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

Simple 3D spheroid cell culture model for studies of prion infection

Mouse neuronal CAD 5 cell line effectively propagates various strains of prions. Previously, we have shown that it can also be differentiated into the cells morphologically resembling neurons. Here, we demonstrate that CAD 5 cells chronically infected with prions undergo differentiation under the same conditions. To make our model more realistic, we triggered the differentiation in the 3D culture created by gentle rocking of CAD 5 cell suspension. Spheroids formed within 1 week and were fully developed in less than 3 weeks of culture. The mature spheroids had a median size of ~300 μm and could be cultured for up to 12 weeks. Increased expression of differentiation markers GAP 43, tyrosine hydroxylase, β-III-tubulin and SNAP 25 supported the differentiated status of the spheroid cells. The majority of them were found in the G0/G1 phase of the cell cycle, which is typical for differentiated cells. Moreover, half of the PrPC on the cell membrane was N-terminally truncated, similarly as in differentiated CAD 5 adherent cells. Finally, we demonstrated that spheroids could be created from prion-infected CAD 5 cells. The presence of prions was verified by immunohistochemistry, western blot and seed amplification assay. We also confirmed that the spheroids can be infected with the prions de novo. Our 3D culture model of differentiated CAD 5 cells is low cost, easy to produce and cultivable for weeks. We foresee its possible use in the testing of anti-prion compounds and future studies of prion formation dynamics.

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

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

A PrP EGFR signaling axis controls neural stem cell senescence through modulating cellular energy pathways

Mis-folding of the prion protein (PrP) is known to cause neurodegenerative disease; however, the native function of this protein remains poorly defined. PrP has been linked with many cellular functions, including cellular proliferation and senescence. It is also known to influence epidermal growth factor receptor (EGFR) signaling, a pathway that is itself linked with both cell growth and senescence. Adult neural stem cells (NSCs) persist at low levels in the brain throughout life and retain the ability to proliferate and differentiate into new neural lineage cells. KO of PrP has previously been shown to reduce NSC proliferative capacity. We used PrP KO and WT NSCs from adult mouse brain to examine the influence of PrP on cellular senescence, EGFR signaling, and the downstream cellular processes. PrP KO NSCs showed decreased cell proliferation and increased senescence in in vitro cultures. Expression of EGFR was decreased in PrP KO NSCs compared with WT NSCs and additional supplementation of EGF was sufficient to reduce senescence. RNA-seq analysis confirmed that significant changes were occurring at the mRNA level within the EGFR signaling pathway and these were associated with reduced expression of mitochondrial components and correspondingly reduced mitochondrial function. Metabolomic analysis of cellular energy pathways showed that blockages were occurring at critical sites for production of energy and biomass, including catabolism of pyruvate. We conclude that, in the absence of PrP, NSC growth pathways are downregulated as a consequence of insufficient energy and growth intermediates.

Huntingtin and Other Neurodegeneration-Associated Proteins in the Development of Intracellular Pathologies: Potential Target Search for Therapeutic Intervention

Neurodegenerative diseases are currently incurable. Numerous experimental data accumulated over the past fifty years have brought us closer to understanding the molecular and cell mechanisms responsible for their development. However, these data are not enough for a complete understanding of the genesis of these diseases, nor to suggest treatment methods. It turns out that many cellular pathologies developing during neurodegeneration coincide from disease to disease. These observations give hope to finding a common intracellular target(s) and to offering a universal method of treatment. In this review, we attempt to analyze data on similar cellular disorders among neurodegenerative diseases in general, and polyglutamine neurodegenerative diseases in particular, focusing on the interaction of various proteins involved in the development of neurodegenerative diseases with various cellular organelles. The main purposes of this review are: (1) to outline the spectrum of common intracellular pathologies and to answer the question of whether it is possible to find potential universal target(s) for therapeutic intervention; (2) to identify specific intracellular pathologies and to speculate about a possible general approach for their treatment.

The multiple functions of PrPC in physiological, cancer, and neurodegenerative contexts

Cellular prion protein (PrP^C) is a highly conserved glycoprotein, present both anchored in the cell membrane and soluble in the extracellular medium. It has a diversity of ligands and is variably expressed in numerous tissues and cell subtypes, most notably in the central nervous system (CNS). Its importance has been brought to light over the years both under physiological conditions, such as embryogenesis and immune system homeostasis, and in pathologies, such as cancer and neurodegenerative diseases. During development, PrP^C plays an important role in CNS, participating in axonal growth and guidance and differentiation of glial cells, but also in other organs such as the heart, lung, and digestive system. In diseases, PrP^C has been related to several types of tumors, modulating cancer stem cells, enhancing malignant properties, and inducing drug resistance. Also, in non-neoplastic diseases, such as Alzheimer's and Parkinson's diseases, PrP^C seems to alter the dynamics of neurotoxic aggregate formation and, consequently, the progression of the disease. In this review, we explore in detail the multiple functions of this protein, which proved to be relevant for understanding the dynamics of organism homeostasis, as well as a promising target in the treatment of both neoplastic and degenerative diseases.

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.

Melatonin: Regulation of Prion Protein Phase Separation in Cancer Multidrug Resistance

The unique ability to adapt and thrive in inhospitable, stressful tumor microenvironments (TME) also renders cancer cells resistant to traditional chemotherapeutic treatments and/or novel pharmaceuticals. Cancer cells exhibit extensive metabolic alterations involving hypoxia, accelerated glycolysis, oxidative stress, and increased extracellular ATP that may activate ancient, conserved prion adaptive response strategies that exacerbate multidrug resistance (MDR) by exploiting cellular stress to increase cancer metastatic potential and stemness, balance proliferation and differentiation, and amplify resistance to apoptosis. The regulation of prions in MDR is further complicated by important, putative physiological functions of ligand-binding and signal transduction. Melatonin is capable of both enhancing physiological functions and inhibiting oncogenic properties of prion proteins. Through regulation of phase separation of the prion N-terminal domain which targets and interacts with lipid rafts, melatonin may prevent conformational changes that can result in aggregation and/or conversion to pathological, infectious isoforms. As a cancer therapy adjuvant, melatonin could modulate TME oxidative stress levels and hypoxia, reverse pH gradient changes, reduce lipid peroxidation, and protect lipid raft compositions to suppress prion-mediated, non-Mendelian, heritable, but often reversible epigenetic adaptations that facilitate cancer heterogeneity, stemness, metastasis, and drug resistance. This review examines some of the mechanisms that may balance physiological and pathological effects of prions and prion-like proteins achieved through the synergistic use of melatonin to ameliorate MDR, which remains a challenge in cancer treatment.

Prions and Neurodegenerative Diseases: A Focus on Alzheimer's Disease

Specific protein misfolding and aggregation are mechanisms underlying various neurodegenerative diseases such as prion disease and Alzheimer's disease (AD). The misfolded proteins are involved in prions, amyloid-β (Aβ), tau, and α-synuclein disorders; they share common structural, biological, and biochemical characteristics, as well as similar mechanisms of aggregation and self-propagation. Pathological features of AD include the appearance of plaques consisting of deposition of protein Aβ and neurofibrillary tangles formed by the hyperphosphorylated tau protein. Although it is not clear how protein aggregation leads to AD, we are learning that the cellular prion protein (PrPC) plays an important role in the pathogenesis of AD. Herein, we first examined the pathogenesis of prion and AD with a focus on the contribution of PrPC to the development of AD. We analyzed the mechanisms that lead to the formation of a high affinity bond between Aβ oligomers (AβOs) and PrPC. Also, we studied the role of PrPC as an AβO receptor that initiates an AβO-induced signal cascade involving mGluR5, Fyn, Pyk2, and eEF2K linking Aβ and tau pathologies, resulting in the death of neurons in the central nervous system. Finally, we have described how the PrPC-AβOs interaction can be used as a new potential therapeutic target for the treatment of PrPC-dependent AD.

The Role of Cellular Prion Protein in Cancer Biology: A Potential Therapeutic Target

Prion protein has two isoforms including cellular prion protein (PrP^C) and scrapie prion protein (PrP^Sc). PrP^Sc is the pathological aggregated form of prion protein and it plays an important role in neurodegenerative diseases. PrP^C is a glycosylphosphatidylinositol (GPI)-anchored protein that can attach to a membrane. Its expression begins at embryogenesis and reaches the highest level in adulthood. PrP^C is expressed in the neurons of the nervous system as well as other peripheral organs. Studies in recent years have disclosed the involvement of PrP^C in various aspects of cancer biology. In this review, we provide an overview of the current understanding of the roles of PrP^C in proliferation, cell survival, invasion/metastasis, and stem cells of cancer cells, as well as its role as a potential therapeutic target.

Cross-Linking Cellular Prion Protein Induces Neuronal Type 2-Like Hypersensitivity

Background

Previous reports identified proteins associated with ‘apoptosis’ following cross-linking PrP^C with motif-specific anti-PrP antibodies in vivo and in vitro. The molecular mechanisms underlying this IgG-mediated neurotoxicity and the role of the activated proteins in the apoptotic pathways leading to neuronal death has not been properly defined. Previous reports implicated a number of proteins, including apolipoprotein E, cytoplasmic phospholipase A2, prostaglandin and calpain with anti-PrP antibody-mediated ‘apoptosis’, however, these proteins are also known to play an important role in allergy. In this study, we investigated whether cross-linking PrP^C with anti-PrP antibodies stimulates a neuronal allergenic response.

Methods

Initially, we predicted the allergenicity of the epitope sequences associated with ‘neurotoxic’ anti-PrP antibodies using allergenicity prediction servers. We then investigated whether anti-PrP antibody treatment of mouse primary neurons (MPN), neuroblastoma cells (N2a) and microglia (N11) cell lines lead to a neuronal allergenic response.

Results

In-Silico studies showed that both tail- and globular-epitopes were allergenic. Specifically, binding regions that contain epitopes for previously reported ‘neurotoxic’ antibodies such as ICSM18 (146-159), ICSM35 (91-110), POM 1 (138-147) and POM 3 (95-100) lead to activation of allergenic related proteins. Following direct application of anti-PrP^C antibodies on N2a cells, we identified 4 neuronal allergenic-related proteins when compared with untreated cells. Furthermore, we identified 8 neuronal allergenic-related proteins following treatment of N11 cells with anti-PrP^C antibodies prior to co-culture with N2a cells when compared with untreated cells. Antibody treatment of MPN or MPN co-cultured with antibody-treated N11 led to identifying 10 and 7 allergenic-related proteins when compared with untreated cells. However, comparison with 3F4 antibody treatment revealed 5 and 4 allergenic-related proteins respectively. Of importance, we showed that the allergenic effects triggered by the anti-PrP antibodies were more potent when antibody-treated microglia were co-cultured with the neuroblastoma cell line. Finally, co-culture of N2a or MPN with N11-treated with anti-PrP antibodies resulted in significant accumulation of NO and IL6 but not TNF-α in the cell culture media supernatant.

Conclusions

This study showed for the first time that anti-PrP antibody binding to PrP^C triggers a neuronal hypersensitivity response and highlights the important role of microglia in triggering an IgG-mediated neuronal hypersensitivity response. Moreover, this study provides an important impetus for including allergenic assessment of therapeutic antibodies for neurodegenerative disorders to derive safe and targeted biotherapeutics.

Tau Exon 10 Inclusion by PrPC through Downregulating GSK3β Activity

Tau protein is largely responsible for tauopathies, including Alzheimer’s disease (AD), where it accumulates in the brain as insoluble aggregates. Tau mRNA is regulated by alternative splicing, and inclusion or exclusion of exon 10 gives rise to the 3R and 4R isoforms respectively, whose balance is physiologically regulated. In this sense, one of the several factors that regulate alternative splicing of tau is GSK3β, whose activity is inhibited by the cellular prion protein (PrP^C), which has different physiological functions in neuroprotection and neuronal differentiation. Moreover, a relationship between PrP^C and tau expression levels has been reported during AD evolution. For this reason, in this study we aimed to analyze the role of PrP^C and the implication of GSK3β in the regulation of tau exon 10 alternative splicing. We used AD human samples and mouse models of PrP^C ablation and tau overexpression. In addition, we used primary neuronal cultures to develop functional studies. Our results revealed a paralleled association between PrP^C expression and tau 4R isoforms in all models analyzed. In this sense, reduction or ablation of PrP^C levels induces an increase in tau 3R/4R balance. More relevantly, our data points to GSK3β activity downstream from PrP^C in this phenomenon. Our results indicate that PrP^C plays a role in tau exon 10 inclusion through the inhibitory capacity of GSK3β.

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

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

Cellular prion protein activates Caspase 3 for apoptotic defense mechanism in astrocytes

The cellular prion protein (PrP^C) is anchored in the plasma membrane of cells, and it is highly present in cells of brain tissue, exerting numerous cellular and cognitive functions. The present study proves the importance of PrP^C in the cellular defense mechanism and metal homeostasis in astrocytes cells. Through experimental studies using cell lines of immortalized mice astrocytes (wild type and knockout for PrP^C), we showed that PrPc is involved in the apoptosis cell death process by the activation of Caspase 3, downregulation of p53, and cell cycle maintenance. Metal homeostasis was determined by inductively coupled plasma mass spectrometry technique, indicating the crucial role of PrP^C to lower intracellular calcium. The lowered calcium concentration and the Caspase 3 downregulation in the PrP^C-null astrocytes resulted in a faster growth rate in cells, comparing with PrP^C wild-type one. The presence of PrP^C shows to be essential to cell death and healthy growth. In conclusion, our results show for the first time that astrocyte knockout cells for the cellular prion protein could modulate apoptosis-dependent cell death pathways.

Prion Protein in Stem Cells: A Lipid Raft Component Involved in the Cellular Differentiation Process

The prion protein (PrP) is an enigmatic molecule with a pleiotropic effect on different cell types; it is localized stably in lipid raft microdomains and it is able to recruit downstream signal transduction pathways by its interaction with various biochemical partners. Since its discovery, this lipid raft component has been involved in several functions, although most of the publications focused on the pathological role of the protein. Recent studies report a key role of cellular prion protein (PrP^C) in physiological processes, including cellular differentiation. Indeed, the PrP^C, whose expression is modulated according to the cell differentiation degree, appears to be part of the multimolecular signaling pathways of the neuronal differentiation process. In this review, we aim to summarize the main findings that report the link between PrP^C and stem cells.

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

BACKGROUND INFORMATION

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

RESULTS

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

CONCLUSIONS

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

SIGNIFICANCE

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

Pioglitazone Improves the Function of Human Mesenchymal Stem Cells in Chronic Kidney Disease Patients

Mesenchymal stem cells (MSCs) are optimal sources of autologous stem cells for cell-based therapy in chronic kidney disease (CKD). However, CKD-associated pathophysiological conditions, such as endoplasmic reticulum (ER) stress and oxidative stress, decrease MSC function. In this work, we study the protective effect of pioglitazone on MSCs isolated from CKD patients (CKD-MSCs) against CKD-induced ER stress. In CKD-MSCs, ER stress is found to induce mitochondrial reactive oxygen species generation and mitochondrial dysfunction. Treatment with pioglitazone reduces the expression of ER stress markers and mitochondrial fusion proteins. Pioglitazone increases the expression of cellular prion protein (PrP^C) in CKD-MSCs, which is dependent on the expression levels of proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α). Treatment with pioglitazone is found to protect CKD-MSCs against reactive oxygen species generation, aberrant mitochondrial oxidative phosphorylation of complexes I and IV, and aberrant proliferation capacity through the PGC-1α-PrP^C axis. These results indicate that pioglitazone protects the mitochondria of MSCs from CKD-induced ER stress. Pioglitazone treatment of CKD-MSCs may be a potential therapeutic strategy for CKD patients.

Role of Prion protein-EGFR multimolecular complex during neuronal differentiation of human dental pulp-derived stem cells

Cellular prion protein (PrP^C) is expressed in a wide variety of stem cells in which regulates their self-renewal as well as differentiation potential. In this study we investigated the presence of PrP^C in human dental pulp-derived stem cells (hDPSCs) and its role in neuronal differentiation process. We show that hDPSCs expresses early PrP^C at low concentration and its expression increases after two weeks of treatment with EGF/bFGF. Then, we analyzed the association of PrP^C with gangliosides and EGF receptor (EGF-R) during neuronal differentiation process. PrP^C associates constitutively with GM2 in control hDPSCs and with GD3 only after neuronal differentiation. Otherwise, EGF-R associates weakly in control hDPSCs and more markedly after neuronal differentiation. To analyze the functional role of PrP^C in the signal pathway mediated by EGF/EGF-R, a siRNA PrP was applied to ablate PrP^C and its function. The treatment with siRNA PrP significantly prevented Akt and ERK1/2 phosphorylation induced by EGF. Moreover, siRNA PrP treatment significantly prevented neuronal-specific antigens expression induced by EGF/bFGF, indicating that cellular prion protein is essential for EGF/bFGF-induced hDPSCs differentiation. These results suggest that PrP^C interact with EGF-R within lipid rafts, playing a role in the multimolecular signaling complexes involved in hDPSCs neuronal differentiation.

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.

Physiological Functions of the Cellular Prion Protein

The prion protein, PrP^C, is a small, cell-surface glycoprotein notable primarily for its critical role in pathogenesis of the neurodegenerative disorders known as prion diseases. A hallmark of prion diseases is the conversion of PrP^C into an abnormally folded isoform, which provides a template for further pathogenic conversion of PrP^C, allowing disease to spread from cell to cell and, in some circumstances, to transfer to a new host. In addition to the putative neurotoxicity caused by the misfolded form(s), loss of normal PrP^C function could be an integral part of the neurodegenerative processes and, consequently, significant research efforts have been directed toward determining the physiological functions of PrP^C. In this review, we first summarise important aspects of the biochemistry of PrP^C before moving on to address the current understanding of the various proposed functions of the protein, including details of the underlying molecular mechanisms potentially involved in these functions. Over years of study, PrP^C has been associated with a wide array of different cellular processes and many interacting partners have been suggested. However, recent studies have cast doubt on the previously well-established links between PrP^C and processes such as stress-protection, copper homeostasis and neuronal excitability. Instead, the functions best-supported by the current literature include regulation of myelin maintenance and of processes linked to cellular differentiation, including proliferation, adhesion, and control of cell morphology. Intriguing connections have also been made between PrP^C and the modulation of circadian rhythm, glucose homeostasis, immune function and cellular iron uptake, all of which warrant further investigation.

The Biological Function of the Prion Protein: A Cell Surface Scaffold of Signaling Modules

The prion glycoprotein (PrP^C) is mostly located at the cell surface, tethered to the plasma membrane through a glycosyl-phosphatydil inositol (GPI) anchor. Misfolding of PrP^C is associated with the transmissible spongiform encephalopathies (TSEs), whereas its normal conformer serves as a receptor for oligomers of the β-amyloid peptide, which play a major role in the pathogenesis of Alzheimer’s Disease (AD). PrP^C is highly expressed in both the nervous and immune systems, as well as in other organs, but its functions are controversial. Extensive experimental work disclosed multiple physiological roles of PrP^C at the molecular, cellular and systemic levels, affecting the homeostasis of copper, neuroprotection, stem cell renewal and memory mechanisms, among others. Often each such process has been heralded as the bona fide function of PrP^C, despite restricted attention paid to a selected phenotypic trait, associated with either modulation of gene expression or to the engagement of PrP^C with a single ligand. In contrast, the GPI-anchored prion protein was shown to bind several extracellular and transmembrane ligands, which are required to endow that protein with the ability to play various roles in transmembrane signal transduction. In addition, differing sets of those ligands are available in cell type- and context-dependent scenarios. To account for such properties, we proposed that PrP^C serves as a dynamic platform for the assembly of signaling modules at the cell surface, with widespread consequences for both physiology and behavior. The current review advances the hypothesis that the biological function of the prion protein is that of a cell surface scaffold protein, based on the striking similarities of its functional properties with those of scaffold proteins involved in the organization of intracellular signal transduction pathways. Those properties are: the ability to recruit spatially restricted sets of binding molecules involved in specific signaling; mediation of the crosstalk of signaling pathways; reciprocal allosteric regulation with binding partners; compartmentalized responses; dependence of signaling properties upon posttranslational modification; and stoichiometric requirements and/or oligomerization-dependent impact on signaling. The scaffold concept may contribute to novel approaches to the development of effective treatments to hitherto incurable neurodegenerative diseases, through informed modulation of prion protein-ligand interactions.

Potentiation of biological effects of mesenchymal stem cells in ischemic conditions by melatonin via upregulation of cellular prion protein expression

Mesenchymal stem cells (MSCs) are promising candidates for stem cell-based therapy in ischemic diseases. However, ischemic injury induces pathophysiological conditions, such as oxidative stress and inflammation, which diminish therapeutic efficacy of MSC-based therapy by reducing survival and functionality of transplanted MSCs. To overcome this problem, we explored the effects of melatonin on the proliferation, resistance to oxidative stress, and immunomodulatory properties of MSCs. Treatment with melatonin enhanced MSC proliferation and self-renewal via upregulation of cellular prion protein (PrP^C ) expression. Melatonin diminished the extent of MSC apoptosis in oxidative stress conditions by regulating the levels of apoptosis-associated proteins, such as BCL-2, BAX, PARP-1, and caspase-3, in a PrP^C -dependent manner. In addition, melatonin regulated the immunomodulatory effects of MSCs via the PrP^C -IDO axis. In a murine hind-limb ischemia model, melatonin-stimulated MSCs improved the blood flow perfusion, limb salvage, and vessel regeneration by lowering the extent of apoptosis of affected local cells and transplanted MSCs as well as by reducing infiltration of macrophages. These melatonin-mediated therapeutic effects were inhibited by silencing of PrP^C expression. Our findings for the first time indicate that melatonin promotes MSC functionality and enhances MSC-mediated neovascularization in ischemic tissues through the upregulation of PrP^C expression. In conclusion, melatonin-treated MSCs could provide a therapeutic strategy for vessel regeneration in ischemic disease, and the targeting of PrP^C levels may prove instrumental for MSC-based therapies.

Cellular Analysis of Adult Neural Stem Cells for Investigating Prion Biology

Traditional primary and secondary cell cultures have been used for the investigation of prion biology and disease for many years. While both types of cultures produce highly valid and immensely valuable results, they also have their limitations; traditional cell lines are often derived from cancers, therefore subject to numerous DNA changes, and primary cultures are labor-intensive and expensive to produce requiring sacrifice of many animals. Neural stem cell (NSC) cultures are a relatively new technology to be used for the study of prion biology and disease. While NSCs are subject to their own limitations-they are generally cultured ex vivo in environments that artificially force their growth-they also have their own unique advantages. NSCs retain the ability for self-renewal and can therefore be propagated in culture similarly to secondary cultures without genetic manipulation. In addition, NSCs are multipotent; they can be induced to differentiate into mature cells of central nervous system (CNS) linage. The combination of self-renewal and multipotency allows NSCs to be used as a primary cell line over multiple generations saving time, costs, and animal harvests, thus providing a valuable addition to the existing cell culture repertoire used for investigation of prion biology and disease. Furthermore, NSC cultures can be generated from mice of any genotype, either by embryonic harvest or harvest from adult brain, allowing gene expression to be studied without further genetic manipulation. This chapter describes a standard method of culturing adult NSCs and assays for monitoring NSC growth, migration, and differentiation and revisits basic reactive oxygen species detection in the context of NSC cultures.

Hypoxia-induced expression of cellular prion protein improves the therapeutic potential of mesenchymal stem cells

Mesenchymal stem cells (MSCs) are ‘adult' multipotent cells that promote regeneration of injured tissues in vivo. However, differences in oxygenation levels between normoxic culture conditions (21% oxygen) and both the MSC niche (2–8% oxygen) and ischemic injury-induced oxidative stress conditions in vivo have resulted in low efficacy of MSC therapies in both pre-clinical and clinical studies. To address this issue, we examined the effectiveness of hypoxia preconditioning (2% oxygen) for enhancing the bioactivity and tissue-regenerative potential of adipose-derived MSCs. Hypoxia preconditioning enhanced the proliferative potential of MSCs by promoting the expression of normal cellular prion protein (PrP^C). In particular, hypoxia preconditioning-mediated MSC proliferation was regulated by PrP^C-dependent JAK2 and STAT3 activation. In addition, hypoxia preconditioning-induced PrP^C regulated superoxide dismutase and catalase activity, and inhibited oxidative stress-induced apoptosis via inactivation of cleaved caspase-3. In a murine hindlimb ischemia model, hypoxia preconditioning enhanced the survival and proliferation of transplanted MSCs, ultimately resulting in improved functional recovery of the ischemic tissue, including the ratio of blood flow perfusion, limb salvage, and neovascularization. These results suggest that Hypo-MSC offer a therapeutic strategy for accelerated neovasculogenesis in ischemic diseases, and that PrP^C comprises a potential target for MSC-based therapies.

The N Terminus of the Prion Protein Mediates Functional Interactions with the Neuronal Cell Adhesion Molecule (NCAM) Fibronectin Domain

The cellular form of the prion protein (PrP^C) is a highly conserved glycoprotein mostly expressed in the central and peripheral nervous systems by different cell types in mammals. A misfolded, pathogenic isoform, denoted as prion, is related to a class of neurodegenerative diseases known as transmissible spongiform encephalopathy. PrP^C function has not been unequivocally clarified, and it is rather defined as a pleiotropic protein likely acting as a dynamic cell surface scaffolding protein for the assembly of different signaling modules. Among the variety of PrP^C protein interactors, the neuronal cell adhesion molecule (NCAM) has been studied in vivo, but the structural basis of this functional interaction is still a matter of debate. Here we focused on the structural determinants responsible for human PrP^C (HuPrP) and NCAM interaction using stimulated emission depletion (STED) nanoscopy, SPR, and NMR spectroscopy approaches. PrP^C co-localizes with NCAM in mouse hippocampal neurons, and this interaction is mainly mediated by the intrinsically disordered PrP^C N-terminal tail, which binds with high affinity to the NCAM fibronectin type-3 domain. NMR structural investigations revealed surface-interacting epitopes governing the interaction between HuPrP N terminus and the second module of the NCAM fibronectin type-3 domain. Our data provided molecular details about the interaction between HuPrP and the NCAM fibronectin domain, and revealed a new role of PrP^C N terminus as a dynamic and functional element responsible for protein-protein interaction.

Multifaceted Role of Sialylation in Prion Diseases

Mammalian prion or PrP(Sc) is a proteinaceous infectious agent that consists of a misfolded, self-replicating state of a sialoglycoprotein called the prion protein, or PrP(C). Sialylation of the prion protein N-linked glycans was discovered more than 30 years ago, yet the role of sialylation in prion pathogenesis remains poorly understood. Recent years have witnessed extraordinary growth in interest in sialylation and established a critical role for sialic acids in host invasion and host-pathogen interactions. This review article summarizes current knowledge on the role of sialylation of the prion protein in prion diseases. First, we discuss the correlation between sialylation of PrP(Sc) glycans and prion infectivity and describe the factors that control sialylation of PrP(Sc). Second, we explain how glycan sialylation contributes to the prion replication barrier, defines strain-specific glycoform ratios, and imposes constraints for PrP(Sc) structure. Third, several topics, including a possible role for sialylation in animal-to-human prion transmission, prion lymphotropism, toxicity, strain interference, and normal function of PrP(C), are critically reviewed. Finally, a metabolic hypothesis on the role of sialylation in the etiology of sporadic prion diseases is proposed.

Characterization of mesenchymal stem cells in sheep naturally infected with scrapie

Mesenchymal stem cells (MSCs) can be infected with prions and have been proposed as in vitro cell-based models for prion replication. In addition, autologous MSCs are of interest for cell therapy in neurodegenerative diseases. To the best of our knowledge, the effect of prion diseases on the characteristics of these cells has never been investigated. Here, we analysed the properties of MSCs obtained from bone marrow (BM-MSCs) and peripheral blood (PB-MSCs) of sheep naturally infected with scrapie — a large mammal model for the study of prion diseases. After three passages of expansion, MSCs derived from scrapie animals displayed similar adipogenic, chondrogenic and osteogenic differentiation ability as cells from healthy controls, although a subtle decrease in the proliferation potential was observed. Exceptionally, mesenchymal markers such as CD29 were significantly upregulated at the transcript level compared with controls. Scrapie MSCs were able to transdifferentiate into neuron-like cells, but displayed lower levels of neurogenic markers at basal conditions, which could limit this potential .The expression levels of cellular prion protein (PrPC) were highly variable between cultures, and no significant differences were observed between control and scrapie-derived MSCs. However, during neurogenic differentiation the expression of PrPC was upregulated in MSCs. This characteristic could be useful for developing in vitro models for prion replication. Despite the infectivity reported for MSCs obtained from scrapie-infected mice and Creutzfeldt–Jakob disease patients, protein misfolding cyclic amplification did not detect PrPSc in BM- or PB-MSCs from scrapie-infected sheep, which limits their use for in vivo diagnosis for scrapie.

Prion Protein and Stage Specific Embryo Antigen 1 as Selection Markers to Enrich the Fraction of Murine Embryonic Stem Cell-Derived Cardiomyocytes

BACKGROUND

The prion protein (PrP) might be useful as a tool to collect cardiac progenitor cells derived from embryonic stem (ES) cells. It is also possible that PrP(+) cells include undifferentiated cells with a capacity to develop into tumors.

METHODS

PrP(+) cells isolated from embryoid bodies (EB) formed by mouse AB1 ES cells were examined using RT-PCR analysis and clonogeneic cell assay. To assess their potential to differentiate into cardiomyocytes, Nkx2.5(GFP/+) (hcgp7) cells, another ES cell line that carries the GFP reporter gene in the Nkx2.5 loci, were used.

RESULTS

PrP(+) cells isolated from EB of day 7 and 14 did not express pluripotency markers, but expressed cardiac cell markers, while PrP(+) cells isolated from EB of day 21 expressed pluripotency markers. Cultured PrP(+) cells isolated from EB of day 21 expressed pluripotency markers to form colonies, whereas those isolated from EB of day 7 and 14 did not. To exclude proliferating cells from PrP(+) cells, stage specific embryo antigen 1 (SSEA1) was employed as a second marker. PrP(+)/SSEA1(-) cells did not proliferate and expressed cardiac cell markers, while PrP(+)/SSEA1(+) did proliferate.

CONCLUSION

PrP(+) cells isolated from EB included undifferentiated cells in day 21. PrP(+)/SSEA1(-) cells included cardiomyoctes, suggesting PrP and SSEA1 may be useful as markers to enrich the fraction of cardiomyocytes.

Prion protein localizes at the ciliary base during neural and cardiovascular development, and its depletion affects α-tubulin post-translational modifications

Although conversion of the cellular form of the prion protein (PrP^C) into a misfolded isoform is the underlying cause of prion diseases, understanding PrP^C physiological functions has remained challenging. PrP^C depletion or overexpression alters the proliferation and differentiation properties of various types of stem and progenitor cells in vitro by unknown mechanisms. Such involvement remains uncertain in vivo in the absence of any drastic phenotype of mice lacking PrP^C. Here, we report PrP^C enrichment at the base of the primary cilium in stem and progenitor cells from the central nervous system and cardiovascular system of developing mouse embryos. PrP^C depletion in a neuroepithelial cell line dramatically altered key cilium-dependent processes, such as Sonic hedgehog signalling and α-tubulin post-translational modifications. These processes were also affected over a limited time window in PrP^C–ablated embryos. Thus, our study reveals PrP^C as a potential actor in the developmental regulation of microtubule dynamics and ciliary functions.

Loss of Cellular Sialidases Does Not Affect the Sialylation Status of the Prion Protein but Increases the Amounts of Its Proteolytic Fragment C1

The central molecular event underlying prion diseases involves conformational change of the cellular form of the prion protein (PrPC), which is a sialoglycoprotein, into the disease-associated, transmissible form denoted PrPSc. Recent studies revealed a correlation between the sialylation status of PrPSc and incubation time to disease and introduced a new hypothesis that progression of prion diseases could be controlled or reversed by altering the sialylation level of PrPC. Of the four known mammalian sialidases, the enzymes that cleave off sialic acid residues, only NEU1, NEU3 and NEU4 are expressed in the brain. To test whether cellular sialidases control the steady-state sialylation level of PrPC and to identify the putative sialidase responsible for desialylating PrPC, we analyzed brain-derived PrPC from knockout mice deficient in Neu1, Neu3, Neu4, or from Neu3/Neu4 double knockouts. Surprisingly, no differences in the sialylation of PrPC or its proteolytic product C1 were noticed in any of the knockout mice tested as compared to the age-matched controls. However, significantly higher amounts of the C1 fragment relative to full-length PrPC were detected in the brains of Neu1 knockout mice as compared to WT mice or to the other knockout mice. Additional experiments revealed that in neuroblastoma cell line the sialylation pattern of C1 could be changed by an inhibitor of sialylatransferases. In summary, this study suggests that targeting cellular sialidases is apparently not the correct strategy for altering the sialylation levels of PrPC, whereas modulating the activity of sialylatransferases might offer a more promising approach. Our findings also suggest that catabolism of PrPC involves its α-cleavage followed by desialylation of the resulting C1 fragments by NEU1 and consequent fast degradation of the desialylated products.

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 Signaling in the Nervous System—A Review and Perspective

Prion protein (PrPC) was originally known as the causative agent of transmissible spongiform encephalopathy (TSE) but with recent research, its true function in cells is becoming clearer. It is known to act as a scaffolding protein, binding multiple ligands at the cell membrane and to be involved in signal transduction, passing information from the extracellular matrix (ECM) to the cytoplasm. Its role in the coordination of transmitters at the synapse, glyapse, and gap junction and in short- and long-range neurotrophic signaling gives PrPC a major part in neural transmission and nervous system signaling. It acts to regulate cellular function in multiple targets through its role as a controller of redox status and calcium ion flux. Given the importance of PrPC in cell physiology, this review considers its potential role in disease apart from TSE. The putative functions of PrPC point to involvement in neurodegenerative disease, neuropathic pain, chronic headache, and inflammatory disease including neuroinflammatory disease of the nervous system. Potential targets for the treatment of disease influenced by PrPC are discussed.

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 prion protein family: a view from the placenta

Based on its developmental pattern of expression, early studies suggested the implication of the mammalian Prion protein PrP, a glycosylphosphatidylinositol-anchored ubiquitously expressed and evolutionary conserved glycoprotein encoded by the Prnp gene, in early embryogenesis. However, gene invalidation in several species did not result in obvious developmental abnormalities and it was only recently that it was associated in mice with intra-uterine growth retardation and placental dysfunction. A proposed explanation for this lack of easily detectable developmental-related phenotype is the existence in the genome of one or more gene (s) able to compensate for the absence of PrP. Indeed, two other members of the Prnp gene family have been recently described, Doppel and Shadoo, and the consequences of their invalidation alongside that of PrP tested in mice. No embryonic defect was observed in mice depleted for Doppel and PrP. Interestingly, the co-invalidation of PrP and Shadoo in two independent studies led to apparently conflicting observations, with no apparent consequences in one report and the observation of a developmental defect of the ectoplacental cone that leads to early embryonic lethality in the other. This short review aims at summarizing these recent, apparently conflicting data highlighting the related biological questions and associated implications in terms of animal and human health.

The potential of mesenchymal stem cell in prion research

Scrapie and bovine spongiform encephalopathy are fatal neurodegenerative diseases caused by the accumulation of a misfolded protein (PrP(res)), the pathological form of the cellular prion protein (PrP(C)). For the last decades, prion research has greatly progressed, but many questions need to be solved about prion replication mechanisms, cell toxicity, differences in genetic susceptibility, species barrier or the nature of prion strains. These studies can be developed in murine models of transmissible spongiform encephalopathies, although development of cell models for prion replication and sample titration could reduce economic and timing costs and also serve for basic research and treatment testing. Some murine cell lines can replicate scrapie strains previously adapted in mice and very few show the toxic effects of prion accumulation. Brain cell primary cultures can be more accurate models but are difficult to develop in naturally susceptible species like humans or domestic ruminants. Stem cells can be differentiated into neuron-like cells and be infected by prions. However, the use of embryo stem cells causes ethical problems in humans. Mesenchymal stem cells (MSCs) can be isolated from many adult tissues, including bone marrow, adipose tissue or even peripheral blood. These cells differentiate into neuronal cells, express PrP(C) and can be infected by prions in vitro. In addition, in the last years, these cells are being used to develop therapies for many diseases, including neurodegenerative diseases. We review here the use of cell models in prion research with a special interest in the potential use of MSCs.

Prion diseases and adult neurogenesis: how do prions counteract the brain's endogenous repair machinery?

Scientific advances in stem cell biology and adult neurogenesis have raised the hope that neurodegenerative disorders could benefit from stem cell-based therapy. Adult neurogenesis might be part of the physiological regenerative process, however it might become impaired by the disease's mechanism and therefore contribute to neurodegeneration. In prion disorders this endogenous repair system has rarely been studied. Whether adult neurogenesis plays a role or not in brain repair or in the propagation of prion pathology remains unclear. We have recently investigated the status of adult neural stem cells isolated from prion-infected mice. We were able to show that neural stem cells accumulate and replicate prions thus resulting in an alteration of their neuronal destiny. We also reproduced these results in adult neural stem cells, which were infected in vitro. The fact that endogenous adult neurogenesis could be altered by the accumulation of misfolded prion protein represents another great challenge. Inhibiting prion propagation in these cells would thus help the endogenous neurogenesis to compensate for the injured neuronal system. Moreover, understanding the endogenous modulation of the neurogenesis system would help develop effective neural stem cell-based therapies.

Cancer stem cells display extremely large evolvability: alternating plastic and rigid networks as a potential Mechanism: network models, novel therapeutic target strategies, and the contributions of hypoxia, inflammation and cellular senescence

Cancer is increasingly perceived as a systems-level, network phenomenon. The major trend of malignant transformation can be described as a two-phase process, where an initial increase of network plasticity is followed by a decrease of plasticity at late stages of tumor development. The fluctuating intensity of stress factors, like hypoxia, inflammation and the either cooperative or hostile interactions of tumor inter-cellular networks, all increase the adaptation potential of cancer cells. This may lead to the bypass of cellular senescence, and to the development of cancer stem cells. We propose that the central tenet of cancer stem cell definition lies exactly in the indefinability of cancer stem cells. Actual properties of cancer stem cells depend on the individual "stress-history" of the given tumor. Cancer stem cells are characterized by an extremely large evolvability (i.e. a capacity to generate heritable phenotypic variation), which corresponds well with the defining hallmarks of cancer stem cells: the possession of the capacity to self-renew and to repeatedly re-build the heterogeneous lineages of cancer cells that comprise a tumor in new environments. Cancer stem cells represent a cell population, which is adapted to adapt. We argue that the high evolvability of cancer stem cells is helped by their repeated transitions between plastic (proliferative, symmetrically dividing) and rigid (quiescent, asymmetrically dividing, often more invasive) phenotypes having plastic and rigid networks. Thus, cancer stem cells reverse and replay cancer development multiple times. We describe network models potentially explaining cancer stem cell-like behavior. Finally, we propose novel strategies including combination therapies and multi-target drugs to overcome the Nietzschean dilemma of cancer stem cell targeting: "what does not kill me makes me stronger".

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.