Brain transcriptional stability upon prion protein-encoding gene invalidation in zygotic or adult mouse

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.

Loss of prion protein control of glucose metabolism promotes neurodegeneration in model of prion diseases

Corruption of cellular prion protein (PrPC) function(s) at the plasma membrane of neurons is at the root of prion diseases, such as Creutzfeldt-Jakob disease and its variant in humans, and Bovine Spongiform Encephalopathies, better known as mad cow disease, in cattle. The roles exerted by PrPC, however, remain poorly elucidated. With the perspective to grasp the molecular pathways of neurodegeneration occurring in prion diseases, and to identify therapeutic targets, achieving a better understanding of PrPC roles is a priority. Based on global approaches that compare the proteome and metabolome of the PrPC expressing 1C11 neuronal stem cell line to those of PrPnull-1C11 cells stably repressed for PrPC expression, we here unravel that PrPC contributes to the regulation of the energetic metabolism by orienting cells towards mitochondrial oxidative degradation of glucose. Through its coupling to cAMP/protein kinase A signaling, PrPC tones down the expression of the pyruvate dehydrogenase kinase 4 (PDK4). Such an event favors the transfer of pyruvate into mitochondria and its conversion into acetyl-CoA by the pyruvate dehydrogenase complex and, thereby, limits fatty acids β-oxidation and subsequent onset of oxidative stress conditions. The corruption of PrPC metabolic role by pathogenic prions PrPSc causes in the mouse hippocampus an imbalance between glucose oxidative degradation and fatty acids β-oxidation in a PDK4-dependent manner. The inhibition of PDK4 extends the survival of prion-infected mice, supporting that PrPSc-induced deregulation of PDK4 activity and subsequent metabolic derangements contribute to prion diseases. Our study posits PDK4 as a potential therapeutic target to fight against prion diseases.

The Cellular Prion Protein-ROCK Connection: Contribution to Neuronal Homeostasis and Neurodegenerative Diseases

Amyloid-based neurodegenerative diseases such as prion, Alzheimer's, and Parkinson's diseases have distinct etiologies and clinical manifestations, but they share common pathological events. These diseases are caused by abnormally folded proteins (pathogenic prions PrP^Sc in prion diseases, β-amyloids/Aβ and Tau in Alzheimer's disease, α-synuclein in Parkinson's disease) that display β-sheet-enriched structures, propagate and accumulate in the nervous central system, and trigger neuronal death. In prion diseases, PrP^Sc-induced corruption of the physiological functions exerted by normal cellular prion proteins (PrP^C) present at the cell surface of neurons is at the root of neuronal death. For a decade, PrP^C emerges as a common cell surface receptor for other amyloids such as Aβ and α-synuclein, which relays, at least in part, their toxicity. In lipid-rafts of the plasma membrane, PrP^C exerts a signaling function and controls a set of effectors involved in neuronal homeostasis, among which are the RhoA-associated coiled-coil containing kinases (ROCKs). Here we review (i) how PrP^C controls ROCKs, (ii) how PrP^C-ROCK coupling contributes to neuronal homeostasis, and (iii) how the deregulation of the PrP^C-ROCK connection in amyloid-based neurodegenerative diseases triggers a loss of neuronal polarity, affects neurotransmitter-associated functions, contributes to the endoplasmic reticulum stress cascade, renders diseased neurons highly sensitive to neuroinflammation, and amplifies the production of neurotoxic amyloids.

The Prion Protein Regulates Synaptic Transmission by Controlling the Expression of Proteins Key to Synaptic Vesicle Recycling and Exocytosis

The cellular prion protein (PrP^C), whose misfolded conformers are implicated in prion diseases, localizes to both the presynaptic membrane and postsynaptic density. To explore possible molecular contributions of PrP^C to synaptic transmission, we utilized a mass spectrometry approach to quantify the release of glutamate from primary cerebellar granule neurons (CGN) expressing, or deprived of (PrP-KO), PrP^C, following a depolarizing stimulus. Under the same conditions, we also tracked recycling of synaptic vesicles (SVs) in the two neuronal populations. We found that in PrP-KO CGN these processes decreased by 40 and 60%, respectively, compared to PrP^C-expressing neurons. Unbiased quantitative mass spectrometry was then employed to compare the whole proteome of CGN with the two PrP genotypes. This approach allowed us to assess that, relative to the PrP^C-expressing counterpart, the absence of PrP^C modified the protein expression profile, including diminution of some components of SV recycling and fusion machinery. Subsequent quantitative RT-PCR closely reproduced proteomic data, indicating that PrP^C is committed to ensuring optimal synaptic transmission by regulating genes involved in SV dynamics and neurotransmitter release. These novel molecular and cellular aspects of PrP^C add insight into the underlying mechanisms for synaptic dysfunctions occurring in neurodegenerative disorders in which a compromised PrP^C is likely to intervene.

Omics of Prion Diseases

Prion diseases are unique neurodegenerative pathologies that can occur with sporadic, genetic, and acquired etiologies. Human and animal prion diseases can be recapitulated in laboratory animals with good reproducibility providing highly controlled models for studying molecular mechanisms of neurodegeneration. In this chapter the overall area of omics research in prion diseases is described. The term omics includes all fields of studies that employ a comprehensive, unbiased, and high-throughput approach to areas of research such as functional genomics, transcriptomics, and proteomics. These kind of approaches can be extremely helpful in identifying disease susceptibility factors and pathways that are dysregulated upon the onset and the progression of the disease. Herein, the most important research about the various forms of prion pathologies in human and in models of prion diseases in animals is presented and discussed.

Loss of prion protein induces a primed state of type I interferon-responsive genes

The cellular prion protein (PrPC) has been extensively studied because of its pivotal role in prion diseases; however, its functions remain incompletely understood. A unique line of goats has been identified that carries a nonsense mutation that abolishes synthesis of PrPC. In these animals, the PrP-encoding mRNA is rapidly degraded. Goats without PrPC are valuable in re-addressing loss-of-function phenotypes observed in Prnp knockout mice. As PrPC has been ascribed various roles in immune cells, we analyzed transcriptomic responses to loss of PrPC in peripheral blood mononuclear cells (PBMCs) from normal goat kids (n = 8, PRNP+/+) and goat kids without PrPC (n = 8, PRNPTer/Ter) by mRNA sequencing. PBMCs normally express moderate levels of PrPC. The vast majority of genes were similarly expressed in the two groups. However, a curated list of 86 differentially expressed genes delineated the two genotypes. About 70% of these were classified as interferon-responsive genes. In goats without PrPC, the majority of type I interferon-responsive genes were in a primed, modestly upregulated state, with fold changes ranging from 1.4 to 3.7. Among these were ISG15, DDX58 (RIG-1), MX1, MX2, OAS1, OAS2 and DRAM1, all of which have important roles in pathogen defense, cell proliferation, apoptosis, immunomodulation and DNA damage response. Our data suggest that PrPC contributes to the fine-tuning of resting state PBMCs expression level of type I interferon-responsive genes. The molecular mechanism by which this is achieved will be an important topic for further research into PrPC physiology.

The biological function of the cellular prion protein: an update

The misfolding of the cellular prion protein (PrP^C) causes fatal neurodegenerative diseases. Yet PrP^C is highly conserved in mammals, suggesting that it exerts beneficial functions preventing its evolutionary elimination. Ablation of PrP^C in mice results in well-defined structural and functional alterations in the peripheral nervous system. Many additional phenotypes were ascribed to the lack of PrP^C, but some of these were found to arise from genetic artifacts of the underlying mouse models. Here, we revisit the proposed physiological roles of PrP^C in the central and peripheral nervous systems and highlight the need for their critical reassessment using new, rigorously controlled animal models.

Strictly co-isogenic C57BL/6J-Prnp-/- mice: A rigorous resource for prion science

Although its involvement in prion replication and neurotoxicity during transmissible spongiform encephalopathies is undisputed, the physiological role of the cellular prion protein (PrP(C)) remains enigmatic. A plethora of functions have been ascribed to PrP(C) based on phenotypes of Prnp(-/-) mice. However, all currently available Prnp(-/-) lines were generated in embryonic stem cells from the 129 strain of the laboratory mouse and mostly crossed to non-129 strains. Therefore, Prnp-linked loci polymorphic between 129 and the backcrossing strain resulted in systematic genetic confounders and led to erroneous conclusions. We used TALEN-mediated genome editing in fertilized mouse oocytes to create the Zurich-3 (ZH3) Prnp-ablated allele on a pure C57BL/6J genetic background. Genomic, transcriptional, and phenotypic characterization of Prnp(ZH3/ZH3) mice failed to identify phenotypes previously described in non-co-isogenic Prnp(-/-) mice. However, aged Prnp(ZH3/ZH3) mice developed a chronic demyelinating peripheral neuropathy, confirming the crucial involvement of PrP(C) in peripheral myelin maintenance. This new line represents a rigorous genetic resource for studying the role of PrP(C) in physiology and disease.

Mutated but Not Deleted Ovine PrP(C) N-Terminal Polybasic Region Strongly Interferes with Prion Propagation in Transgenic Mice

Mammalian prions are proteinaceous infectious agents composed of misfolded assemblies of the host-encoded, cellular prion protein (PrP). Physiologically, the N-terminal polybasic region of residues 23 to 31 of PrP has been shown to be involved in its endocytic trafficking and interactions with glycosaminoglycans or putative ectodomains of membrane-associated proteins. Several recent reports also describe this PrP region as important for the toxicity of mutant prion proteins and the efficiency of prion propagation, both in vitro and in vivo. The question remains as to whether the latter observations made with mouse PrP and mouse prions would be relevant to other PrP species/prion strain combinations given the dramatic impact on prion susceptibility of minimal amino acid substitutions and structural variations in PrP. Here, we report that transgenic mouse lines expressing ovine PrP with a deletion of residues 23 to 26 (KKRP) or mutated in this N-terminal region (KQHPH instead of KKRPK) exhibited a variable, strain-dependent susceptibility to prion infection with regard to the proportion of affected mice and disease tempo relative to findings in their wild-type counterparts. Deletion has no major effect on 127S scrapie prion pathogenesis, whereas mutation increased by almost 3-fold the survival time of the mice. Deletion marginally affected the incubation time of scrapie LA19K and ovine bovine spongiform encephalopathy (BSE) prions, whereas mutation caused apparent resistance to disease.

IMPORTANCE

Recent reports suggested that the N-terminal polybasic region of the prion protein could be a therapeutic target to prevent prion propagation or toxic signaling associated with more common neurodegenerative diseases such as Alzheimer's disease. Mutating or deleting this region in ovine PrP completes the data previously obtained with the mouse protein by identifying the key amino acid residues involved.

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.

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.

Gene expression resulting from PrPC ablation and PrPC overexpression in murine and cellular models

The cellular prion protein (PrP(C)) plays a key role in prion diseases when it converts to the pathogenic form scrapie prion protein. Increasing knowledge of its participation in prion infection contrasts with the elusive and controversial data regarding its physiological role probably related to its pleiotropy, cell-specific functions, and cellular-specific milieu. Multiple approaches have been made to the increasing understanding of the molecular mechanisms and cellular functions modulated by PrP(C) at the transcriptomic and proteomic levels. Gene expression analyses have been made in several mouse and cellular models with regulated expression of PrP(C) resulting in PrP(C) ablation or PrP(C) overexpression. These analyses support previous functional data and have yielded clues about new potential functions. However, experiments on animal models have shown moderate and varied results which are difficult to interpret. Moreover, studies in cell cultures correlate little with in vivo counterparts. Yet, both animal and cell models have provided some insights on how to proceed in the future by using more refined methods and selected functional experiments.

PrP(C) regulates epidermal growth factor receptor function and cell shape dynamics in Neuro2a cells

The prion protein (PrP) plays a key role in prion disease pathogenesis. Although the misfolded and pathologic variant of this protein (PrP(SC)) has been studied in depth, the physiological role of PrP(C) remains elusive and controversial. PrP(C) is a cell-surface glycoprotein involved in multiple cellular functions at the plasma membrane, where it interacts with a myriad of partners and regulates several intracellular signal transduction cascades. However, little is known about the gene expression changes modulated by PrP(C) in animals and in cellular models. In this article, we present PrP(C)-dependent gene expression signature in N2a cells and its implication in the most overrepresented functions: cell cycle, cell growth and proliferation, and maintenance of cell shape. PrP(C) over-expression enhances cell proliferation and cell cycle re-entrance after serum stimulation, while PrP(C) silencing slows down cell cycle progression. In addition, MAP kinase and protein kinase B (AKT) pathway activation are under the regulation of PrP(C) in asynchronous cells and following mitogenic stimulation. These effects are due in part to the modulation of epidermal growth factor receptor (EGFR) by PrP(C) in the plasma membrane, where the two proteins interact in a multimeric complex. We also describe how PrP(C) over-expression modulates filopodia formation by Rho GTPase regulation mainly in an AKT-Cdc42-N-WASP-dependent pathway.

An epigenetic framework for neurodevelopmental disorders: from pathogenesis to potential therapy

Neurodevelopmental disorders (NDDs) are characterized by aberrant and delayed early-life development of the brain, leading to deficits in language, cognition, motor behaviour and other functional domains, often accompanied by somatic symptoms. Environmental factors like perinatal infection, malnutrition and trauma can increase the risk of the heterogeneous, multifactorial and polygenic disorders, autism and schizophrenia. Conversely, discrete genetic anomalies are involved in Down, Rett and Fragile X syndromes, tuberous sclerosis and neurofibromatosis, the less familiar Phelan-McDermid, Sotos, Kleefstra, Coffin-Lowry and "ATRX" syndromes, and the disorders of imprinting, Angelman and Prader-Willi syndromes. NDDs have been termed "synaptopathies" in reference to structural and functional disturbance of synaptic plasticity, several involve abnormal Ras-Kinase signalling ("rasopathies"), and many are characterized by disrupted cerebral connectivity and an imbalance between excitatory and inhibitory transmission. However, at a different level of integration, NDDs are accompanied by aberrant "epigenetic" regulation of processes critical for normal and orderly development of the brain. Epigenetics refers to potentially-heritable (by mitosis and/or meiosis) mechanisms controlling gene expression without changes in DNA sequence. In certain NDDs, prototypical epigenetic processes of DNA methylation and covalent histone marking are impacted. Conversely, others involve anomalies in chromatin-modelling, mRNA splicing/editing, mRNA translation, ribosome biogenesis and/or the regulatory actions of small nucleolar RNAs and micro-RNAs. Since epigenetic mechanisms are modifiable, this raises the hope of novel therapy, though questions remain concerning efficacy and safety. The above issues are critically surveyed in this review, which advocates a broad-based epigenetic framework for understanding and ultimately treating a diverse assemblage of NDDs ("epigenopathies") lying at the interface of genetic, developmental and environmental processes. This article is part of the Special Issue entitled 'Neurodevelopmental Disorders'.

Prion protein and Shadoo are involved in overlapping embryonic pathways and trophoblastic development

The potential requirement of either the Prion or Shadoo protein for early mouse embryogenesis was recently suggested. However, the current data did not allow to precise the developmental process that was affected in the absence of both proteins and that led to the observed early lethal phenotype. In the present study, using various Prnp transgenic mouse lines and lentiviral vectors expressing shRNAs that target the Shadoo-encoding mRNA, we further demonstrate the specific requirement of at least one of these two PrP-related proteins at early developmental stages. Histological analysis reveals developmental defect of the ectoplacental cone and important hemorrhage surrounding the Prnp-knockout-Sprn-knockdown E7.5 embryos. By restricting the RNA interference to the trophoblastic cell lineages, the observed lethal phenotype could be attributed to the sole role of these proteins in this trophectoderm-derived compartment. RNAseq analysis performed on early embryos of various Prnp and Sprn genotypes indicated that the simultaneous down-regulation of these two proteins affects cell-adhesion and inflammatory pathways as well as the expression of ectoplacental-specific genes. Overall, our data provide biological clues in favor of a crucial and complementary embryonic role of the prion protein family in Eutherians and emphasizes the need to further evaluate its implication in normal and pathological human placenta biology.

Neuritogenesis: the prion protein controls β1 integrin signaling activity

Cytoskeleton modifications are required for neuronal stem cells to acquire neuronal polarization. Little is known, however, about mechanisms that orchestrate cytoskeleton remodeling along neuritogenesis. Here, we show that the silencing of the cellular prion protein (PrP(C)) impairs the initial sprouting of neurites upon induction of differentiation of the 1C11 neuroectodermal cell line, indicating that PrP(C) is necessary to neuritogenesis. Such PrP(C) function relies on its capacity to negatively regulate the clustering, activation, and signaling activity of β1 integrins at the plasma membrane. β1 Integrin aggregation caused by PrP(C) depletion triggers overactivation of the RhoA-Rho kinase-LIMK-cofilin pathway, which, in turn, alters the turnover of focal adhesions, increases the stability of actin microfilaments, and in fine impairs neurite formation. Inhibition of Rho kinases is sufficient to compensate for the lack of PrP(C) and to restore neurite sprouting. We also observe an increased secretion of fibronectin in the surrounding milieu of PrP(C)-depleted 1C11 cells, which likely self-sustains β1 integrin signaling overactivation and contributes to neuritogenesis defect. Our overall data reveal that PrP(C) contributes to the acquisition of neuronal polarization by modulating β1 integrin activity, cell interaction with fibronectin, and cytoskeleton dynamics.

Transcriptomic analysis brings new insight into the biological role of the prion protein during mouse embryogenesis

The biological function of the Prion protein remains largely unknown but recent data revealed its implication in early zebrafish and mammalian embryogenesis. To gain further insight into its biological function, comparative transcriptomic analysis between FVB/N and FVB/N Prnp knockout mice was performed at early embryonic stages. RNAseq analysis revealed the differential expression of 73 and 263 genes at E6.5 and E7.5, respectively. The related metabolic pathways identified in this analysis partially overlap with those described in PrP1 and PrP2 knockdown zebrafish embryos and prion-infected mammalian brains and emphasize a potentially important role for the PrP family genes in early developmental processes.

Prion protein function and the disturbance of early embryonic development in zebrafish

Transmissible Spongiform Encephalopathies (TSE) or prion diseases are a threat to food safety and to human and animal health. The molecular mechanisms responsible for prion diseases share similarities with a wider group of neurodegenerative disorders including Alzheimer disease and Parkinson disease and the central pathological event is a disturbance of protein folding of a normal cellular protein that is eventually accompanied by neuronal cell death and the death of the host. Prion protein (PrP) is a constituent of most normal mammalian cells and its presence is essential in the pathogenesis of TSE. However, the function of this normal cellular protein remains unclear. The prevention of PRNP gene expression in mammalian species has been undramatic, implying a functional redundancy. Yet PrP is conserved from mammals to fish. Recent studies of PrP in zebrafish have yielded novel findings showing that PrP has essential roles in early embryonic development. The amenability of zebrafish to global technologies has generated data indicating the existence of "anchorless" splice variants of PrP in the early embryo. This paper will discuss the possibility that the experimentalist's view of PrP functions might be clearer at a greater phylogenetic distance.

Developmental influence of the cellular prion protein on the gene expression profile in mouse hippocampus

The conversion of the cellular prion protein (PrP(C)) to an abnormal and protease-resistant isoform is the key event in prion diseases. Mice lacking PrP(C) are resistant to prion infection, and downregulation of PrP(C) during prion infection prevents neuronal loss and the progression to clinical disease. These results are suggestive of the potential beneficial effect of silencing PrP(C) during prion diseases. However, the silencing of a protein that is widely expressed throughout the central nervous system could be detrimental to brain homeostasis. The physiological role of PrP(C) remains still unclear, but several putative functions (e.g., neuronal development and maintenance) have been proposed. To assess the influence of PrP(C) on gene expression profile in the mouse brain, we undertook a microarray analysis by using RNA isolated from the hippocampus at two different developmental stages: newborn (4.5-day-old) and adult (3-mo-old) mice, both from wild-type and Prnp(0/0) animals. Comparing the different datasets allowed us to identify "commonly" co-regulated genes and "uniquely" deregulated genes during postnatal development. The absence of PrP(C) affected several biological pathways, the most representative being cell signaling, cell-cell communication and transduction processes, calcium homeostasis, nervous system development, synaptic transmission, and cell adhesion. However, there was only a moderate alteration of the gene expression profile in our animal models. PrP(C) deficiency did not lead to a dramatic alteration of gene expression profile and produced moderately altered gene expression levels from young to adult animals. Thus, our results may provide additional support to silencing endogenous PrP(C) levels as therapeutic approach to prion diseases.