Doxycycline control of prion protein transgene expression modulates prion disease in mice

Doxycycline for transgene control disrupts gut microbiome diversity without compromising acute neuroinflammatory response

The tetracycline transactivator (tTA) system provides controllable transgene expression through oral administration of the broad-spectrum antibiotic doxycycline. Antibiotic treatment for transgene control in mouse models of disease might have undesirable systemic effects resulting from changes in the gut microbiome. Here we assessed the impact of doxycycline on gut microbiome diversity in a tTA-controlled model of Alzheimer's disease and then examined neuroimmune effects of these microbiome alterations following acute LPS challenge. We show that doxycycline decreased microbiome diversity in both transgenic and wild-type mice and that these changes persisted long after drug withdrawal. Despite the change in microbiome composition, doxycycline treatment had minimal effect on basal transcriptional signatures of inflammation the brain or on the neuroimmune response to LPS challenge. Our findings suggest that central neuroimmune responses may be less affected by doxycycline at doses needed for transgene control than by antibiotic cocktails at doses used for experimental microbiome disruption.

Neurodegeneration, Mitochondria, and Antibiotics

Neurodegenerative diseases are characterized by the progressive loss of neurons, synapses, dendrites, and myelin in the central and/or peripheral nervous system. Actual therapeutic options for patients are scarce and merely palliative. Although they affect millions of patients worldwide, the molecular mechanisms underlying these conditions remain unclear. Mitochondrial dysfunction is generally found in neurodegenerative diseases and is believed to be involved in the pathomechanisms of these disorders. Therefore, therapies aiming to improve mitochondrial function are promising approaches for neurodegeneration. Although mitochondrial-targeted treatments are limited, new research findings have unraveled the therapeutic potential of several groups of antibiotics. These drugs possess pleiotropic effects beyond their anti-microbial activity, such as anti-inflammatory or mitochondrial enhancer function. In this review, we will discuss the controversial use of antibiotics as potential therapies in neurodegenerative diseases.

Prion protein and prion disease at a glance

Prion diseases are neurodegenerative disorders caused by conformational conversion of the cellular prion protein (PrPC) into scrapie prion protein (PrPSc). As the main component of prion, PrPSc acts as an infectious template that recruits and converts normal cellular PrPC into its pathogenic, misfolded isoform. Intriguingly, the phenomenon of prionoid, or prion-like, spread has also been observed in many other disease-associated proteins, such as amyloid β (Aβ), tau and α-synuclein. This Cell Science at a Glance and the accompanying poster highlight recently described physiological roles of prion protein and the advanced understanding of pathogenesis of prion disease they have afforded. Importantly, prion protein may also be involved in the pathogenesis of other neurodegenerative disorders such as Alzheimer's and Parkinson's disease. Therapeutic studies of prion disease have also exploited novel strategies to combat these devastating diseases. Future studies on prion protein and prion disease will deepen our understanding of the pathogenesis of a broad spectrum of neurodegenerative conditions.

Novel regulators of PrPC expression as potential therapeutic targets in prion diseases

INTRODUCTION

Prion diseases are rare and fatal neurodegenerative disorders. The key molecular event in these disorders is the misfolding of the physiological form of the cellular prion protein, PrP^C, leading to the accumulation of a pathological isoform, PrP^Sc, with unique features. Both isoforms share the same primary sequence, lacking detectable differences in posttranslational modification, a major hurdle for their biochemical or biophysical independent characterization. The mechanism underlying the conversion of PrP^C to PrP^Sc is not completely understood, so finding an effective therapy to cure prion disorders is extremely challenging.

AREAS COVERED

This review discusses the strategies for decreasing prion replication and throws a spotlight on the relevance of PrP^C in the prion accumulation process.

EXPERT OPINION

PrP^C is the key substrate for prion pathology; hence, the most promising therapeutic approach appears to be the targeting of PrP^C to block the production of the infectious isoform. The use of RNA interference and antisense oligonucleotide technologies may offer opportunities for treatment because of their success in clinical trials for other neurodegenerative diseases.

Death-associated protein kinase 1 suppresses hepatocellular carcinoma cell migration and invasion by upregulation of DEAD-box helicase 20

Death‐associated protein kinase 1 (DAPK) is a calcium/calmodulin kinase that plays a vital role as a suppressor gene in various cancers. Yet its role and target gene independent of p53 is still unknown in hepatocellular carcinoma (HCC). In this study, we discovered that DAPK suppressed HCC cell migration and invasion instead of proliferation or colony formation. Using a proteomics approach, we identified DEAD‐box helicase 20 (DDX20) as an important downstream target of DAPK in HCC cells and critical for DAPK‐mediated inhibition of HCC cell migration and invasion. Using integrin inhibitor RGD and GTPase activity assays, we discovered that DDX20 suppressed HCC cell migration and invasion through the CDC42‐integrin pathway, which was previously reported as an important downstream pathway of DAPK in cancer. Further research using cycloheximide found that DAPK attenuates the proteasomal degradation of DDX20 protein, which is dependent on the kinase activity of DAPK. Our results shed light on new functions and regulation for both DAPK and DDX20 in carcinogenesis and identifies new potential therapeutic targets for HCC.

Lrg1 Regulates β (1,3)-Glucan Masking in Candida albicans through the Cek1 MAP Kinase Pathway

Candida albicans is among the most prevalent opportunistic human fungal pathogens. The ability to mask the immunogenic polysaccharide β (1,3)-glucan from immune detection via a layer of mannosylated proteins is a key virulence factor of C. albicans We previously reported that hyperactivation of the Cek1 mitogen-activated protein (MAP) kinase pathway promotes β (1,3)-glucan exposure. In this communication, we report a novel upstream regulator of Cek1 activation and characterize the impact of Cek1 activity on fungal virulence. Lrg1 encodes a GTPase-activating protein (GAP) that has been suggested to inhibit the GTPase Rho1. We found that disruption of LRG1 causes Cek1 hyperactivation and β (1,3)-glucan unmasking. However, when GTPase activation was measured for a panel of GTPases, the lrg1ΔΔ mutant exhibited increased activation of Cdc42 and Ras1 but not Rho1 or Rac1. Unmasking and Cek1 activation in the lrg1ΔΔ mutant can be blocked by inhibition of the Ste11 MAP kinase kinase kinase (MAPKKK), indicating that the lrg1ΔΔ mutant acts through the canonical Cek1 MAP kinase cascade. In order to determine how Cek1 hyperactivation specifically impacts virulence, a doxycycline-repressible hyperactive STE11ΔN467 allele was expressed in C. albicans In the absence of doxycycline, this allele overexpressed STE11ΔN467 , which induced production of proinflammatory tumor necrosis factor alpha (TNF-α) from murine macrophages. This in vitro phenotype correlates with decreased colonization and virulence in a mouse model of systemic infection. The mechanism by which Ste11ΔN467 causes unmasking was explored with RNA sequencing (RNA-Seq) analysis. Overexpression of Ste11ΔN467 caused upregulation of the Cph1 transcription factor and of a group of cell wall-modifying proteins which are predicted to impact cell wall architecture.IMPORTANCECandida albicans is an important source of systemic infections in humans. The ability to mask the immunogenic cell wall polymer β (1,3)-glucan from host immune surveillance contributes to fungal virulence. We previously reported that the hyperactivation of the Cek1 MAP kinase cascade promotes cell wall unmasking, thus increasing strain immunogenicity. In this study, we identified a novel regulator of the Cek1 pathway called Lrg1. Lrg1 is a predicted GTPase-activating protein (GAP) that represses Cek1 activity by downregulating the GTPase Cdc42 and its downstream MAPKKK, Ste11. Upregulation of Cek1 activity diminished fungal virulence in the mouse model of infection, and this correlates with increased cytokine responses from macrophages. We also analyzed the transcriptional profile determined during β (1,3)-glucan exposure driven by Cek1 hyperactivation. Our report provides a model where Cek1 hyperactivation causes β (1,3)-glucan exposure by upregulation of cell wall proteins and leads to more robust immune detection in vivo, promoting more effective clearance.

Brain-wide distribution of reporter expression in five transgenic tetracycline-transactivator mouse lines

The spatial pattern of transgene expression in tetracycline-controlled mouse models is governed primarily by the driver line used to introduce the tetracycline-controlled transactivator (tTA). Detailed maps showing where each tTA driver activates expression are therefore essential for designing and using tet-regulated models, particularly in brain research where cell type and regional specificity determine the circuits affected by conditional gene expression. We have compiled a comprehensive online repository of serial microscopic images showing brain-wide reporter expression for five commonly used tTA driver lines. We have spatially registered all images to a common three-dimensional mouse brain anatomical reference atlas for direct comparison of spatial distribution across lines. The high-resolution images and associated metadata are shared via the web page of the EU Human Brain Project. Images can be inspected using an interactive viewing tool that includes an optional overlay feature providing anatomical delineations and reference atlas coordinates. Interactive viewing is supplemented by semi-quantitative analyses of expression levels within anatomical subregions for each tTA driver line.

Therapies for prion diseases

Recent advances in understanding of the molecular biology of prion diseases and improved clinical diagnostic techniques might allow researchers to think about therapeutic trials in Creutzfeldt-Jakob disease (CJD) patients. Some attempts have been made in the past and various compounds have been tested in single case reports and patient series. Controlled trials are rare. However, in the past few years, it has been demonstrated that clinical trials are feasible. The clinicians might face several specific problems when evaluating the efficacy of the drug in CJD, such as rareness of the disease, lack of appropriate preclinical tests and heterogeneous clinical presentation in humans. These problems have to be carefully addressed in future.

The function of the cellular prion protein in health and disease

The essential role of the cellular prion protein (PrP^C) in prion disorders such as Creutzfeldt-Jakob disease is well documented. Moreover, evidence is accumulating that PrP^C may act as a receptor for protein aggregates and transduce neurotoxic signals in more common neurodegenerative disorders, such as Alzheimer's disease. Although the pathological roles of PrP^C have been thoroughly characterized, a general consensus on its physiological function within the brain has not yet been established. Knockout studies in various organisms, ranging from zebrafish to mice, have implicated PrP^C in a diverse range of nervous system-related activities that include a key role in the maintenance of peripheral nerve myelination as well as a general ability to protect against neurotoxic stimuli. Thus, the function of PrP^C may be multifaceted, with different cell types taking advantage of unique aspects of its biology. Deciphering the cellular function(s) of PrP^C and the consequences of its absence is not simply an academic curiosity, since lowering PrP^C levels in the brain is predicted to be a powerful therapeutic strategy for the treatment of prion disease. In this review, we outline the various approaches that have been employed in an effort to uncover the physiological and pathological functions of PrP^C. While these studies have revealed important clues about the biology of the prion protein, the precise reason for PrP^C's existence remains enigmatic.

Treatment during a vulnerable developmental period rescues a genetic epilepsy

The nervous system is vulnerable to perturbations during specific developmental periods. Insults during such susceptible time windows can have long-term consequences, including the development of neurological diseases such as epilepsy. Here we report that a pharmacological intervention timed during a vulnerable neonatal period of cortical development prevents pathology in a genetic epilepsy model. By using mice with dysfunctional Kv7 voltage-gated K(+) channels, which are mutated in human neonatal epilepsy syndromes, we demonstrate the safety and efficacy of the sodium-potassium-chloride cotransporter NKCC1 antagonist bumetanide, which was administered during the first two postnatal weeks. In Kv7 current-deficient mice, which normally display epilepsy, hyperactivity and stereotypies as adults, transient bumetanide treatment normalized neonatal in vivo cortical network and hippocampal neuronal activity, prevented structural damage in the hippocampus and restored wild-type adult behavioral phenotypes. Furthermore, bumetanide treatment did not adversely affect control mice. These results suggest that in individuals with disease susceptibility, timing prophylactically safe interventions to specific windows during development may prevent or arrest disease progression.

Dewetting transition assisted clearance of (NFGAILS) amyloid fibrils from cell membranes by graphene

Clearance of partially ordered oligomers and monomers deposited on cell membrane surfaces is believed to be an effective route to alleviate many potential protein conformational diseases (PCDs). With large-scale all-atom molecular dynamics simulations, here we show that graphene nanosheets can easily and quickly win a competitive adsorption of human islet amyloid polypeptides (hIAPP22-28) NFGAILS and associated fibrils against cell membrane, due to graphene's unique two-dimensional, highly hydrophobic surface with its all-sp(2) hybrid structure. A nanoscale dewetting transition was observed at the interfacial region between the fibril (originally deposited on the membrane) and the graphene nanosheet, which significantly assisted the adsorption of fibrils onto graphene from the membrane. The π-π stacking interaction between Phe23 and graphene played a crucial role, providing the driving force for the adsorption at the graphene surface. This study renders new insight towards the importance of water during the interactions between amyloid peptides, the phospholipidic membrane, and graphene, which might shed some light on future developments of graphene-based nanomedicine for preventing/curing PCDs like type II diabetes mellitus.

Fluorescent Calcium Indicator Protein Expression in the Brain Using Tetracycline-Responsive Transgenic Mice

To achieve robust long-term fluorescent calcium indicator protein (FCIP) expression in mammalian neurons in vivo, classical mouse transgenesis by pronuclear DNA injection using tetracycline (Tet)-controlled genetic switches can be deployed. This protocol describes methods for regulated expression of FCIP using Tet-responsive transgenic mice. The Tet-inducible system requires three components for inducible and reversible control of gene expression: (1) a potent transcriptional activator protein, either Tet transactivator (tTA) or reverse tTA (rtTA); (2) a minimal Tet-promoter (P(tet)) or a bidirectional Tet-promoter (P(tet)bi) to express one or more responder genes; and (3) Tet or one of its derivatives such as doxycycline (Dox) as an inducer. To ensure a high level of FCIP expression in neurons, transgenic founder mice are screened using an ear fibroblast culture method to identify those that are responsive to Dox treatment before use in experiments. The protocol describes the use of Dox to regulate gene expression and provides a short description of in vivo recording of luciferase activity.

A combination of doxycycline and ribavirin alleviated chikungunya infection

Lack of vaccine and effective antiviral drugs against chikungunya virus (CHIKV) outbreaks have led to significant impact on health care in the developing world. Here, we evaluated the antiviral effects of tetracycline (TETRA) derivatives and other common antiviral agents against CHIKV. Our results showed that within the TETRA derivatives group, Doxycycline (DOXY) exhibited the highest inhibitory effect against CHIKV replication in Vero cells. On the other hand, in the antiviral group Ribavirin (RIBA) showed higher inhibitory effects against CHIKV replication compared to Aciclovir (ACIC). Interestingly, RIBA inhibitory effects were also higher than all but DOXY within the TETRA derivatives group. Docking studies of DOXY to viral cysteine protease and E2 envelope protein showed non-competitive interaction with docking energy of -6.6±0.1 and -6.4±0.1 kcal/mol respectively. The 50% effective concentration (EC50) of DOXY and RIBA was determined to be 10.95±2.12 μM and 15.51±1.62 μM respectively, while DOXY+RIBA (1:1 combination) showed an EC₅₀ of 4.52±1.42 μM. When compared, DOXY showed higher inhibition of viral infectivity and entry than RIBA. In contrast however, RIBA showed higher inhibition against viral replication in target cells compared to DOXY. Assays using mice as animal models revealed that DOXY+RIBA effectively inhibited CHIKV replication and attenuated its infectivity in vivo. Further experimental and clinical studies are warranted to investigate their potential application for clinical intervention of CHIKV disease.

Animal models for prion-like diseases

Prion diseases or Transmissible Spongiform Encephalopathies (TSEs) are a group of fatal neurodegenerative disorders affecting several mammalian species being Creutzfeldt-Jacob Disease (CJD) the most representative in human beings, scrapie in ovine, Bovine Spongiform Encephalopathy (BSE) in bovine and Chronic Wasting Disease (CWD) in cervids. As stated by the "protein-only hypothesis", the causal agent of TSEs is a self-propagating aberrant form of the prion protein (PrP) that through a misfolding event acquires a β-sheet rich conformation known as PrP(Sc) (from scrapie). This isoform is neurotoxic, aggregation prone and induces misfolding of native cellular PrP. Compelling evidence indicates that disease-specific protein misfolding in amyloid deposits could be shared by other disorders showing aberrant protein aggregates such as Alzheimer's Disease (AD), Parkinson's Disease (PD), Amyotrophic lateral sclerosis (ALS) and systemic Amyloid A amyloidosis (AA amyloidosis). Evidences of shared mechanisms of the proteins related to each disease with prions will be reviewed through the available in vivo models. Taking prion research as reference, typical prion-like features such as seeding and propagation ability, neurotoxic species causing disease, infectivity, transmission barrier and strain evidences will be analyzed for other protein-related diseases. Thus, prion-like features of amyloid β peptide and tau present in AD, α-synuclein in PD, SOD-1, TDP-43 and others in ALS and serum α-amyloid (SAA) in systemic AA amyloidosis will be reviewed through models available for each disease.

Synthesis and antibacterial activity of doxycycline neoglycosides

A set of 37 doxycycline neoglycosides were prepared, mediated via a C-9 alkoxyamino-glycyl-based spacer reminiscent of that of tigecycline. Subsequent in vitro antibacterial assays against representative drug-resistant Gram negative and Gram positive strains revealed a sugar-dependent activity profile and one doxycycline neoglycoside, the 2'-amino-α-D-glucoside conjugate, to rival that of the parent pharmacophore. In contrast, the representative tetracycline-susceptible strain E. coli 25922 was found to be relatively responsive to a range of doxycycline neoglycosides. This study also extends the use of aminosugars in the context of neoglycosylation via a simple two-step strategy anticipated to be broadly applicable for neoglycorandomization.

Candida albicans ENO1 null mutants exhibit altered drug susceptibility, hyphal formation, and virulence

We previously showed that the expression of ENO1 (enolase) in the fungal pathogen Candida albicans is critical for cell growth. In this study, we investigate the contribution of the ENO1 gene to virulence. We conducted our functional study of ENO1 in C. albicans by constructing an eno1/eno1 null mutant strain in which both ENO1 alleles in the genome were knockouted with the SAT1 flipper cassette that contains the nourseothricin-resistance marker. Although the null mutant failed to grow on synthetic media containing glucose, it was capable of growth on media containing yeast extract, peptone, and non-fermentable carbon sources. The null mutant was more susceptible to certain antifungal drugs. It also exhibited defective hyphal formation, and was avirulent in BALB/c mice.

Unique drug screening approach for prion diseases identifies tacrolimus and astemizole as antiprion agents

Prion diseases such as Creutzfeldt-Jakob disease (CJD) are incurable and rapidly fatal neurodegenerative diseases. Because prion protein (PrP) is necessary for prion replication but dispensable for the host, we developed the PrP-FRET-enabled high throughput assay (PrP-FEHTA) to screen for compounds that decrease PrP expression. We screened a collection of drugs approved for human use and identified astemizole and tacrolimus, which reduced cell-surface PrP and inhibited prion replication in neuroblastoma cells. Tacrolimus reduced total cellular PrP levels by a nontranscriptional mechanism. Astemizole stimulated autophagy, a hitherto unreported mode of action for this pharmacophore. Astemizole, but not tacrolimus, prolonged the survival time of prion-infected mice. Astemizole is used in humans to treat seasonal allergic rhinitis in a chronic setting. Given the absence of any treatment option for CJD patients and the favorable drug characteristics of astemizole, including its ability to cross the blood-brain barrier, it may be considered as therapy for CJD patients and for prophylactic use in familial prion diseases. Importantly, our results validate PrP-FEHTA as a method to identify antiprion compounds and, more generally, FEHTA as a unique drug discovery platform.

Strain background influences neurotoxicity and behavioral abnormalities in mice expressing the tetracycline transactivator

The tet-off system has been widely used to create transgenic models of neurological disorders including Alzheimer's, Parkinson's, Huntington's, and prion disease. The utility of this system lies in the assumption that the tetracycline transactivator (TTA) acts as an inert control element and does not contribute to phenotypes under study. Here we report that neuronal expression of TTA can affect hippocampal cytoarchitecture and behavior in a strain-dependent manner. While studying neurodegeneration in two tet-off Alzheimer's disease models, we unexpectedly discovered neuronal loss within the dentate gyrus of single transgenic TTA controls. Granule neurons appeared most sensitive to TTA exposure during postnatal development, and doxycycline treatment during this period was neuroprotective. TTA-induced degeneration could be rescued by moving the transgene onto a congenic C57BL/6J background and recurred on reintroduction of either CBA or C3H/He backgrounds. Quantitative trait analysis of B6C3 F2 TTA mice identified a region on Chromosome 14 that contains a major modifier of the neurodegenerative phenotype. Although B6 mice were resistant to degeneration, they were not ideal for cognitive testing. F1 offspring of TTA C57BL/6J and 129X1/SvJ, FVB/NJ, or DBA/1J showed improved spatial learning, but TTA expression caused subtle differences in contextual fear conditioning on two of these backgrounds, indicating that strain and genotype can interact independently under different behavioral settings. All model systems have limitations that should be recognized and mitigated where possible; our findings stress the importance of mapping the effects caused by TTA alone when working with tet-off models.

Tetracycline inducible gene manipulation in serotonergic neurons

The serotonergic (5-HT) neuronal system has important and diverse physiological functions throughout development and adulthood. Its dysregulation during development or later in adulthood has been implicated in many neuropsychiatric disorders. Transgenic animal models designed to study the contribution of serotonergic susceptibility genes to a pathological phenotype should ideally allow to study candidate gene overexpression or gene knockout selectively in serotonergic neurons at any desired time during life. For this purpose, conditional expression systems such as the tet-system are preferable. Here, we generated a transactivator (tTA) mouse line (TPH2-tTA) that allows temporal and spatial control of tetracycline (Ptet) controlled transgene expression as well as gene deletion in 5-HT neurons. The tTA cDNA was inserted into a 196 kb PAC containing a genomic mouse Tph2 fragment (177 kb) by homologous recombination in E. coli. For functional analysis of Ptet-controlled transgene expression, TPH2-tTA mice were crossed to a Ptet-regulated lacZ reporter line (Ptet-nLacZ). In adult double-transgenic TPH2-tTA/Ptet-nLacZ mice, TPH2-tTA founder line L62-20 showed strong serotonergic β-galactosidase expression which could be completely suppressed with doxycycline (Dox). Furthermore, Ptet-regulated gene expression could be reversibly activated or inactivated when Dox was either withdrawn or added to the system. For functional analysis of Ptet-controlled, Cre-mediated gene deletion, TPH2-tTA mice (L62-20) were crossed to double transgenic Ptet-Cre/R26R reporter mice to generate TPH2-tTA/Ptet-Cre/R26R mice. Without Dox, 5-HT specific recombination started at E12.5. With permanent Dox administration, Ptet-controlled Cre-mediated recombination was absent. Dox withdrawal either postnatally or during adulthood induced efficient recombination in serotonergic neurons of all raphe nuclei, respectively. In the enteric nervous system, recombination could not be detected. We generated a transgenic mouse tTA line (TPH2-tTA) which allows both inducible and reversible transgene expression and inducible Cre-mediated gene deletion selectively in 5-HT neurons throughout life. This will allow precise delineation of serotonergic gene functions during development and adulthood.

Mouse models of Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia, affecting 35 million people today. The search for new treatments is made ever more urgent by prospects for increasing prevalence due to population aging. Mouse models are one of the most important research tools for finding new treatments for AD. Here, we review those models. We begin by briefly reviewing the AD genetics on which mouse models are based and then consider the most common mouse models of AD, including mice transgenic for human amyloid precursor protein (hAPP) and beta-amyloid (Aβ), mice expressing mutant presenilin genes, mice modeling tau's role in AD, and apolipoprotein E models. The discussion highlights key features and important differences between these mouse models. We conclude with a discussion about the role of AD mouse models in the translational pipeline.

α-Syn suppression reverses synaptic and memory defects in a mouse model of dementia with Lewy bodies

Abnormally accumulated α-synuclein (α-syn) is a pathological hallmark of Lewy body-related disorders such as Parkinson's disease (PD) and dementia with Lewy body disease (DLB). However, it is not well understood whether and how abnormal accumulation of α-syn leads to cognitive impairment or dementia in PD and DLB. Furthermore, it is not known whether targeted removal of α-syn pathology can reverse cognitive decline. Here, we found that the distribution of α-syn pathology in an inducible α-syn transgenic mouse model recapitulates that in human DLB. Abnormal accumulation of α-syn in the limbic system, particularly in the hippocampus, correlated with memory impairment and led to structural synaptic deficits. Furthermore, when α-syn expression was suppressed, we observed partial clearing of pre-existing α-syn pathology and reversal of structural synaptic defects, resulting in an improvement in memory function.

Evidence that microglia mediate the neurobiological effects of chronic psychological stress on the medial prefrontal cortex

Psychological stress contributes to the development of clinical depression. This has prompted many preclinical studies to investigate the neurobiology of this relationship, however, the effects of stress on glia remain unclear. In this study, we wished to determine, first, how exposure to chronic psychological stress affects microglial activity within the prefrontal cortex (PFC) and, second, whether the observed changes were meaningfully related to corresponding changes in local neuronal activity and PFC-regulated behavior. Therefore, we examined markers of microglial activation, antigen presentation, apoptosis, and persistent neuronal activation within the PFC after exposure to repeated restraint stress. We also examined the effect of stress on spatial working memory, a PFC-dependent function. Finally, we tested the ability of a microglial activation inhibitor (minocycline) to alter the impact of chronic stress on all of these endpoints. Stressor exposure produced positively correlated increases in microglial and long-term neuronal activation in the PFC but not antigen presentation or apoptosis. As expected, it also impaired spatial working memory. Importantly, minocycline reduced the impact of stress on neuronal activation and working memory, as well as microglial activation. These results suggest a role for microglia in mediating the effects of stress on PFC neuronal function and PFC-regulated behavior.

Development of a tightly-regulated tetracycline-dependent transcriptional activator and repressor co-expression system for the strong induction of transgene expression

The teteracycline (Tc)-dependent and -inducible transcriptional activator (rtTA) system has been used to express regulated transgene expression in vitro and in vivo. However, previous reports have demonstrated that, even in the absence of Tc, the rtTA binds weakly to the tetracycline response element (TRE), leading to a low level of background activity. In order to reduce the leaky gene expression induced by rtTA, we previously established a tightly regulated system (A-IRES-R system) that makes use of both the rtTA (A) and a Tc-dependent repressor (TetR-Kruppel-associated box; KRAB) (R). In addition, others have described a transactivator rtTA2-M2 (M2) that displays higher sensitivity to Dox than rtTA. In this study, to further develop the A-IRES-R system, we generated a derivative Tc system (M2-IRES-R system) that co-expresses both rtTA-M2 and TetR-KRAB from a single vector. We show that compared to the A-IRES-R system, the M2-IRES-R system leads to a greater level of induced TRE-mediated transcription in the presence of doxycycline (Dox) and yet displays a similar level of basal TRE-mediated transcription in the absence of Dox. Furthermore, the M2-IRES-R system also displays less leaky gene expression in the absence of Dox compared to rtTA-M2 and rtTA systems. Taken together, our results suggest that the M2-IRES-R system enables to tightly regulate and highly induce the expression of transgene compared to other systems.

Atlas of transgenic Tet-Off Ca2+/calmodulin-dependent protein kinase II and prion protein promoter activity in the mouse brain

Conditional transgenic mouse models are important tools for investigations of neurodegenerative diseases and evaluation of potential therapeutic interventions. A popular conditional transgenic system is the binary tetracycline-responsive gene (Tet-Off) system, in which the expression of the gene of interest depends on a tetracycline-regulatable transactivator (tTA) under the control of a specific promoter construct. The most frequently used Tet-Off promoter mouse lines are the Ca(2+)/calmodulin-dependent protein kinase II (CamKII) and prion protein (PrP) promoter lines, respectively. To target the regulated gene of interest to relevant brain regions, a priori knowledge about the spatial distribution of the regulated gene expression in the brain is important. Such distribution patterns can be investigated using double transgenic mice in which the promoter construct regulates a LacZ reporter gene encoding the marker β-galactosidase which can be histologically detected using its substrate X-gal. We have previously published an atlas showing the brain-wide expression mediated by the Tet-Off PrP promoter mouse line, but the distribution of activity in the Tet-Off CamKII promoter mouse line is less well known. To compare promoter activity distributions in these two Tet-Off mouse lines, we have developed an online digital atlas tailored for side-by-side comparison of histological section images. The atlas provides a comprehensive list of brain regions containing X-gal labeling and an interactive dual image viewer tool for panning and zooming of corresponding section images. Comparison of spatial expression patterns between the two lines show considerable regional and cellular differences, relevant in context of generation and analysis of inducible models based on these two tetracycline responsive promoter mouse lines.

New approaches for modelling sporadic genetic disease in the mouse

Sporadic diseases, which occur as single, scattered cases, are among the commonest causes of human morbidity and death. They result in a variety of diseases, including many cancers, premature aging, neurodegeneration and skeletal defects. They are often pathogenetically complex, involving a mosaic distribution of affected cells, and are difficult to model in the mouse. Faithful models of sporadic diseases require innovative forms of genetic manipulation to accurately recreate their initiation and pathogenesis. Such modelling is crucial to understanding these diseases and, by extension, to the development of therapeutic approaches to treat them. This article focuses on sporadic diseases with a genetic aetiology, the challenges they pose to biomedical researchers, and the different current and developing approaches used to model such disorders in the mouse.

Reversibility of symptoms in a conditional mouse model of spinocerebellar ataxia type 3

Spinocerebellar ataxia type 3 (SCA3) is caused by the expansion of a CAG repeat tract that affects the MJD1 gene which encodes the ataxin-3 protein. In order to analyze whether symptoms caused by ataxin-3 with an expanded repeat are reversible in vivo, we generated a conditional mouse model of SCA3 using the Tet-Off system. We used a full-length human ataxin-3 cDNA with 77 repeats in order to generate the responder mouse line. After crossbreeding with a PrP promoter mouse line, double transgenic mice developed a progressive neurological phenotype characterized by neuronal dysfunction in the cerebellum, reduced anxiety, hyperactivity, impaired Rotarod performance and lower body weight gain. When ataxin-3 expression was turned off in symptomatic mice in an early disease state, the transgenic mice were indistinguishable from negative controls after 5 months of treatment. These results show that reducing the production of pathogenic ataxin-3 indeed may be a promising approach to treat SCA3, provided that such treatment is applied before irreversible damage has taken place and that it is continued for a sufficiently long time.

Unexpected expression pattern of tetracycline-regulated transgenes in mice

In generating a conditional transgenic murine model based on a tetracycline-regulated system, we obtained unexpected patterns of expression due to the transcriptional inactivity of the tet-responder promoter. Here we show strong cell-type-restricted expression that was variegated to an extent determined by the number of responder transgene copies integrated into the host genome.

Quorum-Sensing-Based Toolbox for Regulatable Transgene and siRNA Expression in Mammalian Cells

Technologies for regulated expression of multiple transgenes in mammalian cells have gathered momentum for bioengineering, gene therapy, drug discovery, and gene-function analyses. Capitalizing on recently developed mammalian transgene modalities (QuoRex) derived from Streptomyces coelicolor, we have designed a flexible and highly compatible expression vector set that enables desired transgene/siRNA control in response to the nontoxic butyrolactone SCB1. The construction-kit-like expression portfolio includes (i) multicistronic (pTRIDENT), (ii) autoregulated, (iii) bidirectional (pBiRex), (iv) oncoretro- and lentiviral transduction, and (v) RNA polymerase II-based siRNA transcription-fine-tuning vectors for straightforward implementation of QuoRex-controlled (trans)gene modulation in mammalian cells.

Spatial and temporal down-regulation of transgene expression using the TRSID-silencer in mice: application to Prnp

Spatial and temporal control of ovine prion protein (Prnp) gene expression was achieved in mice using two transgenes: a Prnp minigene with tet-operator sequences inserted 5' to exon 1 and a mouse neurofilament genomic clone carrying the chimeric-repressor TRSID cDNA. In bi-transgenic mice, ovine PrP(C) expression could be reversibly controlled in neuronal cells by doxycycline treatment whereas it remains constant in other cell types. Overall, this model opens opportunities to assess the involvement of cell types in prion diseases and PrP physiological function. It demonstrates the potentiality of the TRSID-silencer to precisely control temporal and spatial gene expression in vivo.

Neurodegeneration and motor dysfunction in a conditional model of Parkinson's disease

Alpha-synuclein (alpha-syn) has been implicated in the pathogenesis of many neurodegenerative disorders, including Parkinson's disease. These disorders are characterized by various neurological and psychiatric symptoms based on progressive neuropathological alterations. Whether the neurodegenerative process might be halted or even reversed is presently unknown. Therefore, conditional mouse models are powerful tools to analyze the relationship between transgene expression and progression of the disease. To explore whether alpha-syn solely originates and further incites these alterations, we generated conditional mouse models by using the tet-regulatable system. Mice expressing high levels of human wild-type alpha-syn in midbrain and forebrain regions developed nigral and hippocampal neuropathology, including reduced neurogenesis and neurodegeneration in absence of fibrillary inclusions, leading to cognitive impairment and progressive motor decline. Turning off transgene expression in symptomatic mice halted progression but did not reverse the symptoms. Thus, our data suggest that approaches targeting alpha-syn-induced pathological pathways might be of benefit rather in early disease stages. Furthermore, alpha-syn-associated cytotoxicity is independent of filamentous inclusion body formation in our conditional mouse model.

Conditional transgenic mouse models: from the basics to genome-wide sets of knockouts and current studies of tissue regeneration

Many mouse models are currently available, providing avenues to elucidate gene function and to recapitulate specific pathological conditions. To a large extent, successful translation of clinical evidence or analytical data into appropriate mouse models is possible through progress in transgenic or gene-targeting technology. Beginning with a review of standard mouse transgenics and conventional gene targeting, this article will move on to discussing the basics of conditional gene expression: the tetracycline (tet)-off and tet-on systems based on the transactivators tet-controlled transactivator (Tta) and reverse tet-on transactivator (rtTA) that allow downregulation or induction of gene expression; Cre or Flp recombinase-mediated modifications, including excision, inversion, insertion and interchromosomal translocation; combination of the tet and Cre systems, permitting inducible knockout, reporter gene activation or activation of point mutations; the avian retroviral system based on delivery of rtTA specifically into cells expressing the avian retroviral receptor, which enables cell type-specific, inducible gene expression; the tamoxifen system, one of the most frequently applied steroid receptor-based systems, allows rapid activation of a fusion protein between the gene of interest and a mutant domain of the estrogen receptor, whereby activation does not depend on transcription; and techniques for cell type-specific ablation. The diphtheria toxin receptor system offers the advantage that it can be combined with the ‘zoo’ of Cre recombinase driver mice. Having described the basics we move on to the cutting edge: generation of genome-wide sets of conditional knockout mice. To this end, large ongoing projects apply two strategies: gene trapping based on random integration of trapping vectors into introns leading to truncation of the transcript, and gene targeting, representing the directed approach using homologous recombination. It can be expected that in the near future genome-wide sets of such mice will be available. Finally, the possibilities of conditional expression systems for investigating gene function in tissue regeneration will be illustrated by examples for neurodegenerative disease, liver regeneration and wound healing of the skin.

Evaluation of the human transmission risk of an atypical bovine spongiform encephalopathy prion strain

Bovine spongiform encephalopathy (BSE), the prion disease in cattle, was widely believed to be caused by only one strain, BSE-C. BSE-C causes the fatal prion disease named new variant Creutzfeldt-Jacob disease in humans. Two atypical BSE strains, bovine amyloidotic spongiform encephalopathy (BASE, also named BSE-L) and BSE-H, have been discovered in several countries since 2004; their transmissibility and phenotypes in humans are unknown. We investigated the infectivity and human phenotype of BASE strains by inoculating transgenic (Tg) mice expressing the human prion protein with brain homogenates from two BASE strain-infected cattle. Sixty percent of the inoculated Tg mice became infected after 20 to 22 months of incubation, a transmission rate higher than those reported for BSE-C. A quarter of BASE strain-infected Tg mice, but none of the Tg mice infected with prions causing a sporadic human prion disease, showed the presence of pathogenic prion protein isoforms in the spleen, indicating that the BASE prion is intrinsically lymphotropic. The pathological prion protein isoforms in BASE strain-infected humanized Tg mouse brains are different from those from the original cattle BASE or sporadic human prion disease. Minimal brain spongiosis and long incubation times are observed for the BASE strain-infected Tg mice. These results suggest that in humans, the BASE strain is a more virulent BSE strain and likely lymphotropic.

Cell culture models to unravel prion protein function and aberrancies in prion diseases

From an early stage of prion research, tissue cultures that could support and propagate the scrapie agent were sought after. The earliest attempts were explants from brains of infected mice, and their growth and morphological characteristics were compared with those from uninfected mice. Using the explant technique, several investigators reported increased cell growth in cultures established from scrapie-sick brain compared with cultures from normal mice. These are odd findings in the light of the massive neuronal cell death known to occur in scrapie-infected brains; however, the cell types responsible for the increased cell growth in the scrapie-explants most probably were not neuronal. The first successful cell culture established in this way, in which the scrapie agent was serially and continuously passaged beyond the initial explant, was in the scrapie mouse brain culture, which is still used today. This chapter describes the generation and use of chronically prion-infected cell lines as cell culture models of prion diseases. These cell lines have been crucial for the current understanding of the cell biology of both the normal (PrP(C)) and the pathogenic isoform (PrP(Sc)) of the prion protein. They also have been useful in the development of antiprion drugs, prospectively used for therapy of prion diseases, and they offer an alternative approach for transmission/infectivity assays normally performed by mouse bioassay. Cell culture models also have been used to study prion-induced cytopathological changes, which could explain the typical spongiform neurodegeneration in prion diseases.

Transgenic mouse models of prion diseases

Prions represent a new biological paradigm of protein-mediated information transfer. In mammals, prions are the cause of fatal, transmissible neurodegenerative diseases, often referred to as transmissible spongiform encephalopathies. Many unresolved issues remain, including the exact molecular nature of the prion, the detailed mechanism of prion propagation, and the mechanism by which prion diseases can be both genetic and infectious. In addition, we know little about the mechanism by which neurons degenerate during prion diseases. Tied to this, the physiological function of the normal form of the prion protein remains unclear, and it is uncertain whether loss of this function contributes to prion pathogenesis. The factors governing the transmission of prions between species remain unclear, in particular the means by which prion strains and PrP primary structure interact to affect interspecies prion transmission. Despite all these unknowns, dramatic advances in our understanding of prions have occurred because of their transmissibility to experimental animals and the development of transgenic mouse models has done much to further our understanding about various aspects of prion biology. In this chapter, I review recent advances in our understanding of prion biology that derive from this powerful and informative approach.

Transgenic mice expressing an inhibitory truncated form of p300 exhibit long-term memory deficits

The formation of many forms of long-term memory requires several molecular mechanisms including regulation of gene expression. The mechanisms directing transcription require not only activation of individual transcription factors but also recruitment of transcriptional coactivators. CBP and p300 are transcriptional coactivators that interact with a large number of transcription factors and regulate transcription through multiple mechanisms, including an intrinsic histone acetyltransferase (HAT) activity. HAT activity mediates acetylation of lysine residues on the amino-terminal tails of histone proteins, thereby increasing DNA accessibility for transcription factors to activate gene expression. CBP has been shown to play an important role in long-term memory formation. We have investigated whether p300 is also required for certain forms of memory. p300 shares a high degree of homology with CBP and has been shown to interact with transcription factors known to be critical for long-term memory formation. Here we demonstrate that conditional transgenic mice expressing an inhibitory truncated form of p300 (p300Delta1), which lacks the carboxy-terminal HAT and activation domains, have impaired long-term recognition memory and contextual fear memory. Thus, our study demonstrates that p300 is required for certain forms of memory and that the HAT and carboxy-terminal domains play a critical role.

Vav promoter-tTA conditional transgene expression system for hematopoietic cells drives high level expression in developing B and T cells

OBJECTIVE

We previously showed that Vav promoter-tetracycline transactivator (Vav-tTA)-driven tetracycline-regulated element (TRE)-NRAS(V12) expression resulted in mastocytosis development in mice. To investigate which hematopoietic cells express TRE-driven transgenes when combined with Vav-tTA, we assayed hematopoietic cells, including bone marrow-derived mast cells (BMMC) and CD34-positive hematopoietic progenitor cells (HPC) as well as myeloid and lymphoid lineages. To determine if suppression of NRAS(V12) expression early in life would delay mastocytosis we treated developing and juvenile mice with doxycycline (Dox).

MATERIALS AND METHODS

Vav-tTA-driven luciferase expression was assayed by live mouse imaging and relative light unit measurement before or after treating Vav-tTA and TRE-luciferase (TRE-Luc) cotransgenic mice with Dox. Magnetic cell sorting and fluorescence-activating cell sorting methods were used to sort hematopoietic cells. To suppress TRE-mediated luciferase or NRAS(V12) expression in Vav-tTA cotransgenic mice, we added Dox to the drinking water.

RESULTS

B cells in the bone marrow and T cells in the thymus expressed Vav-tTA-driven luciferase at much higher levels than in myeloid cells, BMMC, and CD34-positive HPC, which showed relatively low levels. Dox treatment completely eliminated the luciferase expression from all hematopoietic cells. Repression of TRE-NRAS(V12) expression early in life was sufficient to increase the latency of mastocytosis development.

CONCLUSION

The Vav-tTA transgenic line will be very useful for conditional transgene expression in developing B and T cells. Vav-tTA-driven NRAS(V12) expression is sufficient for mastocytosis development, but not for myeloid leukemia. Lymphoid cells are resistant to NRAS(V12) transformation despite high level of expression.

Conditional NF-L transgene expression in mice for in vivo analysis of turnover and transport rate of neurofilaments

We generated mice with doxycycline control of a human neurofilament light (NF-L) transgene in the context of the absence (tTA;hNF-L;NF-L(-/-)) or presence (tTA;hNF-L;NF-L(+/-)) of endogenous mouse NF-L proteins. Doxycycline treatment caused the rapid disappearance of human NF-L (hNF-L) mRNA in tTA;hNF-L mice, but the hNF-L proteins remained with a half-life of 3 weeks in the brain. In the sciatic nerve, the disappearance of hNF-L proteins after doxycycline treatment occurred in synchrony along the sciatic nerve, suggesting a proteolysis of NF proteins along the entire axon. The presence of permanent NF network in tTA;hNF-L;NF-L(+/-) mice further stabilized and extended longevity of hNF-L proteins by several months. Surprisingly, after cessation of doxycycline treatment, there was no evidence of leading front of newly synthesized hNF-L proteins migrating into sciatic nerve axons devoid of NF structures. The hNF-L proteins detected at weekly intervals reappeared and accumulated in synchrony at similar rate along nerve segments, a phenomenon consistent with a fast hNF-L transport into axons. We estimated the hNF-L transport rate to be of approximately 10 mm/d in axons devoid of NF structures based on the use of an adenovirus encoding tet-responsive transcriptional activator to transactivate the hNF-L transgene in hypoglossal motor neurons. These results provide in vivo evidence that the stationary NF network in axons is a key determinant of half-life and transport rate of NF proteins.

Conditional mutant mice using tetracycline-controlled gene expression system in the brain

Generation of knockout mice with targeted mutations in desired genes is one of the most important technologies available for determining the functions of gene products in the brain. However, conventional knockout technology has limitations, such as when conventional knockout results in a lethal phenotype or when gene function at a certain developmental stage must be elucidated. To circumvent these limitations, a tetracycline-controlled gene expression system has been exploited to generate conditional mutant mice in which expression of desired genes can be switched on or off by oral administration of tetracycline derivatives. This up-date article introduces conditional mutant mice obtained using the tetracycline-controlled gene expression system, and presents several examples including our versatile mouse line, the mGluR1 conditional knockout mouse.

Motor behavioral and neuropathological deficits in mice deficient for normal prion protein expression

It has been difficult to reconcile the absence of pathology and apparently normal behavior of mice lacking prion protein (PrP), referred to as Prnp(0/0) mice, with a mechanism of prion pathogenesis involving progressive loss of PrP(C)-mediated neuroprotection. However, here we report that Prnp(0/0) mice exhibit significant age-related defects in motor coordination and balance compared with mice expressing wild type Prnp on a syngeneic background, and that the brains of behaviorally-impaired Prnp(0/0) mice display the cardinal neuropathological hallmarks of spongiform pathology and reactive astrocytic gliosis that normally accompany prion disease. Consistent with the appearance of cerebellar ataxia as an early symptom in patients with Gerstmann-Sträussler-Scheinker syndrome (GSS), an inherited form of human prion disease, motor coordination and balance defects manifested in a transgenic (Tg) mouse model of GSS considerably earlier than the onset of end-stage neurodegenerative disease. Our results are consistent with a mechanism in which loss of normal PrP(C) function is an important pathological component of prion diseases.

New movements in neurofilament transport, turnover and disease

Revealing the mechanisms by which neurofilament transport and turnover are regulated has proven difficult over the years but recent studies have given new insight into these processes. Mature neurofilament fibers may incorporate a fourth functional subunit, alpha-internexin, as new evidence suggests. Recent findings have made the role of phosphorylation in regulating neurofilament transport velocity controversial. Kinesin and dynein may transport neurofilaments in slow axonal transport as they have been found to associate with neurofilaments. Neurofilament transport and turnover rates may be reduced depending on the existing stationary neurofilament network. Finally, mutations in neurofilament light that have been linked to Charcot-Marie-Tooth disease as well as other neurofilament abnormalities in human disease are discussed.

Inducible and reversible Clock gene expression in brain using the tTA system for the study of circadian behavior

The mechanism of circadian oscillations in mammals is cell autonomous and is generated by a set of genes that form a transcriptional autoregulatory feedback loop. While these “clock genes” are well conserved among animals, their specific functions remain to be fully understood and their roles in central versus peripheral circadian oscillators remain to be defined. We utilized the in vivo inducible tetracycline-controlled transactivator (tTA) system to regulate Clock gene expression conditionally in a tissue-specific and temporally controlled manner. Through the use of Secretogranin II to drive tTA expression, suprachiasmatic nucleus– and brain-directed expression of a tetO::Clock^Δ19 dominant-negative transgene lengthened the period of circadian locomotor rhythms in mice, whereas overexpression of a tetO::Clock^wt wild-type transgene shortened the period. Low doses (10 μg/ml) of doxycycline (Dox) in the drinking water efficiently inactivated the tTA protein to silence the tetO transgenes and caused the circadian periodicity to return to a wild-type state. Importantly, low, but not high, doses of Dox were completely reversible and led to a rapid reactivation of the tetO transgenes. The rapid time course of tTA-regulated transgene expression demonstrates that the CLOCK protein is an excellent indicator for the kinetics of Dox-dependent induction/repression in the brain. Interestingly, the daily readout of circadian period in this system provides a real-time readout of the tTA transactivation state in vivo. In summary, the tTA system can manipulate circadian clock gene expression in a tissue-specific, conditional, and reversible manner in the central nervous system. The specific methods developed here should have general applicability for the study of brain and behavior in the mouse.

Although significant progress has been made in unraveling the molecular mechanism of circadian clocks in mammals, previous work has focused on germline mutations and in vitro methods for analysis. To address the function of clock genes, it is necessary to develop tools to manipulate circadian genes in a conditional and tissue-specific manner in vivo. We report such an approach using the tetracycline transactivator system. Despite the development of the “tet” system in transgenic mice over 10 y ago by Bujard and colleagues, there are still relatively few examples of the successful use of the tet system in the central nervous system. Transgenic expression of the Clock gene in the suprachiasmatic nucleus and brain of mice regulated the period length of circadian locomotor rhythms. These effects could be inhibited by low doses of doxycycline in the drinking water. Importantly, low, but not high, doses of doxycycline were completely reversible and led to a rapid reactivation of the Clock transgenes. In summary, the tetracycline-controlled transactivator system can manipulate circadian clock gene expression in a tissue-specific, conditional, and reversible manner in the central nervous system. The specific methods developed here should have general applicability for the study of brain and behavior in the mouse.

Conditional transgenesis and recombination to study the molecular mechanisms of brain plasticity and memory

In the postgenomic era, a primary focus of mouse genetics is to elucidate the role of individual genes in vivo. However, in the nervous system, studying the contribution of specific genes to brain functions is difficult because the brain is a highly complex organ with multiple neuroanatomical structures, orchestrating virtually every function in the body. Further, higher-order brain functions such as learning and memory simultaneously recruit several signaling cascades in different subcellular compartments and have highly fine-tuned spatial and temporal components. Conditional transgenic and gene targeting methodologies, however, now offer valuable tools with improved spatial and temporal resolution for appropriate studies of these functions. This chapter provides an overview of these tools and describes how they have helped gain better understanding of the role of candidate genes such as the NMDA receptor, the protein kinase CaMKIIIalpha, the protein phosphatases calcineurin and PP1, or the transcription factor CREB, in the processes of learning and memory. This review illustrates the broad and innovative applicability of these methodologies to the study of brain plasticity and cognitive functions.

The role of the cellular prion protein in the immune system

Prion protein (PrP) plays a key role in the pathogenesis of prion diseases. However, the normal function of the protein remains unclear. The cellular isoform (PrP(C)) is expressed widely in the immune system, in haematopoietic stem cells and mature lymphoid and myeloid compartments in addition to cells of the central nervous system. It is up-regulated in T cell activation and may be expressed at higher levels by specialized classes of lymphocyte. Furthermore, antibody cross-linking of surface PrP modulates T cell activation and leads to rearrangements of lipid raft constituents and increased phosphorylation of signalling proteins. These findings appear to indicate an important but, as yet, ill-defined role in T cell function. Although PrP(-/-) mice have been reported to have only minor alterations in immune function, recent work has suggested that PrP is required for self-renewal of haematopoietic stem cells. Here, we consider the evidence for a distinctive role for PrP(C) in the immune system and what the effects of anti-prion therapeutics may be on immune function.