Search for a nuclear localization signal in the prion protein

(Dys)functional insights into nucleic acids and RNA-binding proteins modulation of the prion protein and α-synuclein phase separation

Prion diseases are prototype of infectious diseases transmitted by a protein, the prion protein (PrP), and are still not understandable at the molecular level. Heterogenous species of aggregated PrP can be generated from its monomer. α-synuclein (αSyn), related to Parkinson's disease, has also shown a prion-like pathogenic character, and likewise PrP interacts with nucleic acids (NAs), which in turn modulate their aggregation. Recently, our group and others have characterized that NAs and/or RNA-binding proteins (RBPs) modulate recombinant PrP and/or αSyn condensates formation, and uncontrolled condensation might precede pathological aggregation. Tackling abnormal phase separation of neurodegenerative disease-related proteins has been proposed as a promising therapeutic target. Therefore, understanding the mechanism by which polyanions, like NAs, modulate phase transitions intracellularly, is key to assess their role on toxicity promotion and neuronal death. Herein we discuss data on the nucleic acids binding properties and phase separation ability of PrP and αSyn with a special focus on their modulation by NAs and RBPs. Furthermore, we provide insights into condensation of PrP and/or αSyn in the light of non-trivial subcellular locations such as the nuclear and cytosolic environments.

Intrinsic disorder and phase transitions: Pieces in the puzzling role of the prion protein in health and disease

After four decades of prion protein research, the pressing questions in the literature remain similar to the common existential dilemmas. Who am I? Some structural characteristics of the cellular prion protein (PrP^C) and scrapie PrP (PrP^Sc) remain unknown: there are no high-resolution atomic structures for either full-length endogenous human PrP^C or isolated infectious PrP^Sc particles. Why am I here? It is not known why PrP^C and PrP^Sc are found in specific cellular compartments such as the nucleus; while the physiological functions of PrP^C are still being uncovered, the misfolding site remains obscure. Where am I going? The subcellular distribution of PrP^C and PrP^Sc is wide (reported in 10 different locations in the cell). This complexity is further exacerbated by the eight different PrP fragments yielded from conserved proteolytic cleavages and by reversible post-translational modifications, such as glycosylation, phosphorylation, and ubiquitination. Moreover, about 55 pathological mutations and 16 polymorphisms on the PrP gene (PRNP) have been described. Prion diseases also share unique, challenging features: strain phenomenon (associated with the heterogeneity of PrP^Sc conformations) and the possible transmissibility between species, factors which contribute to PrP undruggability. However, two recent concepts in biochemistry-intrinsically disordered proteins and phase transitions-may shed light on the molecular basis of PrP's role in physiology and disease.

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

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

Cellular prion protein is present in mitochondria of healthy mice

Cellular prion protein (PrP^C) is a mammalian glycoprotein which is usually found anchored to the plasma membrane via a glycophosphatidylinositol (GPI) anchor. PrP^C misfolds to a pathogenic isoform PrP^Sc, the causative agent of neurodegenerative prion diseases. The precise function of PrP^C remains elusive but may depend upon its cellular localization. Here we show that PrP^C is present in brain mitochondria from 6–12 week old wild-type and transgenic mice in the absence of disease. Mitochondrial PrP^C was fully processed with mature N-linked glycans and did not require the GPI anchor for localization. Protease treatment of purified mitochondria suggested that mitochondrial PrP^C exists as a transmembrane isoform with the C-terminus facing the mitochondrial matrix and the N-terminus facing the intermembrane space. Taken together, our data suggest that PrP^C can be found in mitochondria in the absence of disease, old age, mutation, or overexpression and that PrP^C may affect mitochondrial function.

The cellular prion protein PrPc is a partner of the Wnt pathway in intestinal epithelial cells

We reported previously that the cellular prion protein (PrP(c)) is a component of desmosomes and contributes to the intestinal barrier function. We demonstrated also the presence of PrP(c) in the nucleus of proliferating intestinal epithelial cells. Here we sought to decipher the function of this nuclear pool. In human intestinal cancer cells Caco-2/TC7 and SW480 and normal crypt-like HIEC-6 cells, PrP(c) interacts, in cytoplasm and nucleus, with γ-catenin, one of its desmosomal partners, and with β-catenin and TCF7L2, effectors of the canonical Wnt pathway. PrP(c) up-regulates the transcriptional activity of the β-catenin/TCF7L2 complex, whereas γ-catenin down-regulates it. Silencing of PrP(c) results in the modulation of several Wnt target gene expressions in human cells, with different effects depending on their Wnt signaling status, and in mouse intestinal crypt cells in vivo. PrP(c) also interacts with the Hippo pathway effector YAP, suggesting that it may contribute to the regulation of gene transcription beyond the β-catenin/TCF7L2 complex. Finally, we demonstrate that PrP(c) is required for proper formation of intestinal organoids, indicating that it contributes to proliferation and survival of intestinal progenitors. In conclusion, PrP(c) must be considered as a new modulator of the Wnt signaling pathway in proliferating intestinal epithelial cells.

The highly conserved, N-terminal (RXXX)8 motif of mouse Shadoo mediates nuclear accumulation

The prion protein (PrP)-known for its central role in transmissible spongiform encephalopathies-has been reported to possess two nuclear localization signals and localize in the nuclei of certain cells in various forms. Although these data are superficially contradictory, it is apparent that nuclear forms of the prion protein can be found in cells in either the healthy or the diseased state. Here we report that Shadoo (Sho)-a member of the prion protein superfamily-is also found in the nucleus of several neural and non-neural cell lines as visualized by using an YFP-Sho construct. This nuclear localization is mediated by the (25-61) fragment of mouse Sho encompassing an (RXXX)8 motif. Bioinformatic analysis shows that the (RXXX)n motif (n=7-8) is a highly conserved and characteristic part of mammalian Shadoo proteins. Experiments to assess if Sho enters the nucleus by facilitated transport gave no decisive results: the inhibition of active processes that require energy in the cell, abolishes nuclear but not nucleolar accumulation. However, the (RXXX)8 motif is not able to mediate the nuclear transport of large fusion constructs exceeding the size limit of the nuclear pore for passive entry. Tracing the journey of various forms of Sho from translation to the nucleus and discerning the potential nuclear function of PrP and Sho requires further studies.

Goat PRND expression pattern suggests its involvement in early sex differentiation

Expression of the goat prion protein gene locus was assessed by reverse transcriptase-polymerase chain reaction on testes and ovaries at various developmental stages. A weak and stochastic expression of the PRNP and PRNT genes was observed. For PRNT, it is consistent with the detected deletions of two single nucleotides within its open reading frame in ruminant genes. PRND was expressed in both tissues at all stages. Whereas its expression is constant in the ovaries, it increases in testes between 36 and 46 days postcoitum (dpc) and remains high thereafter. In testes, Doppel was found in the nucleus of germinal cells and in the cytoplasm of Leydig cells at 44 dpc. It was detected in the cytoplasm of Leydig cells and of some Sertoli and germinal cells at 62 dpc. In the ovaries, it was observed in the nucleus of germinal cells at 44 dpc and mainly in their cytoplasm at 62 dpc. This expression pattern was shown to parallel that of C-kit and suggests Doppel involvement in early testis differentiation.

The truncated 23-230 form of the prion protein localizes to the nuclei of inducible cell lines independently of its nuclear localization signals and is not cytotoxic

The mechanisms of prion-induced neurological dysfunction observed in prion diseases are poorly understood. Transgenic mice expressing a truncated form of the prion protein (23-230 PrP) acquire cerebellar degeneration (Ma and Lindquist, Science, 2002). To decipher the mechanisms of neurodegeneration induced by 23-230 PrP, we established inducible cell lines expressing this truncated form of PrP. We found that 23-230 PrP, expected to be cytosolic, accumulated mostly in the nucleus of the cells and was not cytotoxic. Nuclear localization of this mutant form of PrP is independent of its predicted nuclear localization signals. In contrast to what we previously described for PrPSc, nuclear accumulation of 23-230 PrP does not require a functional microtubule network. We observed that 23-230 PrP interacts with chromatin in vivo, as already described for recombinant PrP and for PrPSc. Our data demonstrate that the 23-230 PrP model does not reflect the situation of a cytosolic PrP but could represent a very useful tool to understand the consequences of the accumulation of the prion protein in the nucleus.

Scrapie-like prion protein is translocated to the nuclei of infected cells independently of proteasome inhibition and interacts with chromatin

Prion diseases are fatal transmissible neurodegenerative disorders characterized by the accumulation of an abnormally folded isoform of the cellular prion protein (PrPC) denoted PrPSc. Recently, wild-type and pathogenic PrP mutants have been shown to be degraded by the endoplasmic reticulum-associated degradation proteasome pathway after translocation into the cytosol. We show here that a protease resistant form of PrP accumulated in the nuclei of prion-infected cells independently of proteasome activity, and that this nuclear translocation required an intact microtubule network. Moreover, our results show for the first time that nuclear PrP interacts with chromatin in vivo, which may have physiopathological consequences in prion diseases

Analysis of the interactions between HIV-1 and the cellular prion protein in a human cell line

The cellular prion protein (PrP(c)) is highly conserved in mammals and expressed widely in different tissues but its physiological role remains elusive. Recently, the human PrP(c) was shown to possess nucleic acid binding and chaperoning properties similar to human immunodeficiency virus type 1 (HIV-1) nucleocapsid protein, a key viral factor in virus structure and replication. These findings prompted us to determine if PrP(c) could influence HIV-1 replication. We used the human 293T cell line as a model system, since only a very low level of PrP(c) accumulates in these cells. Expression of PrP at a high level resulted in a specific decrease of HIV-1 Env and Vpr expression. Despite similar levels of intracellular Gag, virus production was reduced by eightfold and infectivity by three- to fourfold in the presence of PrP(c). A PrP(c) mutant lacking the glycosylphosphatidylinositol (GPI) anchor peptide did not impair HIV-1 production, suggesting that PrP(c) trafficking is critical for this inhibitory effect. Coexpressing HIV-1 and PrP(c) in these cells also caused a fraction of PrP(c) to become partially proteinase K-resistant (PrP(res)), further illustrating the interactions between HIV-1 and PrP(c).

Identification of cryptic nuclear localization signals in the prion protein

Abnormal transport of C-terminally truncated prion protein (PrP) to the nucleus has been reported in cell models of familial prion disorders associated with a stop codon mutation at residues 145 or 160 of the PrP. In both cases, PrP is translocated to the nucleus in an energy-dependent fashion, implying the presence of cryptic nuclear localization signal(s) in this region of PrP. In this report, we describe the presence of two independent nuclear localization signals (NLS) in the N-terminal domain of PrP that differ in the efficiency of nuclear targeting. When acting independently, each NLS sequence mediates the transport of tagged bovine serum albumin into the nucleus of permeabilized cells. When acting together, the two NLS sequences complement each other in transporting the N-terminal fragment of PrP to the nucleus of transfected cells, where it accumulates at steady state. Interestingly, nuclear translocation of PrP is blocked completely if the N-terminal fragment is extended to include one or two N-glycans. The glycosylated PrP fragment, instead, accumulates in the endoplasmic reticulum. Extension of the N-terminal fragment to include both N-glycans and the glycosyl phosphatidylinositol anchor, as expected, directs PrP to the plasma membrane. These observations hold implications for the pathogenesis of familial prion disorders, where truncated and abnormally glycosylated mutant PrP forms may accumulate in the nucleus and initiate neurotoxicity through novel mechanisms.

Tethering the N-terminus of the prion protein compromises the cellular response to oxidative stress

The role of the N-terminal half of the prion protein (PrPC) in normal cellular function and pathology remains enigmatic. To investigate the biological role of the N-terminus of PrP, we examined the cellular properties of a construct of murine PrP, PrP-DA, in which the N-terminus is tethered to the membrane by an uncleaved signal peptide and which retains the glycosyl-phosphatidylinositol anchor. Human neuroblastoma SH-SY5Y cells expressing PrP-DA were more susceptible to hydrogen peroxide and copper induced toxicity than wtPrP expressing cells. The PrP-DA expressing cells had an increased level of intracellular free radicals and reduced levels of superoxide dismutase and glutathione peroxidase as compared to the wtPrP expressing cells. The membrane topology, cell surface location, lipid raft localisation, intracellular trafficking and copper-mediated endocytosis of PrP-DA were not significantly different from wtPrP. However, cells expressing PrP-DA accumulated an N-terminal fragment that was resistant to proteinase K. The data presented here are consistent with the N-terminal region of PrPC having a role in the cellular response to oxidative stress, and that tethering this region of the protein to the membrane compromises this function through the accumulation of a protease-resistant N-terminal fragment, similar to that seen in some forms of human prion disease.

Cell membrane translocation of the N-terminal (1-28) part of the prion protein

The N-terminal (1-28) part of the mouse prion protein (PrP) is a cell penetrating peptide, capable of transporting large hydrophilic cargoes through a cell membrane. Confocal fluorescence microscopy shows that it transports the protein avidin (67kDa) into several cell lines. The (1-28) peptide has a strong tendency for aggregation and beta-structure formation, particularly in interaction with negatively charged phospholipid membranes. The findings have implications for how prion proteins with uncleaved signal peptides in the N-termini may enter into cells, which is important for infection. The secondary structure conversion into beta-structure may be relevant as a seed for the conversion into the scrapie (PrP(Sc)) form of the protein and its amyloidic transformation.

The cellular prion protein: a new partner of the lectin CBP70 in the nucleus of NB4 human promyelocytic leukemia cells

Prion diseases are characterized by the presence of an abnormal isoform of the cellular prion protein (PrPc) whose physiological role still remains elusive. To better understand the function of PrPc, it is important to identify the different subcellular localization(s) of the protein and the different partners with which it might be associated. In this context, the PrPc-lectins interactions are investigated because PrPc is a sialoglycoprotein which can react with lectins which are carbohydrate-binding proteins. We have previously characterized a nuclear lectin CBP70 able to recognize N-acetyl-beta-D-glucosamine residues in HL60 cells. Using confocal immunofluorescence, flow-cytofluorometry, and Western-blotting, we have found that PrPc is expressed in the nucleus of the NB4 human promyelocytic leukemia cell line. It was also found that the lectin CBP70 is localized in NB4 cell nuclei. Moreover, several approaches revealed that PrPc and CBP70 are colocalized in the nucleus. Immunoprecipitation experiments showed that these proteins are coprecipitated and interact via a sugar-dependent binding moiety. In conclusion, PrPc and CBP70 are colocalized in the nuclear compartment of NB4 cells and this interaction may be important to better understand the biological function and possibly the conversion process of PrPc into its pathological form (PrPsc).

Cellular phenotyping of secretory and nuclear prion proteins associated with inherited prion diseases

The pathogenic mechanisms leading from mutations in the prion protein (PrP) gene to infectious disease are not understood. To investigate the possibility that cellular processing of mutant prion protein may contribute to the formation of infectious particles, a mouse PrP model system has been established using the green fluorescent protein. Three novel PrP mutants were examined employing this model system and compared with wild type as well as known mutant PrPs. Two Creutzfeldt-Jakob disease-associated PrP mutants, PrP T188K and PrP T188R, revealed a secretory pathway to the cell membrane and PrP(Sc)-like properties, i.e. enhanced proteinase K resistance and detergent insolubility similar to other mutant PrPs associated with familial prion diseases. Moreover, a recently described disease-related truncated PrP mutant, PrP Q160(Stop), showed an almost exclusive localization in the nucleus and a catabolism along the proteasomal pathway. Therefore, various distinct pathological mechanisms may cause prion diseases, and aberrant cellular processing may be included in the pathogenesis of prion diseases.

Nonneuronal cellular prion protein

The normal cellular prion protein (PrP(c)) is a membrane sialoglycoprotein of unknown function having the unique property of adopting an abnormal tertiary conformation. The pathological conformer PrP(sc) would be the agent of transmissible spongiform encephalopathies or prion diseases. They include scrapie and bovine spongiform encephalopathy in animals and Creutzfeldt-Jakob disease in humans. The conversion of PrP(c) into PrP(sc) in the brain governs the clinical phenotype of the disease. However, the three-dimensional structure change of PrP(c) can also take place outside the central nervous system, in nonneuronal cells particularly of lymphoid tissue where the agent replicates. In natural infection, PrP(c) in nonneuronal cells of peripheral extracerebral organs may play a key role as the receptor required to enable the entry of the infectious agent into the host. In the present review we have undertaken a first evaluation of compelling data concerning the PrP(c)-expressing cells of nonneuronal origin present in cerebral and extracerebral tissues. The analysis of tissue, cellular, and subcellular localization of PrP(c) may help us better understand the biological function of PrP(c) and provide some information on physiopathological processes underlying prion diseases.

The 37-kDa/67-kDa laminin receptor acts as the cell-surface receptor for the cellular prion protein

Recently, we identified the 37-kDa laminin receptor precursor (LRP) as an interactor for the prion protein (PrP). Here, we show the presence of the 37-kDa LRP and its mature 67-kDa form termed high-affinity laminin receptor (LR) in plasma membrane fractions of N2a cells, whereas only the 37-kDa LRP was detected in baby hamster kidney (BHK) cells. PrP co-localizes with LRP/LR on the surface of N2a cells and Semliki Forest virus (SFV) RNA transfected BHK cells. Cell-binding assays reveal the LRP/LR-dependent binding of cellular PrP by neuronal and non-neuronal cells. Hyperexpression of LRP on the surface of BHK cells results in the binding of exogenous PrP. Cell binding is similar in PrP(+/+) and PrP(0/0) primary neurons, demonstrating that PrP does not act as a co-receptor of LRP/LR. LRP/LR-dependent internalization of PrP is blocked at 4 degrees C. Secretion of an LRP mutant lacking the transmembrane domain (aa 86-101) from BHK cells abolishes PrP binding and internalization. Our results show that LRP/LR acts as the receptor for cellular PrP on the surface of mammalian cells.

Nervous and nonnervous cell transduction by recombinant adenoviruses that inducibly express the human PrP

The study of the prion protein (PrP) physiological functions or its specific role in transmissible spongiform encephalopathies (TSE) requires new tools, particularly those able to induce PrP overexpression in a large range of cells, in vivo as well as in vitro. Here we describe the construction of two recombinant adenoviruses encoding the human PrP either with a valine at position 129 (AdTRVal) or a methionine (AdTRMet). Both genes were put under the control of the tetracycline-responsive promoter, allowing tight regulation of PrP expression. AdTRVal and AdTRMet induced high expression of the human PrP in CHO-KI cells and in organotypic brain slices in culture. The proteins expressed from these viruses exhibited a glycosylphosphatidyl inositol (GPI) anchor, proper glycosylation and sensitivity to proteinase K digestion. AdTRVal and AdTRMet will allow future studies on the human PrP and on the role of the codon 129 polyphormism in human TSE.

The prion protein has RNA binding and chaperoning properties characteristic of nucleocapsid protein NCP7 of HIV-1

Transmissible spongiform encephalopathies are fatal neurodegenerative diseases associated with the accumulation of a protease-resistant form of the prion protein (PrP). Although PrP is conserved in vertebrates, its function remains to be identified. In vitro PrP binds large nucleic acids causing the formation of nucleoprotein complexes resembling human immunodeficiency virus type 1 (HIV-1) nucleocapsid-RNA complexes and in vivo MuLV replication accelerates the scrapie infectious process, suggesting possible interactions between retroviruses and PrP. Retroviruses, including HIV-1 encode a major nucleic acid binding protein (NC protein) found within the virus where 2000 NC protein molecules coat the dimeric genome. NC is required in virus assembly and infection to chaperone RNA dimerization and packaging and in proviral DNA synthesis by reverse transcriptase (RT). In HIV-1, 5'-leader RNA/NC interactions appear to control these viral processes. This prompted us to compare and contrast the interactions of human and ovine PrP and HIV-1 NCp7 with HIV-1 5'-leader RNA. Results show that PrP has properties characteristic of NCp7 with respect to viral RNA dimerization and proviral DNA synthesis by RT. The NC-like properties of huPrP map to the N-terminal region of huPrP. Interestingly, PrP localizes in the membrane and cytoplasm of PrP-expressing cells. These findings suggest that PrP is a multifunctional protein possibly participating in nucleic acid metabolism.