identify all PrPC-derived proteoforms. Building on previous knowledge of PrPC endoproteolytic processing, we thus developed an optimized Western blot assay able to obtain the maximum information about PrPC constitutive processing and the relative abundance of PrPC proteoforms in a complex biological sample. This approach led to the concurrent identification of the whole spectrum of known endoproteolytic-derived PrPC proteoforms in brain homogenates, including C-terminal, N-terminal and, most importantly, shed PrPC-derived fragments. Endoproteolytic processing of PrPC was remarkably similar in the brain of widely used wild type and transgenic rodent models, with α-cleavage-derived C1 representing the most abundant proteoform and ADAM10-mediated shedding being an unexpectedly prominent proteolytic event. Interestingly, the relative amount of shed PrPC was higher in WT mice than in most other models. Our results indicate that constitutive endoproteolytic processing of PrPC is not affected by PrPC overexpression or host factors other than PrPC but can be impacted by PrPC primary structure. Finally, this method represents a crucial step in gaining insight into pathophysiological roles, biomarker suitability, and therapeutic potential of shed PrPC and for a comprehensive appraisal of PrPC proteoforms in therapies, drug screening, or in the progression of neurodegenerative diseases.

Cell biology of prion strains in vivo and in vitro.

The properties of infectious prions and the pathology of the diseases they cause are dependent upon the unique conformation of each prion strain. How the pathology of prion disease correlates with different strains and genetic backgrounds has been investigated via in vivo assays, but how interactions between specific prion strains and cell types contribute to the pathology of prion disease has been dissected more effectively using in vitro cell lines. Observations made through in vivo and in vitro assays have informed each other with regard to not only how genetic variation influences prion properties, but also how infectious prions are taken up by cells, modified by cellular processes and propagated, and the cellular components they rely on for persistent infection. These studies suggest that persistent cellular infection results from a balance between prion propagation and degradation. This balance may be shifted depending upon how different cell lines process infectious prions, potentially altering prion stability, and how fast they can be transported to the lysosome. Thus, in vitro studies have given us a deeper understanding of the interactions between different prions and cell types and how they may influence prion disease phenotypes in vivo.

The Role of Cellular Prion Protein in Glioma Tumorigenesis Could Be through the Autophagic Mechanisms: A Narrative Review.

The carcinogenesis of glial tumors appears complex because of the many genetic and epigenetic phenomena involved. Among these, cellular prion protein (PrPC) is considered a key factor in cell-death resistance and important aspect implicated in tumorigenesis. Autophagy also plays an important role in cell death in various pathological conditions. These two cellular phenomena are related and share the same activation by specific alterations in the cellular microenvironment. Furthermore, there is an interdependence between autophagy and prion activity in glioma tumorigenesis. Glioma is one of the most aggressive known cancers, and the fact that such poorly studied processes as autophagy and PrPC activity are so strongly involved in its carcinogenesis suggests that by better understanding their interaction, more can be understood about its origin and treatment. Few studies in the literature relate these two cellular phenomena, much less try to explain their combined activity and role in glioma carcinogenesis. In this study, we explored the recent findings on the molecular mechanism and regulation pathways of autophagy, examining the role of PrPC in autophagy processes and how they may play a central role in glioma tumorigenesis. Among the many molecular interactions that PrP physiologically performs, it appears that processes shared with autophagy activity are those most implicated in glial tumor carcinogeneses such as activity on MAP kinases, PI3K, and mTOR. This work can be supportive and valuable as a basis for further future studies on this topic.

Sigma-2 Receptors-From Basic Biology to Therapeutic Target: A Focus on Age-Related Degenerative Diseases.

There is a large unmet medical need to develop disease-modifying treatment options for individuals with age-related degenerative diseases of the central nervous system. The sigma-2 receptor (S2R), encoded by TMEM97, is expressed in brain and retinal cells, and regulates cell functions via its co-receptor progesterone receptor membrane component 1 (PGRMC1), and through other protein-protein interactions. Studies describing functions of S2R involve the manipulation of expression or pharmacological modulation using exogenous small-molecule ligands. These studies demonstrate that S2R modulates key pathways involved in age-related diseases including autophagy, trafficking, oxidative stress, and amyloid-ß and α-synuclein toxicity. Furthermore, S2R modulation can ameliorate functional deficits in cell-based and animal models of disease. This review summarizes the current evidence-based understanding of S2R biology and function, and its potential as a therapeutic target for age-related degenerative diseases of the central nervous system, including Alzheimer's disease, α-synucleinopathies, and dry age-related macular degeneration.

Titanium dioxide and carbon black nanoparticles disrupt neuronal homeostasis via excessive activation of cellular prion protein signaling.

BACKGROUND: Epidemiological emerging evidence shows that human exposure to some nanosized materials present in the environment would contribute to the onset and/or progression of Alzheimer's disease (AD). The cellular and molecular mechanisms whereby nanoparticles would exert some adverse effects towards neurons and take part in AD pathology are nevertheless unknown. RESULTS: Here, we provide the prime evidence that titanium dioxide (TiO2) and carbon black (CB) nanoparticles (NPs) bind the cellular form of the prion protein (PrPC), a plasma membrane protein well known for its implication in prion diseases and prion-like diseases, such as AD. The interaction between TiO2- or CB-NPs and PrPC at the surface of neuronal cells grown in culture corrupts PrPC signaling function. This triggers PrPC-dependent activation of NADPH oxidase and subsequent production of reactive oxygen species (ROS) that alters redox equilibrium. Through PrPC interaction, NPs also promote the activation of 3-phosphoinositide-dependent kinase 1 (PDK1), which in turn provokes the internalization of the neuroprotective TACE α-secretase. This diverts TACE cleavage activity away from (i) TNFα receptors (TNFR), whose accumulation at the plasma membrane augments the vulnerability of NP-exposed neuronal cells to TNFα -associated inflammation, and (ii) the amyloid precursor protein APP, leading to overproduction of neurotoxic amyloid Aß40/42 peptides. The silencing of PrPC or the pharmacological inhibition of PDK1 protects neuronal cells from TiO2- and CB-NPs effects regarding ROS production, TNFα hypersensitivity, and Aß rise. Finally, we show that dysregulation of the PrPC-PDK1-TACE pathway likely occurs in the brain of mice injected with TiO2-NPs by the intra-cerebro-ventricular route as we monitor a rise of TNFR at the cell surface of several groups of neurons located in distinct brain areas. CONCLUSION: Our in vitro and in vivo study thus posits for the first time normal cellular prion protein PrPC as being a neuronal receptor of TiO2- and CB-NPs and identifies PrPC-coupled signaling pathways by which those nanoparticles alter redox equilibrium, augment the intrinsic sensitivity of neurons to neuroinflammation, and provoke a rise of Aß peptides. By identifying signaling cascades dysregulated by TiO2- and CB-NPs in neurons, our data shed light on how human exposure to some NPs might be related to AD.

A soluble derivative of PrPC activates cell-signaling and regulates cell physiology through LRP1 and the NMDA receptor.

Cellular prion protein (PrPC) is a widely expressed glycosylphosphatidylinositol-anchored membrane protein. Scrapie prion protein is a misfolded and aggregated form of PrPC responsible for prion-induced neurodegenerative diseases. Understanding the function of the nonpathogenic PrPC monomer is an important objective. PrPC may be shed from the cell surface to generate soluble derivatives. Herein, we studied a recombinant derivative of PrPC (soluble cellular prion protein, S-PrP) that corresponds closely in sequence to a soluble form of PrPC shed from the cell surface by proteases in the A Disintegrin And Metalloprotease (ADAM) family. S-PrP activated cell-signaling in PC12 and N2a cells. TrkA was transactivated by Src family kinases and extracellular signal-regulated kinase 1/2 was activated downstream of Trk receptors. These cell-signaling events were dependent on the N-methyl-d-aspartate receptor (NMDA-R) and low-density lipoprotein receptor-related protein-1 (LRP1), which functioned as a cell-signaling receptor system in lipid rafts. Membrane-anchored PrPC and neural cell adhesion molecule were not required for S-PrP-initiated cell-signaling. S-PrP promoted PC12 cell neurite outgrowth. This response required the NMDA-R, LRP1, Src family kinases, and Trk receptors. In Schwann cells, S-PrP interacted with the LRP1/NMDA-R system to activate extracellular signal-regulated kinase 1/2 and promote cell migration. The effects of S-PrP on PC12 cell neurite outgrowth and Schwann cell migration were similar to those caused by other proteins that engage the LRP1/NMDA-R system, including activated α2-macroglobulin and tissue-type plasminogen activator. Collectively, these results demonstrate that shed forms of PrPC may exhibit important biological activities in the central nervous system and the peripheral nervous system by serving as ligands for the LRP1/NMDA-R system.

ADAM10 is involved in the oncogenic process and chemo-resistance of triple-negative breast cancer via regulating Notch1 signaling pathway, CD44 and PrPc.

BACKGROUND: Triple-negative breast cancer (TNBC) is the most challenging breast cancer subtype to treat, because it is so aggressive with shorter survival. Chemotherapy remains the standard treatment due to the lack of specific and effective molecular targets. The aim of the present study is to investigate the potential roles of A Disintegrin and Metalloproteinase 10 (ADAM10) on TNBC cells and the effects of combining ADAM10 expression and neoadjuvant chemotherapy treatment (NACT) to improve the overall survival in breast cancer patients. METHODS: Using a series of breast cancer cell lines, we measured the expression of ADAM10 and its substrates by quantitative real-time PCR assay (qRT-PCR) and western blot analysis. Cell migration and invasion, cell proliferation, drug sensitivity assay, cell cycle and apoptosis were conducted in MDA-MB-231 cells cultured with ADAM10 siRNA. The effect of ADAM10 down-regulation by siRNA on its substrates was assessed by western blot analysis. We performed immunohistochemical staining for ADAM10 in clinical breast cancer tissues in 94 patients receiving NACT. RESULTS: The active form of ADAM10 was highly expressed in TNBC cell lines. Knockdown of ADAM10 in MDA-MB-231 cells led to a significant decrease in cell proliferation, migration, invasion and the IC50 value of paclitaxel and adriamycin, while induced cell cycle arrest and apoptosis. And these changes were correlated with down-regulation of Notch signaling, CD44 and cellular prion protein (PrPc). In clinical breast cancer cases, a high ADAM10 expression in pre-NACT samples was strongly associated with poorer response to NACT and shorter overall survival. CONCLUSIONS: These data suggest the previously unrecognized roles of ADAM10 in contributing to the progression and chemo-resistance of TNBC.

PrPC Aptamer Conjugated-Gold Nanoparticles for Targeted Delivery of Doxorubicin to Colorectal Cancer Cells.

Anticancer drugs, such as fluorouracil (5-FU), oxaliplatin, and doxorubicin (Dox) are commonly used to treat colorectal cancer (CRC); however, owing to their low response rate and adverse effects, the development of efficient drug delivery systems (DDSs) is required. The cellular prion protein PrPC, which is a cell surface glycoprotein, has been demonstrated to be overexpressed in CRC, however, there has been no research on the development of PrPC-targeting DDSs for targeted drug delivery to CRC. In this study, PrPC aptamer (Apt)-conjugated gold nanoparticles (AuNPs) were synthesized for targeted delivery of Dox to CRC. Thiol-terminated PrPC-Apt was conjugated to AuNPs, followed by hybridization of its complementary DNA for drug loading. Finally, Dox was loaded onto the AuNPs to synthesize PrPC-Apt-functionalized doxorubicin-oligomer-AuNPs (PrPC-Apt DOA). The PrPC-Apt DOA were spherical nanoparticles with an average diameter of 20 nm. Treatment of CRC cells with PrPC-Apt DOA induced reactive oxygen species generation by decreasing catalase and superoxide dismutase activities. In addition, treatment with PrPC-Apt DOA inhibited mitochondrial functions by decreasing the expression of peroxisome proliferator-activated receptor gamma coactivator 1-alpha, complex 4 activity, and oxygen consumption rates. Compared to free Dox, PrPC-Apt DOA decreased proliferation and increased apoptosis of CRC cells to a greater degree. In this study, we demonstrated that PrPC-Apt DOA targeting could effectively deliver Dox to CRC cells. PrPC-Apt DOA can be used as a treatment for CRC, and have the potential to replace existing anticancer drugs, such as 5-FU, oxaliplatin, and Dox.

Interactions of Amyloid-ß with Membrane Proteins.

In developing and developed countries, an increasing elderly population is observed. This affects the growing percentage of people struggling with neurodegenerative diseases, including Alzheimer's disease. Nevertheless, the pathomechanism of this disease is still unknown. This contributes to problems with early diagnosis of the disease as well as with treatment. One of the most popular hypotheses of Alzheimer's disease is related to the pathological deposition of amyloid-ß (Aß) in the brain of ill people. In this paper, we discuss issues related to Aß and its relationship in the development of Alzheimer's disease. The structure of Aß and its interaction with the cell membrane are discussed. Not only do the extracellular plaques affect nerve cells, but other forms of this peptide as well.

Application of high-throughput, capillary-based Western analysis to modulated cleavage of the cellular prion protein.

The cellular prion protein (PrPC) is a glycoprotein that is processed through several proteolytic pathways. Modulators of PrPC proteolysis are of interest because full-length PrPC and its cleavage fragments differ in their propensity to misfold, a process that plays a key role in the pathogenesis of prion diseases. PrPC may also act as a receptor for neurotoxic, oligomeric species of other proteins that are linked to neurodegeneration. Importantly, the PrPC C-terminal fragment C1 does not contain the reported binding sites for these oligomers. Western blotting would be a simple end point detection method for cell-based screening of compound libraries for effects on PrPC proteolysis or overall expression level. However, traditional Western blotting methods provide unreliable quantification and have only low throughput. Consequently, we explored capillary-based Western technology as a potential alternative; we believe that this study is the first to report analysis of PrPC using such an approach. We successfully optimized the detection and quantification of the deglycosylated forms of full-length PrPC and its C-terminal cleavage fragments C1 and C2, including simultaneous quantification of ß-tubulin levels to control for loading error. We also developed and tested a method for performing all cell culture, lysis, and deglycosylation steps in 96-well microplates prior to capillary Western analysis. These advances represent steps along the way to the development of an automated, high-throughput screening pipeline to identify modulators of PrPC expression levels or proteolysis.

Systematic and standardized comparison of reported amyloid-ß receptors for sufficiency, affinity, and Alzheimer's disease relevance.

Oligomeric assemblies of amyloid-ß (Aß) peptide (Aßo) in the brains of individuals with Alzheimer's disease (AD) are toxic to neuronal synapses. More than a dozen Aß receptor candidates have been suggested to be responsible for various aspects of the molecular pathology and memory impairment in mouse models of AD. A lack of consistent experimental design among previous studies of different receptor candidates limits evaluation of the relative roles of these candidates, producing some controversy within the field. Here, using cell-based assays with several Aß species, including Aßo from AD brains obtained by autopsy, we directly compared the Aß-binding capacity of multiple receptor candidates while accounting for variation in expression and confirming cell surface expression. In a survey of 15 reported Aß receptors, only cellular prion protein (PrPC), Nogo receptor 1 (NgR1), and leukocyte immunoglobulin-like receptor subfamily B member 2 (LilrB2) exhibited direct binding to synaptotoxic assemblies of synthetic Aß. Both PrPC and NgR1 preferentially bound synaptotoxic oligomers rather than nontoxic monomers, and the method of oligomer preparation did not significantly alter our binding results. Hippocampal neurons lacking both NgR1 and LilrB2 exhibited a partial reduction of Aßo binding, but this reduction was lower than in neurons lacking PrPC under the same conditions. Finally, binding studies with soluble Aßo from human AD brains revealed a strong affinity for PrPC, weak affinity for NgR1, and no detectable affinity for LilrB2. These findings clarify the relative contributions of previously reported Aß receptors under controlled conditions and highlight the prominence of PrPC as an Aß-binding site.

Fibrinogen-cellular prion protein complex formation on astrocytes.

Traumatic brain injury (TBI) is one of the most common neurological disorders causing memory reduction, particularly short-term memory (STM). We showed that, during TBI-induced inflammation, increased blood content of fibrinogen (Fg) enhanced vascular protein transcytosis and deposition of extravasated Fg in vasculo-astrocyte interfaces. In addition, we found that deposition of cellular prion protein (PrPC) was also increased in the vasculo-astrocyte endfeet interface. However, association of Fg and PrPC was not confirmed. Presently, we aimed to define whether Fg can associate with PrPC on astrocytes and cause their activation. Cultured mouse brain astrocytes were treated with medium alone (control), Fg (2 mg/mL or 4 mg/mL), 4 mg/mL of Fg in the presence of a function-blocking anti-PrPC peptide or anti-mouse IgG, function-blocking anti-PrPC peptide, or anti-mouse IgG alone. After treatment, either cell lysates were collected and analyzed via Western blot or coimmunoprecipitation was performed, or astrocytes were fixed and their activation was assessed with immunohistochemistry. Results showed that Fg dose-dependently activated astrocytes, increased expressions of PrPC and tyrosine (tropomyosin) receptor kinase B (TrkB), and PrP gene. Blocking the function of PrPC reduced these effects. Coimmunoprecipitation demonstrated Fg and PrPC association. Since it is known that prion protein has a greater effect on memory reduction than amyloid beta, and that activation of TrkB is involved in neurodegeneration, our findings confirming the possible formation of Fg-PrPC and Fg-induced overexpression of TrkB on astrocytes suggest a possible triggering mechanism for STM reduction that was seen previously during mild-to-moderate TBI.NEW & NOTEWORTHY For the first time we showed that fibrinogen (Fg) can associate with cellular prion protein (PrPC) on the surface of cultured mouse brain astrocytes. At high levels, Fg causes upregulation of astrocyte PrPC and astrocyte activation accompanied with overexpression of tyrosine receptor kinase B (TrkB), which results in nitric oxide (NO) production and generation of reactive oxygen species (ROS). Fg/PrPC interaction can be a triggering mechanism for TrkB-NO-ROS axis activation and the resultant astrocyte-mediated neurodegeneration.

Identification of compounds inhibiting prion replication and toxicity by removing PrPC from the cell surface.

The vast majority of therapeutic approaches tested so far for prion diseases, transmissible neurodegenerative disorders of human and animals, tackled PrPSc , the aggregated and infectious isoform of the cellular prion protein (PrPC ), with largely unsuccessful results. Conversely, targeting PrPC expression, stability or cell surface localization are poorly explored strategies. We recently characterized the mode of action of chlorpromazine, an anti-psychotic drug known to inhibit prion replication and toxicity by inducing the re-localization of PrPC from the plasma membrane. Unfortunately, chlorpromazine possesses pharmacokinetic properties unsuitable for chronic use in vivo, namely low specificity and high toxicity. Here, we employed HEK293 cells stably expressing EGFP-PrP to carry out a semi-automated high content screening (HCS) of a chemical library directed at identifying non-cytotoxic molecules capable of specifically relocalizing PrPC from the plasma membrane as well as inhibiting prion replication in N2a cell cultures. We identified four candidate hits inducing a significant reduction in cell surface PrPC , one of which also inhibited prion propagation and toxicity in cell cultures in a strain-independent fashion. This study defines a new screening method and novel anti-prion compounds supporting the notion that removing PrPC from the cell surface could represent a viable therapeutic strategy for prion diseases.

Generation, optimization and characterization of novel anti-prion compounds.

Prions are misfolded proteins involved in neurodegenerative diseases of high interest in veterinary and public health. In this work, we report the chemical space exploration around the anti-prion compound BB 0300674 in order to gain an understanding of its Structure Activity Relationships (SARs). A series of 43 novel analogues, based on four different chemical clusters, were synthetized and tested against PrPSc and mutant PrP toxicity assays. From this biological screening, two compounds (59 and 65) emerged with a 10-fold improvement in anti-prion activity compared with the initial lead compound, presenting at the same time interesting cell viability.

The Cellular Prion Protein: A Promising Therapeutic Target for Cancer.

Studies on the cellular prion protein (PrPC) have been actively conducted because misfolded PrPC is known to cause transmissible spongiform encephalopathies or prion disease. PrPC is a glycophosphatidylinositol-anchored cell surface glycoprotein that has been reported to affect several cellular functions such as stress protection, cellular differentiation, mitochondrial homeostasis, circadian rhythm, myelin homeostasis, and immune modulation. Recently, it has also been reported that PrPC mediates tumor progression by enhancing the proliferation, metastasis, and drug resistance of cancer cells. In addition, PrPC regulates cancer stem cell properties by interacting with cancer stem cell marker proteins. In this review, we summarize how PrPC promotes tumor progression in terms of proliferation, metastasis, drug resistance, and cancer stem cell properties. In addition, we discuss strategies to treat tumors by modulating the function and expression of PrPC via the regulation of HSPA1L/HIF-1α expression and using an anti-prion antibody.

A New Take on Prion Protein Dynamics in Cellular Trafficking.

The mobility of cellular prion protein (PrPC) in specific cell membrane domains and among distinct cell compartments dictates its molecular interactions and directs its cell function. PrPC works in concert with several partners to organize signaling platforms implicated in various cellular processes. The scaffold property of PrPC is able to gather a molecular repertoire to create heterogeneous membrane domains that favor endocytic events. Dynamic trafficking of PrPC 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 PrPC traffics across intra- and extracellular compartments and the consequences of this dynamic transport in governing cell functions and contributing to prion disease pathogenesis.

Dimerization of the cellular prion protein inhibits propagation of scrapie prions.

A central step in the pathogenesis of prion diseases is the conformational transition of the cellular prion protein (PrPC) into the scrapie isoform, denoted PrPSc Studies in transgenic mice have indicated that this conversion requires a direct interaction between PrPC and PrPSc; however, insights into the underlying mechanisms are still missing. Interestingly, only a subfraction of PrPC is converted in scrapie-infected cells, suggesting that not all PrPC species are suitable substrates for the conversion. On the basis of the observation that PrPC can form homodimers under physiological conditions with the internal hydrophobic domain (HD) serving as a putative dimerization domain, we wondered whether PrP dimerization is involved in the formation of neurotoxic and/or infectious PrP conformers. Here, we analyzed the possible impact on dimerization of pathogenic mutations in the HD that induce a spontaneous neurodegenerative disease in transgenic mice. Similarly to wildtype (WT) PrPC, the neurotoxic variant PrP(AV3) formed homodimers as well as heterodimers with WTPrPC Notably, forced PrP dimerization via an intermolecular disulfide bond did not interfere with its maturation and intracellular trafficking. Covalently linked PrP dimers were complex glycosylated, GPI-anchored, and sorted to the outer leaflet of the plasma membrane. However, forced PrPC dimerization completely blocked its conversion into PrPSc in chronically scrapie-infected mouse neuroblastoma cells. Moreover, PrPC dimers had a dominant-negative inhibition effect on the conversion of monomeric PrPC Our findings suggest that PrPC monomers are the major substrates for PrPSc propagation and that it may be possible to halt prion formation by stabilizing PrPC dimers.

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

Normally folded prion protein (PrPC) and its functions in healthy brains remain underappreciated compared with the intense study of its misfolded forms ("prions," PrPSc) during the pathobiology of prion diseases. This impedes the development of therapeutic strategies in Alzheimer's and prion diseases. Disrupting the zebrafish homologs of PrPC 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 PrPC 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.

Alzheimer's disease: Key developments support promising perspectives for therapy.

Alzheimer's is the neurodegenerative disease affecting the largest number of patients in the world. In spite of the intense research of the last decades, progress about its knowledge and therapy was limited. In particular, various cytotoxic processes remained debated, while the few drugs approved for therapy were of only marginal relevance. Recent studies have identified key aspects of the disease, such as the mechanisms governing the development of pathology. In order to operate the Aß peptide, known as the key factor, requires a complex assembled by its high affinity binding to PrPc, a cell surface prion protein, and mGluR5, a metabotropic glutamate receptor. Aß and its associates bind also phosphorylated tau transferred to the extracellular space, with final activation of intracellular cytotoxic signals. Pathology is further affected by factors (including genes, receptors and their agonists) and by glial cells governing (via vesicles, cytokines and enzymes) cell immunology, inflammation and oxidative stress. Concomitant to pathology studies, strong attempts have been made for the development of new, effective therapies. Critical for this are biomarkers, by which Alzheimer's patients are recognized even before appearance of their symptoms. The question was whether patients take advantage from drugs not yet approved. The latter, first identified in mice, were found effective also in men, however only before appearance or at early stage of the disease. In other words, the drugs not yet approved induce effective protection of patients still healthy or in a preliminary stage of the disease. In contrast, developed Alzheimer's disease is practically irreversible.