Roles of the cellular prion protein in the regulation of cell-cell junctions and barrier function

Copper drives prion protein phase separation and modulates aggregation

Prion diseases are characterized by prion protein (PrP) transmissible aggregation and neurodegeneration, which has been linked to oxidative stress. The physiological function of PrP seems related to sequestering of redox-active Cu2+, and Cu2+ dyshomeostasis is observed in prion disease brain. It is unclear whether Cu2+ contributes to PrP aggregation, recently shown to be mediated by PrP condensation. This study indicates that Cu2+ promotes PrP condensation in live cells at the cell surface and in vitro through copartitioning. Molecularly, Cu2+ inhibited PrP β-structure and hydrophobic residues exposure. Oxidation, induced by H2O2, triggered liquid-to-solid transition of PrP:Cu2+ condensates and promoted amyloid-like PrP aggregation. In cells, overexpression of PrPC initially protected against Cu2+ cytotoxicity but led to PrPC aggregation upon extended copper exposure. Our data suggest that PrP condensates function as a buffer for copper that prevents copper toxicity but can transition into PrP aggregation at prolonged oxidative stress.

Amyloid precursor protein (APP) and amyloid β (Aβ) interact with cell adhesion molecules: Implications in Alzheimer’s disease and normal physiology

Alzheimer’s disease (AD) is an incurable neurodegenerative disorder in which dysfunction and loss of synapses and neurons lead to cognitive impairment and death. Accumulation and aggregation of neurotoxic amyloid-β (Aβ) peptides generated via amyloidogenic processing of amyloid precursor protein (APP) is considered to play a central role in the disease etiology. APP interacts with cell adhesion molecules, which influence the normal physiological functions of APP, its amyloidogenic and non-amyloidogenic processing, and formation of Aβ aggregates. These cell surface glycoproteins also mediate attachment of Aβ to the neuronal cell surface and induce intracellular signaling contributing to Aβ toxicity. In this review, we discuss the current knowledge surrounding the interactions of cell adhesion molecules with APP and Aβ and analyze the evidence of the critical role these proteins play in regulating the processing and physiological function of APP as well as Aβ toxicity. This is a necessary piece of the complex AD puzzle, which we should understand in order to develop safe and effective therapeutic interventions for AD.

Phase separation of the mammalian prion protein: physiological and pathological perspectives

Abnormal phase transitions have been implicated in the occurrence of proteinopathies. Disordered proteins with nucleic acid binding ability drive the formation of reversible micron-sized condensates capable of controlling nucleic acid processing/transport. This mechanism, achieved via liquid-liquid phase separation (LLPS), underlies the formation of long-studied membraneless organelles (e.g., nucleolus) and various transient condensates formed by driver proteins. The prion protein (PrP) is not a classical nucleic acid-binding protein. However, it binds nucleic acids with high affinity, undergoes nucleocytoplasmic shuttling, contains a long intrinsically disordered region rich in glycines and evenly spaced aromatic residues, among other biochemical/biophysical properties of bona fide drivers of phase transitions. Because of this, our group and others have characterized LLPS of recombinant PrP. In vitro phase separation of PrP is modulated by nucleic acid aptamers, and, depending on the aptamer conformation, the liquid droplets evolve to solid-like species. Herein we discuss recent studies and previous evidence supporting PrP phase transitions. We focus on the central role of LLPS related to PrP physiology and pathology, with a special emphasis on the interaction of PrP with different ligands, such as proteins and nucleic acids, which can play a role in prion disease pathogenesis. Finally, we comment on therapeutic strategies directed at the nonfunctional phase separation that could potentially tackle prion diseases or other protein misfolding disorders.

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.

Demyelinating polyneuropathy in goats lacking prion protein

Studies in mice with ablation of Prnp, the gene that encodes the cellular prion protein (PrP^C ), have led to the hypothesis that PrP^C is important for peripheral nerve myelin maintenance. Here, we have used a nontransgenic animal model to put this idea to the test; namely, goats that, due to a naturally occurring nonsense mutation, lack PrP^C . Teased nerve fiber preparation revealed a demyelinating pathology in goats without PrP^C . Affected nerves were invaded by macrophages and T cells and displayed vacuolated fibers, shrunken axons, and onion bulbs. Peripheral nerve lipid composition was similar in young goats with or without PrP^C , but markedly different between corresponding groups of adult goats, reflecting the progressive nature of the neuropathy. This is the first report of a subclinical demyelinating polyneuropathy caused by loss of PrP^C function in a nontransgenic mammal.

Involvement of the Prion Protein in the Protection of the Human Bronchial Epithelial Barrier Against Oxidative Stress

Aim: Bronchial epithelium acts as a defensive barrier against inhaled pollutants and microorganisms. This barrier is often compromised in inflammatory airway diseases that are characterized by excessive oxidative stress responses, leading to bronchial epithelial shedding, barrier failure, and increased bronchial epithelium permeability. Among proteins expressed in the junctional barrier and participating to the regulation of the response to oxidative and to environmental stresses is the cellular prion protein (PrP^C). However, the role of PrP^C is still unknown in the bronchial epithelium. Herein, we investigated the cellular mechanisms by which PrP^C protein participates into the junctional complexes formation, regulation, and oxidative protection in human bronchial epithelium. Results: Both PrP^C messenger RNA and mature protein were expressed in human epithelial bronchial cells. PrP^C was localized in the apical domain and became lateral, at high degree of cell polarization, where it colocalized and interacted with adherens (E-cadherin/γ-catenin) and desmosomal (desmoglein/desmoplakin) junctional proteins. No interaction was detected with tight junction proteins. Disruption of such interactions induced the loss of the epithelial barrier. Moreover, we demonstrated that PrP^C protection against copper-associated oxidative stress was involved in multiple processes, including the stability of adherens and desmosomal junctional proteins. Innovation: PrP^C is a pivotal protein in the protection against oxidative stress that is associated with the degradation of adherens and desmosomal junctional proteins. Conclusion: Altogether, these results demonstrate that the loss of the integrity of the epithelial barrier by oxidative stress is attenuated by the activation of PrP^C expression, where deregulation might be associated with respiratory diseases.

Inflammatory mediators reduce surface PrPc on human BMVEC resulting in decreased barrier integrity

The cellular prion protein (PrP^c) is a surface adhesion molecule expressed at junctions of various cell types including brain microvascular endothelial cells (BMVEC) that are important components of the blood-brain barrier (BBB). PrP^c is involved in several physiological processes including regulation of epithelial cell barrier function and monocyte migration across BMVEC. BBB dysfunction and disruption are significant events in central nervous system (CNS) inflammatory processes including HIV neuropathogenesis. Tumor necrosis factor (TNF)-α and vascular endothelial growth factor (VEGF) are two inflammatory factors that have been implicated in the processes that affect BBB integrity. To examine the effect of inflammation on PrP^c expression in BMVEC, we used these mediators and found that TNF-α and VEGF decrease surface PrP^c on primary human BMVEC. We also showed that these factors decrease total PrP^c protein as well as mRNA, indicating that they regulate expression of this protein by de novo synthesis. To determine the effect of PrP^c loss from the surface of BMVEC on barrier integrity, we used small hairpin RNAs to knockdown PrP^c. We found that the absence of PrP^c from BMVEC causes increased permeability as determined by a fluorescein isothiocyanate (FITC)-dextran permeability assay. This suggests that cell surface PrP^c is essential for endothelial monolayer integrity. To determine the mechanism by which PrP^c downregulation leads to increased permeability of an endothelial monolayer, we examined changes in expression and localization of tight junction proteins, occludin and claudin-5, and found that decreased PrP^c leads to decreased total and membrane-associated occludin and claudin-5. We propose that an additional mechanism by which inflammatory factors affect endothelial monolayer permeability is by decreasing cell-associated PrP^c. This increase in permeability may have subsequent consequences that lead to CNS damage.

The nucleo-junctional interplay of the cellular prion protein: A new partner in cancer-related signaling pathways?

The cellular prion protein PrP(c) plays important roles in proliferation, cell death and survival, differentiation and adhesion. The participation of PrP(c) in tumor growth and metastasis was pointed out, but the underlying mechanisms were not deciphered completely. In the constantly renewing intestinal epithelium, our group demonstrated a dual localization of PrP(c), which is targeted to cell-cell junctions in interaction with Src kinase and desmosomal proteins in differentiated enterocytes, but is predominantly nuclear in dividing cells. While the role of PrP(c) in the dynamics of intercellular junctions was confirmed in other biological systems, we unraveled its function in the nucleus only recently. We identified several nuclear PrP(c) partners, which comprise γ-catenin, one of its desmosomal partners, β-catenin and TCF7L2, the main effectors of the canonical Wnt pathway, and YAP, one effector of the Hippo pathway. PrP(c) up-regulates the activity of the β-catenin/TCF7L2 complex and its invalidation impairs the proliferation of intestinal progenitors. We discuss how PrP(c) could participate to oncogenic processes through its interaction with Wnt and Hippo pathway effectors, which are controlled by cell-cell junctions and Src family kinases and dysregulated during tumorigenesis. This highlights new potential mechanisms that connect PrP(c) expression and subcellular redistribution to cancer.

Tau phosphorylation induced by severe closed head traumatic brain injury is linked to the cellular prion protein

Studies in vivo and in vitro have suggested that the mechanism underlying Alzheimer's disease (AD) neuropathogenesis is initiated by an interaction between the cellular prion protein (PrPC) and amyloid-β oligomers (Aβo). This PrPC-Aβo complex activates Fyn kinase which, in turn, hyperphosphorylates tau (P-Tau) resulting in synaptic dysfunction, neuronal loss and cognitive deficits. AD transgenic mice lacking PrPC accumulate Aβ, but show normal survival and no loss of spatial learning and memory suggesting that PrPC functions downstream of Aβo production but upstream of intracellular toxicity within neurons. Since AD and traumatic brain injury (TBI)-linked chronic traumatic encephalopathy are tauopathies, we examined whether similar mechanistic pathways are responsible for both AD and TBI pathophysiologies. Using transgenic mice expressing different levels of PrPC, our studies investigated the influence and necessity of PrPC on biomarker (total-tau [T-Tau], P-Tau, GFAP) levels in brain and blood as measured biochemically following severe TBI in the form of severe closed head injury (sCHI). We found that following sCHI, increasing levels of T-Tau and P-Tau in the brain were associated with the PrPC expression levels. A similar relationship between PrPC expression and P-Tau levels following sCHI were found in blood in the absence of significant T-Tau changes. This effect was not seen with GFAP which increased within 24 h following sCHI and progressively decreased by the 7 day time point regardless of the PrPC expression levels. Changes in the levels of all biomarkers were independent of gender. We further enhanced and expanded the quantitation of brain biomarkers with correlative studies using immunohisochemistry. We also demonstrate that a TBI-induced calpain hyperactivation is not required for the generation of P-Tau. A relationship was demonstrated between the presence/absence of PrPC, the levels of P-Tau and cognitive dysfunction. Our studies suggest that PrPC is important in mediating TBI related pathology.

Prions: Beyond a Single Protein

Since the term protein was first coined in 1838 and protein was discovered to be the essential component of fibrin and albumin, all cellular proteins were presumed to play beneficial roles in plants and mammals. However, in 1967, Griffith proposed that proteins could be infectious pathogens and postulated their involvement in scrapie, a universally fatal transmissible spongiform encephalopathy in goats and sheep. Nevertheless, this novel hypothesis had not been evidenced until 1982, when Prusiner and coworkers purified infectious particles from scrapie-infected hamster brains and demonstrated that they consisted of a specific protein that he called a "prion." Unprecedentedly, the infectious prion pathogen is actually derived from its endogenous cellular form in the central nervous system. Unlike other infectious agents, such as bacteria, viruses, and fungi, prions do not contain genetic materials such as DNA or RNA. The unique traits and genetic information of prions are believed to be encoded within the conformational structure and posttranslational modifications of the proteins. Remarkably, prion-like behavior has been recently observed in other cellular proteins-not only in pathogenic roles but also serving physiological functions. The significance of these fascinating developments in prion biology is far beyond the scope of a single cellular protein and its related disease.

Expression of prion protein is closely associated with pathological and clinical progression and abnormalities of p53 in head and neck squamous cell carcinomas

Prion protein (PrP) is a glycosyl-phosphatidylinositol (GPI)-anchored membrane protein that functions as a unique pathogenic agent in transmissible spongiform encephalopathy (TSE). In the past decade, overexpression of PrP was observed in a number of human malignant tumors, such as gastric, breast and pancreatic cancer. However, the role of PrP expression in squamous cell carcinoma is rarely documented. To screen PrP expression in head and neck squamous cell carcinoma (HNSCCs), the paraffin-embedded specimens of 92 pathologically diagnosed HNSCCs were assessed by PrP-specific immunohistochemistry (IHC). A total of 55.43% (51/92) of the tested carcinoma tissues were PrP-positive. The rate of positivity and the staining intensity of PrP were closely related with the pathological degree of the HNSCCs; a higher rate of PrP expression was noted in the group of poorly differentiated cancers. PrP-positivity rates increased along with the progression of the clinical grade of the carcinomas. Further evaluation of the associations between PrP expression and the data concerning p53 abnormalities and human papillomavirus (HPV) infection in these samples as previously described, revealed that PrP-positive staining was more frequently detected in the tissues with p53-positive accumulation and the wild-type TP53 gene. The patients with a proline (Pro) polymorphism in SNP72 of TP53 showed significantly higher PrP-positive rates than those with arginine (Arg). No notable difference in PrP expression was identified between the HPV-positive and HPV-negative group. These data indicate a close association of PrP expression with clinical and histological differentiation of HNSCCs, as well as abnormalities of p53.

Genetics of Prion Disease in Cattle

Bovine spongiform encephalopathy (BSE) is a prion disease that is invariably fatal in cattle and has been implicated as a significant human health risk. As a transmissible disease of livestock, it has impacted food safety, production practices, global trade, and profitability. Genetic polymorphisms that alter the prion protein in humans and sheep are associated with transmissible spongiform encephalopathy susceptibility or resistance. In contrast, there is no strong evidence that nonsynonymous mutations in the bovine prion gene (PRNP) are associated with classical BSE (C-BSE) disease susceptibility, though two bovine PRNP insertion/deletion polymorphisms, in the putative region, are associated with susceptibility to C-BSE. However, these associations do not explain the full extent of BSE susceptibility, and loci outside of PRNP appear to be associated with disease incidence in some cattle populations. This article provides a review of the current state of genetic knowledge regarding prion diseases in cattle.

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 prion protein inhibits monocytic cell migration by stimulating β1 integrin adhesion and uropod formation

The broad tissue distribution and evolutionary conservation of the glycosylphosphatidylinositol (GPI)-anchored prion protein (PrP, also known as PRNP) suggests that it plays a role in cellular homeostasis. Given that integrin adhesion determines cell behavior, the proposed role of PrP in cell adhesion might underlie the various in vitro and in vivo effects associated with PrP loss-of-function, including the immune phenotypes described in PrP(-/-) mice. Here, we investigated the role of PrP in the adhesion and (transendothelial) migration of human (pro)monocytes. We found that PrP regulates β1-integrin-mediated adhesion of monocytes. Additionally, PrP controls the cell morphology and migratory behavior of monocytes: PrP-silenced cells show deficient uropod formation on immobilized VCAM and display bleb-like protrusions on the endothelium. Our data further show that PrP regulates ligand-induced integrin activation. Finally, we found that PrP controls the activation of several proteins involved in cell adhesion and migration, including RhoA and its effector cofilin, as well as proteins of the ERM family. We propose that PrP modulates β1 integrin adhesion and migration of monocytes through RhoA-induced actin remodeling mediated by cofilin, and through the regulation of ERM-mediated membrane-cytoskeleton linkage.

Polo-like kinase 3 (PLK3) mediates the clearance of the accumulated PrP mutants transiently expressed in cultured cells and pathogenic PrP(Sc) in prion infected cell line via protein interaction

Polo-like kinases (PLKs) family has long been known to be critical for cell cycle and recent studies have pointed to new dimensions of PLKs function in the nervous system. Our previous study has verified that the levels of PLK3 in the brain are severely downregulated in prion-related diseases. However, the associations of PLKs with prion protein remain unclear. In the present study, we confirmed that PrP protein constitutively interacts with PLK3 as determined by both in vitro and in vivo assays. Both the kinase domain and polo-box domain of PLK3 were proved to bind PrP proteins expressed in mammalian cell lines. Overexpression of PLK3 did not affect the level of wild-type PrP, but significantly decreased the levels of the mutated PrPs in cultured cells. The kinase domain appeared to be responsible for the clearance of abnormally aggregated PrPs, but this function seemed to be independent of its kinase activity. RNA-mediated knockdown of PLK3 obviously aggravated the accumulation of cytosolic PrPs. Moreover, PLK3 overexpression in a scrapie infected cell line caused notable reduce of PrP(Sc) level in a dose-dependent manner, but had minimal effect on the expression of PrP(C) in its normal partner cell line. Our findings here confirmed the molecular interaction between PLK3 and PrP and outlined the regulatory activity of PLK3 on the degradation of abnormal PrPs, even its pathogenic isoform PrP(Sc). We, therefore, assume that the recovery of PLK3 in the early stage of prion infection may be helpful to prevent the toxic accumulation of PrP(Sc) in the brain tissues.

To develop with or without the prion protein

The deletion of the cellular form of the prion protein (PrP^C) in mouse, goat, and cattle has no drastic phenotypic consequence. This stands in apparent contradiction with PrP^C quasi-ubiquitous expression and conserved primary and tertiary structures in mammals, and its pivotal role in neurodegenerative diseases such as prion and Alzheimer's diseases. In zebrafish embryos, depletion of PrP ortholog leads to a severe loss-of-function phenotype. This raises the question of a potential role of PrP^C in the development of all vertebrates. This view is further supported by the early expression of the PrP^C encoding gene (Prnp) in many tissues of the mouse embryo, the transient disruption of a broad number of cellular pathways in early Prnp^−/− mouse embryos, and a growing body of evidence for PrP^C involvement in the regulation of cell proliferation and differentiation in various types of mammalian stem cells and progenitors. Finally, several studies in both zebrafish embryos and in mammalian cells and tissues in formation support a role for PrP^C in cell adhesion, extra-cellular matrix interactions and cytoskeleton. In this review, we summarize and compare the different models used to decipher PrP^C functions at early developmental stages during embryo- and organo-genesis and discuss their relevance.