Cellular prion protein promotes regeneration of adult muscle tissue

PrPC Glycoprotein Is Indispensable for Maintenance of Skeletal Muscle Homeostasis During Aging

BACKGROUND

The cellular prion protein (PrPC), a glycoprotein encoded by the PRNP gene, is known to modulate muscle mass and exercise capacity. However, the role of PrPC in the maintenance and regeneration of skeletal muscle during ageing remains unclear.

METHODS

This study investigated the change in PrPC expression during muscle formation using C2C12 cells and evaluated muscle function in Prnp wild-type (WT) and knock-out (KO) mice at different ages (1, 9 and 15 months). To determine the role of PrPC in skeletal muscle homeostasis during ageing, we conducted regeneration experiments via cardiotoxin injection in Prnp mice to assess the effects of PrPC deficiency on the senescence of satellite stem cells (SCs) and regenerative capacity in skeletal muscle.

RESULTS

Our data demonstrate that PrPC expression increased significantly during muscle differentiation (p < 0.01), correlating with myogenin (immunofluorescence at the differentiation stage). PrPC deficiency disrupted muscle homeostasis, leading to age-associated mitochondrial autophagy (Pink-1, +180%, p < 0.001; Parkin, +161%, p < 0.01) and endoplasmic reticulum stress (SERCA, -26%, p < 0.05; IRE1α, +195%, p < 0.001) while decreasing the level of mitochondrial biogenesis (SIRT-1, -50%, p < 0.01; PGC-1α, -36%, p < 0.05; VDAC, -27%, p < 0.001), and activated oxidative stress (serum myoglobin, +23%, p < 0.001; MDA, +23%, p < 0.05; NFκB, +117%, p < 0.05) during ageing, which accelerated reduced muscle growth or mass accumulation (tibialis anterior muscle mass, -23%, p < 0.001; gastrocnemius muscle mass, -30%, p < 0.001; muscle fibre size, -48%, p < 0.05; MSTN, +160%, p < 0.01; MAFbx, +83%, p < 0.05). Furthermore, PrPC deficiency induced the senescence (β-galactosidase, +60%, p < 0.05; p16, +103%, p < 0.001) of SCs, which was directly related to the defect in muscle recovery, with the senescence-mediated enhancement of adipogenesis (PPARγ, +74%, p < 0.05) during the regeneration process after cardiotoxin-induced muscle injury.

CONCLUSIONS

Our findings demonstrate that PrPC is indispensable for maintaining skeletal muscle homeostasis during ageing by modulating the functional integrity of mitochondria, ER and SCs.

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.

Excess PrPC inhibits muscle cell differentiation via miRNA-enhanced liquid-liquid phase separation implicated in myopathy

The cellular prion protein (PrPC) is required for skeletal muscle function. Here, we report that a higher level of PrPC accumulates in the cytoplasm of the skeletal muscle of six myopathy patients compared to controls. PrPC inhibits skeletal muscle cell autophagy, and blocks myoblast differentiation. PrPC selectively binds to a subset of miRNAs during myoblast differentiation, and the colocalization of PrPC and miR-214-3p was observed in the skeletal muscle of six myopathy patients with excessive PrPC. We demonstrate that PrPC is overexpressed in skeletal muscle cells under pathological conditions, inhibits muscle cell differentiation by physically interacting with a subset of miRNAs, and selectively recruits these miRNAs into its phase-separated condensate in living myoblasts, which in turn enhances liquid-liquid phase separation of PrPC, promotes pathological aggregation of PrP, and results in the inhibition of autophagy-related protein 5-dependent autophagy and muscle bundle formation in myopathy patients characterized by incomplete muscle regeneration.

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.

Detailing the ultrastructure's increase of prion protein in pancreatic adenocarcinoma

BACKGROUND

Recent evidences have shown a relationship between prion protein (PrPc) expression and pancreatic ductal adenocarcinoma (PDAC). Indeed, PrPc could be one of the markers explaining the aggressiveness of this tumor. However, studies investigating the specific compartmentalization of increased PrPc expression within PDAC cells are lacking, as well as a correlation between ultrastructural evidence, ultrastructural morphometry of PrPc protein and clinical data. These data, as well as the quantitative stoichiometry of this protein detected by immuno-gold, provide a significant advancement in understanding the biology of disease and the outcome of surgical resection.

AIM

To analyze quantitative stoichiometry and compartmentalization of PrPc in PDAC cells and to correlate its presence with prognostic data.

METHODS

Between June 2018 and December 2020, samples from pancreatic tissues of 45 patients treated with pancreatic resection for a preoperative suspicion of PDAC at our Institution were collected. When the frozen section excluded a PDAC diagnosis, or the nodules were too small for adequate sampling, patients were ruled out from the present study. Western blotting was used to detect, quantify and compare the expression of PrPc in PDAC and control tissues, such as those of non-affected neighboring pancreatic tissue of the same patient. To quantify the increase of PrPc and to detect the subcellular compartmentalization of PrPc within PDAC cells, immuno-gold stoichiometry within specific cell compartments was analyzed with electron microscopy. Finally, an analysis of quantitative PrPc expression according to prognostic data, such as cancer stage, recurrence of the disease at 12 mo after surgery and recurrence during adjuvant chemotherapy was made.

RESULTS

The amount of PrPc within specimen from 38 out of 45 patients was determined by semi-quantitative analysis by using Western blotting, which indicates that PrPc increases almost three-fold in tumor pancreatic tissue compared with healthy pancreatic regions [242.41 ± 28.36 optical density (OD) vs 95 ± 17.40 OD, P < 0.0001]. Quantitative morphometry carried out by using immuno-gold detection at transmission electron microscopy confirms an increased PrPc expression in PDAC ductal cells of all patients and allows to detect a specific compartmentalization of PrPc within tumor cells. In particular, the number of immuno-gold particles of PrPc was significantly higher in PDAC cells respect to controls, when considering the whole cell (19.8 ± 0.79 particles vs 9.44 ± 0.45, P < 0.0001). Remarkably, considering PDAC cells, the increase of PrPc was higher in the nucleus than cytosol of tumor cells, which indicates a shift in PrPc compartmentalization within tumor cells. In fact, the increase of immuno-gold within nuclear compartment exceeds at large the augment of PrPc which was detected in the cytosol (nucleus: 12.88 ± 0.59 particles vs 5.12 ± 0.32, P < 0.0001; cytosol: 7.74. ± 0.44 particles vs 4.3 ± 0.24, P < 0.0001). In order to analyze the prognostic impact of PrPc, we found a correlation between PrPc expression and cancer stage according to pathology results, with a significantly higher expression of PrPc for advanced stages. Moreover, 24 patients with a mean follow-up of 16.8 mo were considered. Immuno-blot analysis revealed a significantly higher expression of PrPc in patients with disease recurrence at 12 mo after radical surgery (360.71 ± 69.01 OD vs 170.23 ± 23.06 OD, P = 0.023), also in the subgroup of patients treated with adjuvant CT (368.36 ± 79.26 OD in the recurrence group vs 162.86 ± 24.16 OD, P = 0.028), which indicates a correlation with a higher chemo-resistance.

CONCLUSION

Expression of PrPc is significantly higher in PDAC cells compared with control, with the protein mainly placed in the nucleus. Preliminary clinical data confirm the correlation with a poorer prognosis.

[Prion diseases and the biosecurity problems.]

The review presents the current state of the problem of prions and prion diseases with an emphasis on theepidemiological and epizootological risks of pathogens that cause fatal neurodegenerative diseases in humans and animals. The results of molecular genetic studies of the conversion of normal PrP^c prion protein molecules to infectious forms of PrP^d, resistance to physical disinfection methods, in particular exceptional thermal stability, and their ability to overcome interspecific barriers, while increasing virulence, are described. The possibility of infection not only by nutrition, when eating even heat-treated meat of sick animals, but also due to surgical interventions, especially neurosurgical and ophthalmic, as well as the use of immunobiological preparations, are emphasized. Since there are currently no means for the effective treatment of prion diseases in the world, attention is drawn to the high degree of relevance for the biosafety of the country to develop domestic highly sensitive test systems that can effectively detect prion infectious protein in vivo at the preclinical stage of the disease. The latest methods of automatic protein amplification and identification of prion proteins are briefly described as the most promising areas of research in the field of diagnosis of prion diseases.

Cellular prion protein dysfunction in a prototypical inherited metabolic myopathy

Inherited fatty acid oxidation diseases in their mild forms often present as metabolic myopathies. Carnitine Palmitoyl Transferase 2 (CPT2) deficiency, one such prototypical disorder is associated with compromised myotube differentiation. Here, we show that CPT2-deficient myotubes exhibit defects in focal adhesions and redox balance, exemplified by increased SOD2 expression. We document unprecedented alterations in the cellular prion protein PrPC, which directly arise from the failure in CPT2 enzymatic activity. We also demonstrate that the loss of PrPC function in normal myotubes recapitulates the defects in focal adhesion, redox balance and differentiation hallmarks monitored in CPT2-deficient cells. These results are further corroborated by studies performed in muscles from Prnp-/- mice. Altogether, our results unveil a molecular scenario, whereby PrPC dysfunction governed by faulty CPT2 activity may drive aberrant focal adhesion turnover and hinder proper myotube differentiation. Our study adds a novel facet to the involvement of PrPC in diverse physiopathological situations.

The cellular prion protein promotes neuronal regeneration after acute nasotoxic injury

Adult neurogenesis, analogous to early development, is comprised of several, often concomitant, processes including proliferation, differentiation, and formation of synaptic connections. However, due to continual, asynchronous turn-over, newly-born adult olfactory sensory neurons (OSNs) must integrate into existing circuitry. Additionally, OSNs express high levels of cellular prion protein (PrPC), particularly in the axon, which implies a role in this cell type. The cellular prion has been shown to be important for proper adult OSN neurogenesis primarily by stabilizing mature olfactory neurons within this circuitry. However, the role of PrPC on each specific adult neurogenic processes remains to be investigated in detail. To tease out the subtle effects of prion protein expression level, a large population of regenerating neurons must be investigated. The thyroid drug methimazole (MTZ) causes nearly complete OSN loss in rodents and is used as a model of acute olfactory injury, providing a mechanism to induce synchronized OSN regeneration. This study investigated the effect of PrPC on adult neurogenesis after acute nasotoxic injury. Altered PrPC levels affected olfactory sensory epithelial (OSE) regeneration, cell proliferation, and differentiation. Attempts to investigate the role of PrPC level on axon regeneration did not support previous studies, and glomerular targeting did not recover to vehicle-treated levels, even by 20 weeks. Together, these studies demonstrate that the cellular prion protein is critical for regeneration of neurons, whereby increased PrPC levels promote early neurogenesis, and that lack of PrPC delays the regeneration of this tissue after acute injury.

Microglia in Prion Diseases: Angels or Demons?

Prion diseases are rare transmissible neurodegenerative disorders caused by the accumulation of a misfolded isoform (PrP^Sc) of the cellular prion protein (PrP^C) in the central nervous system (CNS). Neuropathological hallmarks of prion diseases are neuronal loss, astrogliosis, and enhanced microglial proliferation and activation. As immune cells of the CNS, microglia participate both in the maintenance of the normal brain physiology and in driving the neuroinflammatory response to acute or chronic (e.g., neurodegenerative disorders) insults. Microglia involvement in prion diseases, however, is far from being clearly understood. During this review, we summarize and discuss controversial findings, both in patient and animal models, suggesting a neuroprotective role of microglia in prion disease pathogenesis and progression, or—conversely—a microglia-mediated exacerbation of neurotoxicity in later stages of disease. We also will consider the active participation of PrP^C in microglial functions, by discussing previous reports, but also by presenting unpublished results that support a role for PrP^C in cytokine secretion by activated primary microglia.

The occurrence of prion protein in surgically resected pancreatic adenocarcinoma

BACKGROUND

Among the several new targets for the comprehension of the biology of pancreatic ductal adenocarcinoma (PDAC), Prion proteins (PrPc) deserve particular mention, since they share a marked neurotropism. Actually, PrPc could have also a role in tumorigenesis, as recently demonstrated. However, only few in vitro studies in cell cultures showed the occurrence of PrPc in PDAC cells. We aim to evaluate the presence of PrPc in vivo in PDAC tissues as a potential new biomarker.

METHODS

Samples from tumors of 23 patients undergone pancreatic resections from July 2018 to May 2020 at our institution were collected and analyzed. Immunohistochemistry and western blotting of PDAC tissues were compared with control tissues. Immunohistochemistry was used also to evaluate the localization of PrPc and of CD155, a tumoral stem-cell marker.

RESULTS

All cases were moderately differentiated PDAC, with perineural invasion (PNI) in 19/23 cases (83%). According to western-blot analysis, PrPc was markedly expressed in PDAC tissues (273.5 ± 44.63 OD) respect to controls (100 ± 28.35 OD, p = 0.0018). Immunohistochemistry confirmed these findings, with higher linear staining of PrPc in PDAC ducts (127.145 ± 7.56 μm vs 75.21 ± 5.01 μm, p < 0.0001). PrPc and CD155 exactly overlapped in ductal tumoral cells, highlighting the possible relationship of PrPc with cancer stemness. Finally, PrPc expression related with cancer stage and there was a potential correspondence with PNI.

CONCLUSIONS

Our work provides evidence for increased levels of PrPc in PDAC. This might contribute to cancer aggressiveness and provides a potentially new biomarker. Work is in progress to decipher clinical implications.

The cellular prion protein promotes olfactory sensory neuron survival and axon targeting during adult neurogenesis

The cellular prion protein (PrP^C) has been associated with diverse biological processes including cell signaling, neurogenesis, and neuroprotection, but its physiological function(s) remain ambiguous. Here we determine the role of PrP^C in adult neurogenesis using the olfactory system model in transgenic mice. Olfactory sensory neurons (OSNs) within the olfactory sensory epithelium (OSE) undergo neurogenesis, integration, and turnover even into adulthood. The neurogenic processes of proliferation, differentiation/maturation, and axon targeting were evaluated in wild type, PrP-overexpressing, and PrP-null transgenic mice. Our results indicate that PrP^C plays a role in maintaining mature OSNs within the epithelium: overexpression of PrP^C resulted in greater survival of mitotically active cells within the OSE, whereas absence of prion protein resulted in fewer cells being maintained over time. These results are supported by both quantitative PCR analysis of gene expression and protein analysis characteristic of OSN differentiation. Finally, evaluation of axon migration determined that OSN axon targeting in the olfactory bulb is PrP^C dose-dependent. Together, these findings provide new mechanistic insight into the neuroprotective role for PrP^C in adult OSE neurogenesis, whereby more mature neurons are stably maintained in animals expressing PrP^C.

Cellular prion protein controls stem cell-like properties of human glioblastoma tumor-initiating cells

Prion protein (PrP^C) is a cell surface glycoprotein whose misfolding is responsible for prion diseases. Although its physiological role is not completely defined, several lines of evidence propose that PrP^C is involved in self-renewal, pluripotency gene expression, proliferation and differentiation of neural stem cells. Moreover, PrP^C regulates different biological functions in human tumors, including glioblastoma (GBM). We analyzed the role of PrP^C in GBM cell pathogenicity focusing on tumor-initiating cells (TICs, or cancer stem cells, CSCs), the subpopulation responsible for development, progression and recurrence of most malignancies. Analyzing four GBM CSC-enriched cultures, we show that PrP^C expression is directly correlated with the proliferation rate of the cells. To better define its role in CSC biology, we knocked-down PrP^C expression in two of these GBM-derived CSC cultures by specific lentiviral-delivered shRNAs. We provide evidence that CSC proliferation rate, spherogenesis and in vivo tumorigenicity are significantly inhibited in PrP^C down-regulated cells. Moreover, PrP^C down-regulation caused loss of expression of the stemness and self-renewal markers (NANOG, Sox2) and the activation of differentiation pathways (i.e. increased GFAP expression). Our results suggest that PrP^C controls the stemness properties of human GBM CSCs and that its down-regulation induces the acquisition of a more differentiated and less oncogenic phenotype.

Role of hypoxia‑mediated cellular prion protein functional change in stem cells and potential application in angiogenesis (Review)

Cellular prion protein (PrP^C) can replace other pivotal molecules due to its interaction with several partners in performing a variety of important biological functions that may differ between embryonic and mature stem cells. Recent studies have revealed major advances in elucidating the putative role of PrP^C in the regulation of stem cells and its application in stem cell therapy. What is special about PrP^C is that its expression may be regulated by hypoxia-inducible factor (HIF)-1α, which is the transcriptional factor of cellular response to hypoxia. Hypoxic conditions have been known to drive cellular responses that can enhance cell survival, differentiation and angiogenesis through adaptive processes. Our group recently reported hypoxia-enhanced vascular repair of endothelial colony-forming cells on ischemic injury. Hypoxia-induced AKT/signal transducer and activator of transcription 3 phosphorylation eventually increases neovasculogenesis. In stem cell biology, hypoxia promotes the expression of growth factors. According to other studies, aspects of tissue regeneration and cell function are influenced by hypoxia, which serves an essential role in stem cell HIF-1α signaling. All these data suggest the possibility that hypoxia-mediated PrP^C serves an important role in angiogenesis. Therefore, the present review summarizes the characteristics of PrP^C, which is produced by HIF-1α in hypoxia, as it relates to angiogenesis.

Protective role of cellular prion protein against TNFα-mediated inflammation through TACE α-secretase

Although cellular prion protein PrP^C is well known for its implication in Transmissible Spongiform Encephalopathies, its functions remain elusive. Combining in vitro and in vivo approaches, we here show that PrP^C displays the intrinsic capacity to protect neuronal cells from a pro-inflammatory TNFα noxious insult. Mechanistically, PrP^C coupling to the NADPH oxidase-TACE α-secretase signaling pathway promotes TACE-mediated cleavage of transmembrane TNFα receptors (TNFRs) and the release of soluble TNFR, which limits the sensitivity of recipient cells to TNFα. We further show that PrP^C expression is necessary for TACE α-secretase to stay at the plasma membrane in an active state for TNFR shedding. Such PrP^C control of TACE localization depends on PrP^C modulation of β1 integrin signaling and downstream activation of ROCK-I and PDK1 kinases. Loss of PrP^C provokes TACE internalization, which in turn cancels TACE-mediated cleavage of TNFR and renders PrP^C-depleted neuronal cells as well as PrP^C knockout mice highly vulnerable to pro-inflammatory TNFα insult. Our work provides the prime evidence that in an inflammatory context PrP^C adjusts the response of neuronal cells targeted by TNFα through TACE α-secretase. Our data also support the view that abnormal TACE trafficking and activity in prion diseases originate from a-loss-of-PrP^C cytoprotective function.

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.

Cellular prion protein (PrPC) in the development of Merlin-deficient tumours

Loss of function mutations in the neurofibromatosis Type 2 (NF2) gene, coding for a tumour suppressor, Merlin, cause multiple tumours of the nervous system such as schwannomas, meningiomas and ependymomas. These tumours may occur sporadically or as part of the hereditary condition neurofibromatosis Type 2 (NF2). Current treatment is confined to (radio) surgery and no targeted drug therapies exist. NF2 mutations and/or Merlin inactivation are also seen in other cancers including some mesothelioma, breast cancer, colorectal carcinoma, melanoma and glioblastoma. To study the relationship between Merlin deficiency and tumourigenesis, we have developed an in vitro model comprising human primary schwannoma cells, the most common Merlin-deficient tumour and the hallmark for NF2. Using this model, we show increased expression of cellular prion protein (PrP^C) in schwannoma cells and tissues. In addition, a strong overexpression of PrP^C is observed in human Merlin-deficient mesothelioma cell line TRA and in human Merlin-deficient meningiomas. PrP^C contributes to increased proliferation, cell-matrix adhesion and survival in schwannoma cells acting via 37/67 kDa non-integrin laminin receptor (LR/37/67 kDa) and downstream ERK1/2, PI3K/AKT and FAK signalling pathways. PrP^C protein is also strongly released from schwannoma cells via exosomes and as a free peptide suggesting that it may act in an autocrine and/or paracrine manner. We suggest that PrP^C and its interactor, LR/37/67 kDa, could be potential therapeutic targets for schwannomas and other Merlin-deficient tumours.

Absolute quantification of myosin heavy chain isoforms by selected reaction monitoring can underscore skeletal muscle changes in a mouse model of amyotrophic lateral sclerosis

Skeletal muscle fibers contain different isoforms of myosin heavy chain (MyHC) that define distinctive contractile properties. In light of the muscle capacity to adapt MyHC expression to pathophysiological conditions, a rapid and quantitative assessment of MyHC isoforms in small muscle tissue quantities would represent a valuable diagnostic tool for (neuro)muscular diseases. As past protocols did not meet these requirements, in the present study we applied a targeted proteomic approach based on selected reaction monitoring that allowed the absolute quantification of slow and fast MyHC isoforms in different mouse skeletal muscles with high reproducibility. This mass-spectrometry-based method was validated also in a pathological specimen, by comparison of the MyHC expression profiles in different muscles from healthy mice and a genetic mouse model of amyotrophic lateral sclerosis (ALS) expressing the SOD1(G93A) mutant. This analysis showed that terminally ill ALS mice have a fast-to-slow shift in the fiber type composition of the tibialis anterior and gastrocnemius muscles, as previously reported. These results will likely open the way to accurate and rapid diagnoses of human (neuro)muscular diseases by the proposed method. Graphical Abstract Methods for myosin heavy chain (MyHC) quantification: a comparison of classical methods and selected reaction monitoring (SRM)-based mass spectrometry approaches.

Almost a century of prion protein(s): From pathology to physiology, and back to pathology

Prions are one of the few pathogens whose name is renowned at all population levels, after the dramatic years pervaded by the fear of eating prion-infected food. If now this, somehow irrational, scare of bovine meat inexorably transmitting devastating brain disorders is largely subdued, several prion-related issues are still unsolved, precluding the design of therapeutic approaches that could slow, if not halt, prion diseases. One unsolved issue is, for example, the role of the prion protein (PrP^C), whole conformational misfolding originates the prion but whose physiologic reason d'etre in neurons, and in cells at large, remains enigmatic. Preceded by a historical outline, the present review will discuss the functional pleiotropicity ascribed to PrP^C, and whether this aspect could fall, at least in part, into a more concise framework. It will also be devoted to radically different perspectives for PrP^C, which have been recently brought to the attention of the scientific world with unexpected force. Finally, it will discuss the possible reasons allowing an evolutionary conserved and benign protein, as PrP^C is, to turn into a high affinity receptor for pathologic misfolded oligomers, and to transmit their toxic message into neurons.

Cellular Prion Protein Promotes Neuronal Differentiation of Adipose-Derived Stem Cells by Upregulating miRNA-124

The cellular prion protein (PrP(C)) is a highly conserved glycoprotein anchored by glycosylphosphatidylinositol (GPI) to the cell surface and is also the source of pathogenic agent of scrapie prion protein (PrP(Sc)). Numerous researches have suggested putative physiological roles for PrP(C), including protection from ischemic and excitotoxic lesions, and participation in cell signaling and differentiation. Here, we demonstrated that PrP(C) positively regulates neuronal differentiation of mouse adipose-derived stem cells (ADSCs). The small C-terminal domain phosphatase 1 (SCP1) expression was knocked down by gene silencing. The mRNA expression of miRNA-124 and PrP(C) was measured with quantitative PCR. Western blot analysis was used to detect the protein levels of nestin, βIII-tubulin, and SCP1, and dual-luciferase reporter assay was performed to test the target of miRNA-124. The expression level of PrP(C) was found to increase steadily during neuron-like differentiation process, and PrP(C) knockout resulted in the reduction of neuron-like cell markers. We further showed that miRNA-124 could directly target SCP1-3'-untranslated region to decrease small C-terminal domain phosphatase 1 (SCP1) SCP1, and that miRNA-124 expression is regulated by PrP(C). Our results suggest that PrP(C) may play a key role in the neuronal differentiation of ADSC through modulating miRNA-124-SCP1 axis. To date, this is the first time strong evidence for the involvement of PrP(C) in the neuronal differentiation of ADSC is reported.

Activation of zebrafish Src family kinases by the prion protein is an amyloid-β-sensitive signal that prevents the endocytosis and degradation of E-cadherin/β-catenin complexes in vivo

Background

Prions and amyloid-β (Aβ) oligomers trigger neurodegeneration by hijacking a poorly understood cellular signal mediated by the prion protein (PrP) at the plasma membrane. In early zebrafish embryos, PrP-1-dependent signals control cell-cell adhesion via a tyrosine phosphorylation-dependent mechanism.

Results

Here we report that the Src family kinases (SFKs) Fyn and Yes act downstream of PrP-1 to prevent the endocytosis and degradation of E-cadherin/β-catenin adhesion complexes in vivo. Accordingly, knockdown of PrP-1 or Fyn/Yes cause similar zebrafish gastrulation phenotypes, whereas Fyn/Yes expression rescues the PrP-1 knockdown phenotype. We also show that zebrafish and mouse PrPs positively regulate the activity of Src kinases and that these have an unexpected positive effect on E-cadherin-mediated cell adhesion. Interestingly, while PrP knockdown impairs β-catenin adhesive function, PrP overexpression enhances it, thereby antagonizing its nuclear, wnt-related signaling activity and disturbing embryonic dorsoventral specification. The ability of mouse PrP to influence these events in zebrafish embryos requires its neuroprotective, polybasic N-terminus but not its neurotoxicity-associated central region. Remarkably, human Aβ oligomers up-regulate the PrP-1/SFK/E-cadherin/β-catenin pathway in zebrafish embryonic cells, mimicking a PrP gain-of-function scenario.

Conclusions

Our gain- and loss-of-function experiments in zebrafish suggest that PrP and SFKs enhance the cell surface stability of embryonic adherens junctions via the same complex mechanism through which they over-activate neuroreceptors that trigger synaptic damage. The profound impact of this pathway on early zebrafish development makes these embryos an ideal model to study the cellular and molecular events affected by neurotoxic PrP mutations and ligands in vivo. In particular, our finding that human Aβ oligomers activate the zebrafish PrP/SFK/E-cadherin pathway opens the possibility of using fish embryos to rapidly screen for novel therapeutic targets and compounds against prion- and Alzheimer's-related neurodegeneration. Altogether, our data illustrate PrP-dependent signals relevant to embryonic development, neuronal physiology and neurological disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-016-0076-5) contains supplementary material, which is available to authorized users.

Functional cross-talk between the cellular prion protein and the neural cell adhesion molecule is critical for neuronal differentiation of neural stem/precursor cells

Cellular prion protein (PrP) is prominently expressed in brain, in differentiated neurons but also in neural stem/precursor cells (NPCs). The misfolding of PrP is a central event in prion diseases, yet the physiological function of PrP is insufficiently understood. Although PrP has been reported to associate with the neural cell adhesion molecule (NCAM), the consequences of concerted PrP-NCAM action in NPC physiology are unknown. Here, we generated NPCs from the subventricular zone (SVZ) of postnatal day 5 wild-type and PrP null (-/-) mice and observed that PrP is essential for proper NPC proliferation and neuronal differentiation. Moreover, we found that PrP is required for the NPC response to NCAM-induced neuronal differentiation. In the absence of PrP, NCAM not only fails to promote neuronal differentiation but also induces an accumulation of doublecortin-positive neuronal progenitors at the proliferation stage. In agreement, we noted an increase in cycling neuronal progenitors in the SVZ of PrP-/- mice compared with PrP+/+ mice, as evidenced by double labeling for the proliferation marker Ki67 and doublecortin as well as by 5-bromo-2'-deoxyuridine incorporation experiments. Additionally, fewer newly born neurons were detected in the rostral migratory stream of PrP-/- mice. Analysis of the migration of SVZ cells in microexplant cultures from wild-type and PrP-/- mice revealed no differences between genotypes or a role for NCAM in this process. Our data demonstrate that PrP plays a critical role in neuronal differentiation of NPCs and suggest that this function is, at least in part, NCAM-dependent.

Prion protein- and cardiac troponin T-marked interstitial cells from the adult myocardium spontaneously develop into beating cardiomyocytes

Atypically-shaped cardiomyocytes (ACMs) constitute a novel subpopulation of beating heart cells found in the cultures of cardiac myocyte-removed crude fraction cells obtained from adult mouse cardiac ventricles. Although ~500 beating ACMs are observed under microscope in the cell cultures obtained from the hearts of either male or female mice, the origin of these cells in cardiac tissue has yet to be elucidated due to the lack of exclusive markers. In the present study, we demonstrate the efficacy of cellular prion protein (PrP) as a surface marker of ACMs. Cells expressing PrP at the plasma membrane in the culture of the crude fraction cells were found to develop into beating ACMs by themselves or fuse with each other to become larger multinuclear beating ACMs. Combining PrP with a cardiac-specific contractile protein cardiac troponin T (cTnT) allowed us to identify native ACMs in the mouse cardiac ventricles as either clustered or solitary cells. PrP- and cTnT-marked cells were also found in the adult, even aged, human cardiac ventricles. These findings suggest that interstitial cells marked by PrP and cTnT, native ACMs, exhibit life-long survival in the cardiac ventricles of both mice and humans.

The cellular form of the prion protein guides the differentiation of human embryonic stem cells into neuron-, oligodendrocyte-, and astrocyte-committed lineages

Prion protein, PrP(C), is a glycoprotein that is expressed on the cell surface beginning with the early stages of embryonic stem cell differentiation. Previously, we showed that ectopic expression of PrP(C) in human embryonic stem cells (hESCs) triggered differentiation toward endodermal, mesodermal, and ectodermal lineages, whereas silencing of PrP(C) suppressed differentiation toward ectodermal but not endodermal or mesodermal lineages. Considering that PrP(C) might be involved in controlling the balance between cells of different lineages, the current study was designed to test whether PrP(C) controls differentiation of hESCs into cells of neuron-, oligodendrocyte-, and astrocyte-committed lineages. PrP(C) was silenced in hESCs cultured under three sets of conditions that were previously shown to induce hESCs differentiation into predominantly neuron-, oligodendrocyte-, and astrocyte-committed lineages. We found that silencing of PrP(C) suppressed differentiation toward all three lineages. Similar results were observed in all three protocols, arguing that the effect of PrP(C) was independent of differentiation conditions employed. Moreover, switching PrP(C) expression during a differentiation time course revealed that silencing PrP(C) expression during the very initial stage that corresponds to embryonic bodies has a more significant impact than silencing at later stages of differentiation. The current work illustrates that PrP(C) controls differentiation of hESCs toward neuron-, oligodendrocyte-, and astrocyte-committed lineages and is likely involved at the stage of uncommitted neural progenitor cells rather than lineage-committed neural progenitors.

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.

PrP(C) from stem cells to cancer

The cellular prion protein PrP(C) was initially discovered as the normal counterpart of the pathological scrapie prion protein PrP(Sc), the main component of the infectious agent of Transmissible Spongiform Encephalopathies. While clues as to the physiological function of this ubiquitous protein were greatly anticipated from the development of knockout animals, PrP-null mice turned out to be viable and to develop without major phenotypic abnormalities. Notwithstanding, the discovery that hematopoietic stem cells from PrP-null mice have impaired long-term repopulating potential has set the stage for investigating into the role of PrP(C) in stem cell biology. A wealth of data have now exemplified that PrP(C) is expressed in distinct types of stem cells and regulates their self-renewal as well as their differentiation potential. A role for PrP(C) in the fate restriction of embryonic stem cells has further been proposed. Paralleling these observations, an overexpression of PrP(C) has been documented in various types of tumors. In line with the contribution of PrP(C) to stemness and to the proliferation of cancer cells, PrP(C) was recently found to be enriched in subpopulations of tumor-initiating cells. In the present review, we summarize the current knowledge of the role played by PrP(C) in stem cell biology and discuss how the subversion of its function may contribute to cancer progression.

The cellular prion protein counteracts cardiac oxidative stress

AIMS

The cellular prion protein, PrP(C), whose aberrant isoforms are related to prion diseases of humans and animals, has a still obscure physiological function. Having observed an increased expression of PrP(C) in two in vivo paradigms of heart remodelling, we focused on isolated mouse hearts to ascertain the capacity of PrP(C) to antagonize oxidative damage induced by ischaemic and non-ischaemic protocols.

METHODS AND RESULTS

Hearts isolated from mice expressing PrP(C) in variable amounts were subjected to different and complementary oxidative perfusion protocols. Accumulation of reactive oxygen species, oxidation of myofibrillar proteins, and cell death were evaluated. We found that overexpressed PrP(C) reduced oxidative stress and cell death caused by post-ischaemic reperfusion. Conversely, deletion of PrP(C) increased oxidative stress during both ischaemic preconditioning and perfusion (15 min) with H2O2. Supporting its relation with intracellular systems involved in oxidative stress, PrP(C) was found to influence the activity of catalase and, for the first time, the expression of p66(Shc), a protein implicated in oxidative stress-mediated cell death.

CONCLUSIONS

Our data demonstrate that PrP(C) contributes to the cardiac mechanisms antagonizing oxidative insults.

Foodborne transmission of bovine spongiform encephalopathy to non-human primates results in preclinical rapid-onset obesity

Obesity has become one of the largest public health challenges worldwide. Recently, certain bacterial and viral pathogens have been implicated in the pathogenesis of obesity. In the present study, we retrospectively analyzed clinical data, plasma samples and post-mortem tissue specimens derived from a risk assessment study in bovine spongiform encephalopathy (BSE)-infected female cynomolgus monkeys (Macaca fascicularis). The original study design aimed to determine minimal infectious doses after oral or intracerebral (i.c.) infection of macaques to assess the risk for humans. High-dose exposures resulted in 100% attack rates and a median incubation time of 4.7 years as described previously. Retrospective analyses of clinical data from high-dosed macaques revealed that foodborne BSE transmission caused rapid weight gain within 1.5 years post infection (β = 0.915; P<0.0001) which was not seen in age- and sex-matched control animals or i.c. infected animals. The rapid-onset obesity was not associated with impaired pancreatic islet function or glucose metabolism. In the early preclinical phase of oral transmission associated with body weight gain, prion accumulation was confined to the gastrointestinal tract. Intriguingly, immunohistochemical findings suggest that foodborne BSE transmission has a pathophysiological impact on gut endocrine cells which may explain rapid weight gain. To our knowledge, this is the first experimental model which clearly demonstrates that foodborne pathogens can induce obesity.

Prion protein and cancers

The normal cellular prion protein, PrP(C) is a highly conserved and widely expressed cell surface glycoprotein in all mammals. The expression of PrP is pivotal in the pathogenesis of prion diseases; however, the normal physiological functions of PrP(C) remain incompletely understood. Based on the studies in cell models, a plethora of functions have been attributed to PrP(C). In this paper, we reviewed the potential roles that PrP(C) plays in cell physiology and focused on its contribution to tumorigenesis.

Infectious prions accumulate to high levels in non proliferative C2C12 myotubes

Prion diseases are driven by the strain-specific, template-dependent transconformation of the normal cellular prion protein (PrP^C) into a disease specific isoform PrP^Sc. Cell culture models of prion infection generally use replicating cells resulting in lower levels of prion accumulation compared to animals. Using non-replicating cells allows the accumulation of higher levels of PrP^Sc and, thus, greater amounts of infectivity. Here, we infect non-proliferating muscle fiber myotube cultures prepared from differentiated myoblasts. We demonstrate that prion-infected myotubes generate substantial amounts of PrP^Sc and that the level of infectivity produced in these post-mitotic cells, 10^5.5 L.D.₅₀/mg of total protein, approaches that observed in vivo. Exposure of the myotubes to different mouse-adapted agents demonstrates strain-specific replication of infectious agents. Mouse-derived myotubes could not be infected with hamster prions suggesting that the species barrier effect is intact. We suggest that non-proliferating myotubes will be a valuable model system for generating infectious prions and for screening compounds for anti-prion activity.

This manuscript describes the generation of a new cell culture system to study the replication of infectious prions. While numerous cell lines exist that can replicate prions, these systems are usually based upon proliferating cells. As mammalian cell cultures double approximately every day, prions established in the culture must also, at least, double to be maintained. This is problematic, however, as prions replicate relatively slowly and cell replication may outpace prion replication. In fact, many cell culture systems do not replicate prions and those that do often do not replicate all strains of prions. Here we describe the use of differentiated non-proliferative muscle cells to replicate prions without the interfering effect of cell division. We observed that prions accumulate to very high levels in this muscle cell culture with infectivity approaching that observed in animals.

Conserved roles of the prion protein domains on subcellular localization and cell-cell adhesion

Analyses of cultured cells and transgenic mice expressing prion protein (PrP) deletion mutants have revealed that some properties of PrP -such as its ability to misfold, aggregate and trigger neurotoxicity- are controlled by discrete molecular determinants within its protein domains. Although the contributions of these determinants to PrP biosynthesis and turnover are relatively well characterized, it is still unclear how they modulate cellular functions of PrP. To address this question, we used two defined activities of PrP as functional readouts: 1) the recruitment of PrP to cell-cell contacts in Drosophila S2 and human MCF-7 epithelial cells, and 2) the induction of PrP embryonic loss- and gain-of-function phenotypes in zebrafish. Our results show that homologous mutations in mouse and zebrafish PrPs similarly affect their subcellular localization patterns as well as their in vitro and in vivo activities. Among PrP’s essential features, the N-terminal leader peptide was sufficient to drive targeting of our constructs to cell contact sites, whereas lack of GPI-anchoring and N-glycosylation rendered them inactive by blocking their cell surface expression. Importantly, our data suggest that the ability of PrP to homophilically trans-interact and elicit intracellular signaling is primarily encoded in its globular domain, and modulated by its repetitive domain. Thus, while the latter induces the local accumulation of PrPs at discrete punctae along cell contacts, the former counteracts this effect by promoting the continuous distribution of PrP. In early zebrafish embryos, deletion of either domain significantly impaired PrP’s ability to modulate E-cadherin cell adhesion. Altogether, these experiments relate structural features of PrP to its subcellular distribution and in vivo activity. Furthermore, they show that despite their large evolutionary history, the roles of PrP domains and posttranslational modifications are conserved between mouse and zebrafish.

The role of prion protein in stem cell regulation

Cellular prion protein (PrP(C)) has been well described as an essential partner of prion diseases due to the existence of a pathological conformation (PrP(Sc)). Recently, it has also been demonstrated that PrP(C) is an important element of the pluripotency and self-renewal matrix, with an increasing amount of evidence pointing in this direction. Here, we review the data that demonstrate its role in the transcriptional regulation of pluripotency, in the differentiation of stem cells into different lineages (e.g. muscle and neurons), in embryonic development, and its involvement in reproductive cells. Also highlighted are recent results from our laboratory that describe an important regulation by PrP(C) of the major pluripotency gene Nanog. Together, these data support the appearance of new strategies to control stemness, which could represent an important advance in the field of regenerative medicine.

The cellular form of the prion protein is involved in controlling cell cycle dynamics, self-renewal, and the fate of human embryonic stem cell differentiation

Prion protein (PrP(C) ), is a glycoprotein that is expressed on the cell surface. The current study examines the role of PrP(C) in early human embryogenesis using human embryonic stem cells (hESCs) and tetracycline-regulated lentiviral vectors that up-regulate or suppresses PrP(C) expression. Here, we show that expression of PrP(C) in pluripotent hESCs cultured under self-renewal conditions induced cell differentiation toward lineages of three germ layers. Silencing of PrP(C) in hESCs undergoing spontaneous differentiation altered the dynamics of the cell cycle and changed the balance between the lineages of the three germ layers, where differentiation toward ectodermal lineages was suppressed. Moreover, over-expression of PrP(C) in hESCs undergoing spontaneous differentiation inhibited differentiation toward lineages of all three germ layers and helped to preserve high proliferation activity. These results illustrate that PrP(C) is involved in key activities that dictate the status of hESCs including regulation of cell cycle dynamics, controlling the switch between self-renewal and differentiation, and determining the fate of hESCs differentiation. This study suggests that PrP(C) is at the crossroads of several signaling pathways that regulate the switch between preservation of or departure from the self-renewal state, control cell proliferation activity, and define stem cell fate.

Human prion protein binds Argonaute and promotes accumulation of microRNA effector complexes

Despite intense research in the context of neurodegenerative diseases associated with its misfolding, the endogenous human prion protein PrP(C) (or PRNP) has poorly understood physiological functions. Whereas most PrP(C) is exposed to the extracellular environment, conserved domains result in transmembrane forms of PrP(C) that traffic in the endolysosomal system and are linked to inherited and infectious neuropathologies. One transmembrane PrP(C) variant orients the N-terminal 'octarepeat' domain into the cytoplasm. Here we demonstrate that the octarepeat domain of human PrP(C) contains GW/WG motifs that bind Argonaute (AGO) proteins, the essential components of microRNA (miRNA)-induced silencing complexes (miRISCs). Transmembrane PrP(C) preferentially binds AGO, and PrP(C) promotes formation or stability of miRISC effector complexes containing the trinucleotide repeat-containing gene 6 proteins (TNRC6) and miRNA-repressed mRNA. Accordingly, effective repression of several miRNA targets requires PrP(C). We propose that dynamic interactions between PrP(C)-enriched endosomes and subcellular foci of AGO underpin these effects.

Prion potency in stem cells biology

Prion protein (PrP) can be considered a pivotal molecule because it interacts with several partners to perform a diverse range of critical biological functions that might differ in embryonic and adult cells. In recent years, there have been major advances in elucidating the putative role of PrP in the basic biology of stem cells in many different systems. Here, we review the evidence indicating that PrP is a key molecule involved in driving different aspects of the potency of embryonic and tissue-specific stem cells in self-perpetuation and differentiation in many cell types. It has been shown that PrP is involved in stem cell self-renewal, controlling pluripotency gene expression, proliferation, and neural and cardiomyocyte differentiation. PrP also has essential roles in distinct processes that regulate tissue-specific stem cell biology in nervous and hematopoietic systems and during muscle regeneration. Results from our own investigations have shown that PrP is able to modulate self-renewal and proliferation in neural stem cells, processes that are enhanced by PrP interactions with stress inducible protein 1 (STI1). Thus, the available data reveal the influence of PrP in acting upon the maintenance of pluripotent status or the differentiation of stem cells from the early embryogenesis through adulthood.

Cellular prion protein regulates its own α-cleavage through ADAM8 in skeletal muscle

The ubiquitously expressed cellular prion protein (PrP(C)) is subjected to the physiological α-cleavage at a region critical for both PrP toxicity and the conversion of PrP(C) to its pathogenic prion form (PrP(Sc)), generating the C1 and N1 fragments. The C1 fragment can activate caspase 3 while the N1 fragment is neuroprotective. Recent articles indicate that ADAM10, ADAM17, and ADAM9 may not play a prominent role in the α-cleavage of PrP(C) as previously thought, raising questions on the identity of the responsible protease(s). Here we show that, ADAM8 can directly cleave PrP to generate C1 in vitro and PrP C1/full-length ratio is greatly decreased in the skeletal muscles of ADAM8 knock-out mice; in addition, the PrP C1/full-length ratio is linearly correlated with ADAM8 protein level in myoblast cell line C2C12 and in skeletal muscle tissues of transgenic mice. These results indicate that ADAM8 is the primary protease responsible for the α-cleavage of PrP(C) in muscle cells. Moreover, we found that overexpression of PrP(C) led to up-regulation of ADAM8, suggesting that PrP(C) may regulate its own α-cleavage through modulating ADAM8 activity.

Prion protein expression and functional importance in skeletal muscle

Skeletal muscle expresses prion protein (PrP) that buffers oxidant activity in neurons.

AIMS

We hypothesize that PrP deficiency would increase oxidant activity in skeletal muscle and alter redox-sensitive functions, including contraction and glucose uptake. We used real-time polymerase chain reaction and Western blot analysis to measure PrP mRNA and protein in human diaphragm, five murine muscles, and muscle-derived C2C12 cells. Effects of PrP deficiency were tested by comparing PrP-deficient mice versus wild-type mice and morpholino-knockdown versus vehicle-treated myotubes. Oxidant activity (dichlorofluorescin oxidation) and specific force were measured in murine diaphragm fiber bundles.

RESULTS

PrP content differs among mouse muscles (gastrocnemius>extensor digitorum longus, EDL>tibialis anterior, TA; soleus>diaphragm) as does glycosylation (di-, mono-, nonglycosylated; gastrocnemius, EDL, TA=60%, 30%, 10%; soleus, 30%, 40%, 30%; diaphragm, 30%, 30%, 40%). PrP is predominantly di-glycosylated in human diaphragm. PrP deficiency decreases body weight (15%) and EDL mass (9%); increases cytosolic oxidant activity (fiber bundles, 36%; C2C12 myotubes, 7%); and depresses specific force (12%) in adult (8-12 mos) but not adolescent (2 mos) mice.

INNOVATION

This study is the first to directly assess a role of prion protein in skeletal muscle function.

CONCLUSIONS

PrP content varies among murine skeletal muscles and is essential for maintaining normal redox homeostasis, muscle size, and contractile function in adult animals.

Prion protein in ESC regulation

A large number of studies have analysed the putative functions of the prion protein (PrP(C)) in mammals. Although its sequence conservation over a wide range of different animals may indicate that this protein could have a key role in prion diseases, an absolutely accepted involvement has not been found so far. We have recently reported that PrP(C) regulates Nanog mRNA expression, the first non-redundant function of PrP(C) in embryonic stem cells (ESC), which translates into control of pluripotency and early differentiation. Contrary to what it is believed, the other two members of the prion protein family, Doppel and Shadoo, cannot replace the absence of PrP(C), causing the appearance of a new embryoid body (EB) population in our in vitro culture. The similarities between EB and an early post-implantation embryo suggest that this might also occur in vivo, enhancing the importance of this finding. On the other hand, our data may support the hypothesis of a relationship between the loss of PrP(C) function and neuronal degeneration in prion diseases. A reduction in brain stem cells pluripotency after PrP(C) is misfolded into the pathological conformation (PrP(Sc)) could lead to a delay or a disappearance of the normal brain damage recovery.

Possible role for Ca2+ in the pathophysiology of the prion protein?

Transmissible spongiform encephalopathies, or prion diseases, are lethal neurodegenerative disorders caused by the infectious agent named prion, whose main constituent is an aberrant conformational isoform of the cellular prion protein, PrP(C) . The mechanisms of prion-associated neurodegeneration and the physiologic function of PrP(C) are still unclear, although it is now increasingly acknowledged that PrP(C) plays a role in cell differentiation and survival. PrP(C) thus exhibits dichotomic attributes, as it can switch from a benign function under normal conditions to the triggering of neuronal death during disease. By reviewing data from models of prion infection and PrP-knockout paradigms, here we discuss the possibility that Ca(2+) is the hidden factor behind the multifaceted behavior of PrP(C) . By featuring in almost all processes of cell signaling, Ca(2+) might explain diverse aspects of PrP(C) pathophysiology, including the recently proposed one in which PrP(C) acts as a mediator of synaptic degeneration in Alzheimer's disease.

Prion protein expression regulates embryonic stem cell pluripotency and differentiation

Cellular prion protein (PRNP) is a glycoprotein involved in the pathogenesis of transmissible spongiform encephalopathies (TSEs). Although the physiological function of PRNP is largely unknown, its key role in prion infection has been extensively documented. This study examines the functionality of PRNP during the course of embryoid body (EB) differentiation in mouse Prnp-null (KO) and WT embryonic stem cell (ESC) lines. The first feature observed was a new population of EBs that only appeared in the KO line after 5 days of differentiation. These EBs were characterized by their expression of several primordial germ cell (PGC) markers until Day 13. In a comparative mRNA expression analysis of genes playing an important developmental role during ESC differentiation to EBs, Prnp was found to participate in the transcription of a key pluripotency marker such as Nanog. A clear switching off of this gene on Day 5 was observed in the KO line as opposed to the WT line, in which maximum Prnp and Nanog mRNA levels appeared at this time. Using a specific antibody against PRNP to block PRNP pathways, reduced Nanog expression was confirmed in the WT line. In addition, antibody-mediated inhibition of ITGB5 (integrin αvβ5) in the KO line rescued the low expression of Nanog on Day 5, suggesting the regulation of Nanog transcription by Prnp via this Itgb5. mRNA expression analysis of the PRNP-related proteins PRND (Doppel) and SPRN (Shadoo), whose PRNP function is known to be redundant, revealed their incapacity to compensate for the absence of PRNP during early ESC differentiation. Our findings provide strong evidence for a relationship between Prnp and several key pluripotency genes and attribute Prnp a crucial role in regulating self-renewal/differentiation status of ESC, confirming the participation of PRNP during early embryogenesis.

Neuroprotective and neurotoxic signaling by the prion protein

Prion diseases in humans and animals are characterized by progressive neurodegeneration and the formation of infectious particles called prions. Both features are intimately linked to a conformational transition of the cellular prion protein (PrP(C)) into aberrantly folded conformers with neurotoxic and self-replicating activities. Interestingly, there is increasing evidence that the infectious and neurotoxic properties of PrP conformers are not necessarily coupled. Transgenic mouse models revealed that some PrP mutants interfere with neuronal function in the absence of infectious prions. Vice versa, propagation of prions can occur without causing neurotoxicity. Consequently, it appears plausible that two partially independent pathways exist, one pathway leading to the propagation of infectious prions and another one that mediates neurotoxic signaling. In this review we will summarize current knowledge of neurotoxic PrP conformers and discuss the role of PrP(C) as a mediator of both stress-protective and neurotoxic signaling cascades.