Human stem cell-derived astrocytes replicate human prions in a PRNP genotype-dependent manner

Cell adhesion molecule CD44 is dispensable for reactive astrocyte activation during prion disease

Prion diseases are fatal, infectious, neurodegenerative disorders resulting from accumulation of misfolded cellular prion protein in the brain. Early pathological changes during CNS prion disease also include reactive astrocyte activation with increased CD44 expression, microgliosis, as well as loss of dendritic spines and synapses. CD44 is a multifunctional cell surface adhesion and signalling molecule which is considered to play roles in astrocyte morphology and the maintenance of dendritic spine integrity and synaptic plasticity. However, the role of CD44 in prion disease was unknown. Here we used mice deficient in CD44 to determine the role of CD44 during prion disease. We show that CD44-deficient mice displayed no difference in their response to CNS prion infection when compared to wild type mice. Furthermore, the reactive astrocyte activation and microgliosis that accompanies CNS prion infection was unimpaired in the absence of CD44. Together, our data show that although CD44 expression is upregulated in reactive astrocytes during CNS prion disease, it is dispensable for astrocyte and microglial activation and the development of prion neuropathogenesis.

Understanding Creutzfeldt-Jakob disease in Iran: a systematic review of case reports

OBJECTIVE

To systematically review the reported cases of Creutzfeldt-Jakob disease (CJD) in Iran.

METHODS

A comprehensive literature review of CJD cases in Iran was undertaken using the PubMed®, Scopus® and Google Scholar databases. In addition, the Iranian database MagIran was searched for Persian language reports. Case selection used the following criteria: (i) patients of Iranian origin; (ii) publication in peer-reviewed journals or reputable medical databases; (iii) a definitive diagnosis of CJD based on established diagnostic criteria.

RESULTS

Thirteen cases from twelve reports were included in this systematic review. The majority of the cases were female (11 of 13; 84.6%). The mean ± SD age of patients at hospital admission was 59.38 ± 7.44 years. The findings of the case review suggested that the prevalence of CJD in Iran is not fully established. CJD may be misdiagnosed alongside other clinical signs. The most prevalent early indications of the disease were psychiatric and neurological in nature. A considerable delay in diagnosis was observed in some cases and there was a shortage of brain autopsy records.

CONCLUSION

Efforts to improve diagnostic capabilities, promote awareness and establish monitoring systems are necessary for managing the challenges of providing an early diagnosis of CJD in Iran.

Mesenchymal stem cell-derived exosomes: Characteristics and applications in disease pathology and management

Mesenchymal stem cells (MSCs) possess a role in tissue regeneration and homeostasis because of inherent immunomodulatory capacity and the production of factors that encourage healing. There is substantial evidence that MSCs' therapeutic efficacy is primarily determined by their paracrine function including in cancers. Extracellular vesicles (EVs) are basic paracrine effectors of MSCs that reside in numerous bodily fluids and cell homogenates and play an important role in bidirectional communication. MSC-derived EVs (MSC-EVs) offer a wide range of potential therapeutic uses that exceed cell treatment, while maintaining protocell function and having less immunogenicity. We describe characteristics and isolation methods of MSC-EVs, and focus on their therapeutic potential describing its roles in tissue repair, anti-fibrosis, and cancer with an emphasis on the molecular mechanism and immune modulation and clinical trials. We also explain current understanding and challenges in the clinical applications of MSC-EVs as a cell free therapy.

Two-color coincidence single-molecule pulldown for the specific detection of disease-associated protein aggregates

Protein misfolding and aggregation is a characteristic of many neurodegenerative disorders, including Alzheimer's and Parkinson's disease. The oligomers generated during aggregation are likely involved in disease pathogenesis and present promising biomarker candidates. However, owing to their small size and low concentration, specific tools to quantify and characterize aggregates in complex biological samples are still lacking. Here, we present single-molecule two-color aggregate pulldown (STAPull), which overcomes this challenge by probing immobilized proteins using orthogonally labeled detection antibodies. By analyzing colocalized signals, we can eliminate monomeric protein and specifically quantify aggregated proteins. Using the aggregation-prone alpha-synuclein protein as a model, we demonstrate that this approach can specifically detect aggregates with a limit of detection of 5 picomolar. Furthermore, we show that STAPull can be used in a range of samples, including human biofluids. STAPull is applicable to protein aggregates from a variety of disorders and will aid in the identification of biomarkers that are crucial in the effort to diagnose these diseases.

Potential Therapeutic Use of Stem Cells for Prion Diseases

Prion diseases are neurodegenerative disorders that are progressive, incurable, and deadly. The prion consists of PrPSc, the misfolded pathogenic isoform of the cellular prion protein (PrPC). PrPC is involved in a variety of physiological functions, including cellular proliferation, adhesion, differentiation, and neural development. Prion protein is expressed on the membrane surface of a variety of stem cells (SCs), where it plays an important role in the pluripotency and self-renewal matrix, as well as in SC differentiation. SCs have been found to multiply the pathogenic form of the prion protein, implying their potential as an in vitro model for prion diseases. Furthermore, due to their capability to self-renew, differentiate, immunomodulate, and regenerate tissue, SCs are prospective cell treatments in many neurodegenerative conditions, including prion diseases. Regenerative medicine has become a new revolution in disease treatment in recent years, particularly with the introduction of SC therapy. Here, we review the data demonstrating prion diseases' biology and molecular mechanism. SC biology, therapeutic potential, and its role in understanding prion disease mechanisms are highlighted. Moreover, we summarize preclinical studies that use SCs in prion diseases.

Serine metabolism during differentiation of human iPSC-derived astrocytes

Astrocytes are essential players in development and functions, being particularly relevant as regulators of brain energy metabolism, ionic homeostasis and synaptic transmission. They are also the major source of l-serine in the brain, which is synthesized from the glycolytic intermediate 3-phosphoglycerate through the phosphorylated pathway. l-Serine is the precursor of the two main co-agonists of the N-methyl-d-aspartate receptor, glycine and d-serine. Strikingly, dysfunctions in both l- and d-serine metabolism are associated with neurological and psychiatric disorders. Here, we exploited a differentiation protocol, based on the generation of human mature astrocytes from neural stem cells, and investigated the modification of the proteomic and metabolomic profile during the differentiation process. We show that differentiated astrocytes are more similar to mature rather than to reactive ones, and that axogenesis and pyrimidine metabolism increase up to 30 days along with the folate cycle and sphingolipid metabolism. Consistent with the proliferation and cellular maturation processes that are taking place, also the intracellular levels of l-serine, glycine, threonine, l- and d-aspartate (which level is unexpectedly higher than that of d-serine) show the same biosynthetic time course. A significant utilization of l-serine from the medium is apparent while glycine is first consumed and then released with a peak at 30 days, parallel to its intracellular level. These results underline how metabolism changes during astrocyte differentiation, highlight that d-serine synthesis is restricted in differentiated astrocytes and provide a valuable model for developing potential novel therapeutic approaches to address brain diseases, especially the ones related to serine metabolism alterations.

A single-cell map of antisense oligonucleotide activity in the brain

Antisense oligonucleotides (ASOs) dosed into cerebrospinal fluid (CSF) distribute broadly throughout the central nervous system (CNS). By modulating RNA, they hold the promise of targeting root molecular causes of disease and hold potential to treat myriad CNS disorders. Realization of this potential requires that ASOs must be active in the disease-relevant cells, and ideally, that monitorable biomarkers also reflect ASO activity in these cells. The biodistribution and activity of such centrally delivered ASOs have been deeply characterized in rodent and non-human primate (NHP) models, but usually only in bulk tissue, limiting our understanding of the distribution of ASO activity across individual cells and across diverse CNS cell types. Moreover, in human clinical trials, target engagement is usually monitorable only in a single compartment, CSF. We sought a deeper understanding of how individual cells and cell types contribute to bulk tissue signal in the CNS, and how these are linked to CSF biomarker outcomes. We employed single nucleus transcriptomics on tissue from mice treated with RNase H1 ASOs against Prnp and Malat1 and NHPs treated with an ASO against PRNP. Pharmacologic activity was observed in every cell type, though sometimes with substantial differences in magnitude. Single cell RNA count distributions implied target RNA suppression in every single sequenced cell, rather than intense knockdown in only some cells. Duration of action up to 12 weeks post-dose differed across cell types, being shorter in microglia than in neurons. Suppression in neurons was generally similar to, or more robust than, the bulk tissue. In macaques, PrP in CSF was lowered 40% in conjunction with PRNP knockdown across all cell types including neurons, arguing that a CSF biomarker readout is likely to reflect ASO pharmacodynamic effect in disease-relevant cells in a neuronal disorder. Our results provide a reference dataset for ASO activity distribution in the CNS and establish single nucleus sequencing as a method for evaluating cell type specificity of oligonucleotide therapeutics and other modalities.

Strain-Dependent Morphology of Reactive Astrocytes in Human- and Animal-Vole-Adapted Prions

Reactive astrogliosis is one of the pathological hallmarks of prion diseases. Recent studies highlighted the influence of several factors on the astrocyte phenotype in prion diseases, including the brain region involved, the genotype backgrounds of the host, and the prion strain. Elucidating the influence of prion strains on the astrocyte phenotype may provide crucial insights for developing therapeutic strategies. Here, we investigated the relationship between prion strains and astrocyte phenotype in six human- and animal-vole-adapted strains characterized by distinctive neuropathological features. In particular, we compared astrocyte morphology and astrocyte-associated PrP^Sc deposition among strains in the same brain region, the mediodorsal thalamic nucleus (MDTN). Astrogliosis was detected to some extent in the MDTN of all analyzed voles. However, we observed variability in the morphological appearance of astrocytes depending on the strain. Astrocytes displayed variability in thickness and length of cellular processes and cellular body size, suggesting strain-specific phenotypes of reactive astrocytes. Remarkably, four out of six strains displayed astrocyte-associated PrP^Sc deposition, which correlated with the size of astrocytes. Overall, these data show that the heterogeneous reactivity of astrocytes in prion diseases depends at least in part on the infecting prion strains and their specific interaction with astrocytes.

Organoids for modeling prion diseases

Human cerebral organoids are an exciting and novel model system emerging in the field of neurobiology. Cerebral organoids are spheres of self-organizing, neuronal lineage tissue that can be differentiated from human pluripotent stem cells and that present the possibility of on-demand human neuronal cultures that can be used for non-invasively investigating diseases affecting the brain. Compared with existing humanized cell models, they provide a more comprehensive replication of the human cerebral environment. The potential of the human cerebral organoid model is only just beginning to be elucidated, but initial studies have indicated that they could prove to be a valuable model for neurodegenerative diseases such as prion disease. The application of the cerebral organoid model to prion disease, what has been learned so far and the future potential of this model are discussed in this review.

A single-cell map of antisense oligonucleotide activity in the brain

Antisense oligonucleotides (ASOs) dosed into cerebrospinal fluid (CSF) distribute broadly throughout the brain and hold the promise of treating myriad brain diseases by modulating RNA. CNS tissue is not routinely biopsied in living individuals, leading to reliance on CSF biomarkers to inform on drug target engagement. Animal models can link CSF biomarkers to brain parenchyma, but our understanding of how individual cells contribute to bulk tissue signal is limited. Here we employed single nucleus transcriptomics on tissue from mice treated with RNase H1 ASOs against Prnp and Malat1 and macaques treated with an ASO against PRNP . Activity was observed in every cell type, though sometimes with substantial differences in magnitude. Single cell RNA count distributions implied target suppression in every single sequenced cell, rather than intense knockdown in only some cells. Duration of action up to 12 weeks post-dose differed across cell types, being shorter in microglia than in neurons. Suppression in neurons was generally similar to, or more robust than, the bulk tissue. In macaques, PrP in CSF was lowered 40% in conjunction with PRNP knockdown across all cell types including neurons, arguing that a CSF biomarker readout is likely to reflect disease-relevant cells in a neuronal disorder.

Microglia deficiency accelerates prion disease but does not enhance prion accumulation in the brain

Prion diseases are transmissible, neurodegenerative disorders associated with misfolding of the prion protein. Previous studies show that reduction of microglia accelerates central nervous system (CNS) prion disease and increases the accumulation of prions in the brain, suggesting that microglia provide neuroprotection by phagocytosing and destroying prions. In Csf1r^ΔFIRE mice, the deletion of an enhancer within Csf1r specifically blocks microglia development, however, their brains develop normally and show none of the deficits reported in other microglia-deficient models. Csf1r^ΔFIRE mice were used as a refined model in which to study the impact of microglia-deficiency on CNS prion disease. Although Csf1r^ΔFIRE mice succumbed to CNS prion disease much earlier than wild-type mice, the accumulation of prions in their brains was reduced. Instead, astrocytes displayed earlier, non-polarized reactive activation with enhanced phagocytosis of neuronal contents and unfolded protein responses. Our data suggest that rather than simply phagocytosing and destroying prions, the microglia instead provide host-protection during CNS prion disease and restrict the harmful activities of reactive astrocytes.

Recent advances in cellular models for discovering prion disease therapeutics

INTRODUCTION

Prion diseases are a group of rare and lethal rapidly progressive neurodegenerative diseases arising due to conversion of the physiological cellular prion protein into its pathological counterparts, denoted as "prions". These agents are resistant to inactivation by standard decontamination procedures and can be transmitted between individuals, consequently driving the irreversible brain damage typical of the diseases.

AREAS COVERED

Since its infancy, prion research has mainly depended on animal models for untangling the pathogenesis of the disease as well as for the drug development studies. With the advent of prion-infected cell lines, relevant animal models have been complemented by a variety of cell-based models presenting a much faster, ethically acceptable alternative.

EXPERT OPINION

To date, there are still either no effective prophylactic regimens or therapies for human prion diseases. Therefore, there is an urgent need for more relevant cellular models that best approximate in vivo models. Each cellular model presented and discussed in detail in this review has its own benefits and limitations. Once embarking in a drug screening campaign for the identification of molecules that could interfere with prion conversion and replication, one should carefully consider the ideal cellular model.

Astrocyte in prion disease: a double-edged sword

Prion diseases are infectious protein misfolding disorders of the central nervous system that result from misfolding of the cellular prion protein (PrPC) into the pathologic isoform PrPSc. Pathologic hallmarks of prion disease are depositions of pathological prion protein PrPSc, neuronal loss, spongiform degeneration and astrogliosis in the brain. Prion diseases affect human and animals, there is no effective therapy, and they invariably remain fatal. For a long time, neuronal loss was considered the sole reason for neurodegeneration in prion pathogenesis, and the contribution of non-neuronal cells like microglia and astrocytes was considered less important. Recent evidence suggests that neurodegeneration during prion pathogenesis is a consequence of a complex interplay between neuronal and non-neuronal cells in the brain, but the exact role of these non-neuronal cells during prion pathology is still elusive. Astrocytes are non-neuronal cells that regulate brain homeostasis under physiological conditions. However, astrocytes can deposit PrPSc aggregates and propagate prions in prion-infected brains. Additionally, sub-populations of reactive astrocytes that include neurotrophic and neurotoxic species have been identified, differentially expressed in the brain during prion infection. Revealing the exact role of astrocytes in prion disease is hampered by the lack of in vitro models of prion-infected astrocytes. Recently, we established a murine astrocyte cell line persistently infected with mouse-adapted prions, and showed how such astrocytes differentially process various prion strains. Considering the complexity of the role of astrocytes in prion pathogenesis, we need more in vitro and in vivo models for exploring the contribution of sub-populations of reactive astrocytes, their differential regulation of signaling cascades, and the interaction with neurons and microglia during prion pathogenesis. This will help to establish novel in vivo models and define new therapeutic targets against prion diseases. In this review, we will discuss the complex role of astrocytes in prion disease, the existing experimental resources, the challenges to analyze the contribution of astrocytes in prion disease pathogenesis, and future strategies to improve the understanding of their role in prion disease.

Probing TDP-43 condensation using an in silico designed aptamer

Aptamers are artificial oligonucleotides binding to specific molecular targets. They have a promising role in therapeutics and diagnostics but are often difficult to design. Here, we exploited the catRAPID algorithm to generate aptamers targeting TAR DNA-binding protein 43 (TDP-43), whose aggregation is associated with Amyotrophic Lateral Sclerosis. On the pathway to forming insoluble inclusions, TDP-43 adopts a heterogeneous population of assemblies, many smaller than the diffraction-limit of light. We demonstrated that our aptamers bind TDP-43 and used the tightest interactor, Apt-1, as a probe to visualize TDP-43 condensates with super-resolution microscopy. At a resolution of 10 nanometers, we tracked TDP-43 oligomers undetectable by standard approaches. In cells, Apt-1 interacts with both diffuse and condensed forms of TDP-43, indicating that Apt-1 can be exploited to follow TDP-43 phase transition. The de novo generation of aptamers and their use for microscopy opens a new page to study protein condensation.

Genetically engineered cellular models of prion propagation

For over three decades, cultured cells have been a useful tool for dissecting the molecular details of prion replication and the identification of candidate therapeutics for prion disease. A major issue limiting the translatability of these studies has been the inability to reliably propagate disease-relevant, non-mouse strains of prions in cells relevant to prion pathogenesis. In recent years, fueled by advances in gene editing technology, it has become possible to propagate prions from hamsters, cervids, and sheep in immortalized cell lines originating from the central nervous system. In particular, the use of CRISPR-Cas9-mediated gene editing to generate versions of prion-permissive cell lines that lack endogenous PrP expression has provided a blank canvas upon which re-expression of PrP leads to species-matched susceptibility to prion infection. When coupled with the ability to propagate prions in cells or organoids derived from stem cells, these next-generation cellular models should provide an ideal paradigm for identifying small molecules and other biological therapeutics capable of interfering with prion replication in animal and human prion disorders. In this review, we summarize recent advances that have widened the spectrum of prion strains that can be propagated in cultured cells and cutting-edge tissue-based models.

The role of astrocytes in prion-like mechanisms of neurodegeneration

Accumulating evidence suggests that neurodegenerative diseases are not merely neuronal in nature but comprise multicellular involvement, with astrocytes emerging as key players. The pathomechanisms of several neurodegenerative diseases involve the deposition of misfolded protein aggregates in neurons that have characteristic prion-like behaviours such as template-directed seeding, intercellular propagation, distinct conformational strains and protein-mediated toxicity. The role of astrocytes in dealing with these pathological prion-like protein aggregates and whether their responses either protect from or conspire with the disease process is currently unclear. Here we review the existing literature implicating astrocytes in multiple neurodegenerative proteinopathies with a focus on prion-like behaviour in this context.

Propagation and Dissemination Strategies of Transmissible Spongiform Encephalopathy Agents in Mammalian Cells

Transmissible spongiform encephalopathies or prion disorders are fatal infectious diseases that cause characteristic spongiform degeneration in the central nervous system. The causative agent, the so-called prion, is an unconventional infectious agent that propagates by converting the host-encoded cellular prion protein PrP into ordered protein aggregates with infectious properties. Prions are devoid of coding nucleic acid and thus rely on the host cell machinery for propagation. While it is now established that, in addition to PrP, other cellular factors or processes determine the susceptibility of cell lines to prion infection, exact factors and cellular processes remain broadly obscure. Still, cellular models have uncovered important aspects of prion propagation and revealed intercellular dissemination strategies shared with other intracellular pathogens. Here, we summarize what we learned about the processes of prion invasion, intracellular replication and subsequent dissemination from ex vivo cell models.

Therapeutically harnessing extracellular vesicles

The field of extracellular vesicle (EV) research has developed rapidly over the last decade from the study of fundamental biology to a subject of significant clinical relevance. The potential of harnessing EVs in the diagnosis and treatment of diseases - including cancer and neurological and cardiovascular disorders - is now being recognized. Accordingly, the applications of EVs as therapeutic targets, biomarkers, novel drug delivery agents and standalone therapeutics are being actively explored. This Review provides a brief overview of the characteristics and physiological functions of the various classes of EV, focusing on their association with disease and emerging strategies for their therapeutic exploitation.

Glial activation in prion diseases is selectively triggered by neuronal PrPSc

Although prion infections cause cognitive impairment and neuronal death, transcriptional and translational profiling shows progressive derangement within glia but surprisingly little changes within neurons. Here we expressed PrP^C selectively in neurons and astrocytes of mice. After prion infection, both astrocyte and neuron‐restricted PrP^C expression led to copious brain accumulation of PrP^Sc. As expected, neuron‐restricted expression was associated with typical prion disease. However, mice with astrocyte‐restricted PrP^C expression experienced a normal life span, did not develop clinical disease, and did not show astro‐ or microgliosis. Besides confirming that PrP^Sc is innocuous to PrP^C‐deficient neurons, these results show that astrocyte‐born PrP^Sc does not activate the extreme neuroinflammation that accompanies the onset of prion disease and precedes any molecular changes of neurons. This points to a nonautonomous mechanism by which prion‐infected neurons instruct astrocytes and microglia to acquire a specific cellular state that, in turn, drives neural dysfunction.

Primary glia cells from bank vole propagate multiple rodent-adapted scrapie prions

Since the beginning prion research has been largely dependent on animal models for deciphering the disease, drug development or prion detection and quantification. Thereby, ethical as well as cost and labour-saving aspects call for alternatives in vitro. Cell models can replace or at least complement animal studies, but their number is still limited and the application usually restricted to certain strains and host species due to often strong transmission barriers. Bank voles promise to be an exception as they or materials prepared from them are uniquely susceptible to prions from various species in vivo, in vitro and in cell-free applications. Here we present a mainly astrocyte-based primary glia cell assay from bank vole, which is infectible with scrapie strains from bank vole, mouse and hamster. Stable propagation of bank vole-adapted RML, murine 22L and RML, and hamster 263K scrapie is detectable from 20 or 30 days post exposure onwards. Thereby, the infected bank vole glia cells show similar or even faster prion propagation than likewise infected glia cells of the corresponding murine or hamster hosts. We propose that our bank vole glia cell assay could be a versatile tool for studying and comparing multiple prion strains with different species backgrounds combined in one cell assay.

A human iPSC-astroglia neurodevelopmental model reveals divergent transcriptomic patterns in schizophrenia

While neurodevelopmental abnormalities have been associated with schizophrenia (SCZ), the role of astroglia in disease pathophysiology remains poorly understood. In the present study, we used a human induced pluripotent stem cell (iPSC)-derived astrocyte model to investigate the temporal patterns of astroglia differentiation during developmental stages critical for SCZ using RNA sequencing. The model generated astrocyte-specific gene expression patterns during differentiation that corresponded well to astroglia-specific expression signatures of in vivo cortical fetal development. Using this model we identified SCZ-specific expression dynamics, and found that SCZ-associated differentially expressed genes were significantly enriched in the medial prefrontal cortex, striatum, and temporal lobe, targeting VWA5A and ADAMTS19. In addition, SCZ astrocytes displayed alterations in calcium signaling, and significantly decreased glutamate uptake and metalloproteinase activity relative to controls. These results implicate novel transcriptional dynamics in astrocyte differentiation in SCZ together with functional changes that are potentially important biological components of SCZ pathology.

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.

Propagation of CJD Prions in Primary Murine Glia Cells Expressing Human PrPc

There are various existing cell models for the propagation of animal prions. However, in vitro propagation of human prions has been a long-standing challenge. This study presents the establishment of a long-term primary murine glia culture expressing the human prion protein homozygous for methionine at codon 129, which allows in vitro propagation of Creutzfeldt-Jakob disease (CJD) prions (variant CJD (vCJD) and sporadic CJD (sCJD) type MM2). Prion propagation could be detected by Western blotting of pathological proteinase K-resistant prion protein (PrPSc) from 120 days post exposure. The accumulation of PrPSc could be intensified by adding a cationic lipid mixture to the infectious brain homogenate at the time of infection. Stable propagation of human prions in a long-term murine glia cell culture represents a new tool for future drug development and for mechanistic studies in the field of human prion biology. In addition, our cell model can reduce the need for bioassays with human prions and thereby contributes to further implementation of the 3R principles aiming at replacement, reduction and refinement of animal experiments.

Novel Morphological Glial Alterations in the Spectrum of Prion Disease Types: A Focus on Common Findings

Human prion diseases are a group of rare fatal neurodegenerative diseases with sporadic, genetic, and acquired forms. They are neuropathologically characterized by pathological prion protein accumulation, neuronal death, and vacuolation. Classical immunological response has long been known not to play a major in prion diseases; however, gliosis is known to be a common feature although variable in extent and poorly described. In this investigation, astrogliosis and activated microglia in two brain regions were assessed and compared with non-neurologically affected patients in a representative sample across the spectrum of Creutzfeldt–Jakob disease (CJD) forms and subtypes in order to analyze the influence of prion strain on pathological processes. In this report, we choose to focus on features common to all CJD types rather than the diversity among them. Novel pathological changes in both glial cell types were found to be shared by all CJD types. Microglial activation correlated to astrogliosis. Spongiosis, but not pathological prion protein deposition, correlated to both astrogliosis and microgliosis. At the ultrastructural level, astrocytic glial filaments correlated with pathological changes associated with prion disease. These observations confirm that neuroglia play a prominent role in the neurodegenerative process of prion diseases, regardless of the causative prion type.

Human stem cell-based models for studying host-pathogen interactions

The use of human cell lines and primary cells as in vitro models represents a valuable approach to study cellular responses to infection. However, with the advent of new molecular technologies and tools available, there is a growing need to develop more physiologically relevant systems to overcome cell line model limitations and better mimic human disease. Since the discovery of human stem cells, its use has revolutionised the development of in vitro models. This is because after differentiation, these cells have the potential to reflect in vivo cell phenotypes and allow for probing questions in numerous fields of the biological sciences. Moreover, the possibility to combine the advantages of stem cell-derived cell types with genome editing technologies and engineered 3D microenvironments, provides enormous potential for producing in vitro systems to investigate cellular responses to infection that are both relevant and predictive. Here, we discuss recent advances in the use of human stem cells to model host-pathogen interactions, highlighting emerging technologies in the field of stem cell biology that can be exploited to investigate the fundamental biology of infection. TAKE AWAYS: hPSC overcome current limitations to study host-pathogen interactions in vitro. Genome editing can be used in hPSC to study cellular responses to infection. hPSC, 3D models and genome editing can recreate physiological in vitro systems.

Utilising Induced Pluripotent Stem Cells in Neurodegenerative Disease Research: Focus on Glia

Induced pluripotent stem cells (iPSCs) are a self-renewable pool of cells derived from an organism's somatic cells. These can then be programmed to other cell types, including neurons. Use of iPSCs in research has been two-fold as they have been used for human disease modelling as well as for the possibility to generate new therapies. Particularly in complex human diseases, such as neurodegenerative diseases, iPSCs can give advantages over traditional animal models in that they more accurately represent the human genome. Additionally, patient-derived cells can be modified using gene editing technology and further transplanted to the brain. Glial cells have recently become important avenues of research in the field of neurodegenerative diseases, for example, in Alzheimer's disease and Parkinson's disease. This review focuses on using glial cells (astrocytes, microglia, and oligodendrocytes) derived from human iPSCs in order to give a better understanding of how these cells contribute to neurodegenerative disease pathology. Using glia iPSCs in in vitro cell culture, cerebral organoids, and intracranial transplantation may give us future insight into both more accurate models and disease-modifying therapies.

Effect of Scrapie Prion Infection in Ovine Bone Marrow-Derived Mesenchymal Stem Cells and Ovine Mesenchymal Stem Cell-Derived Neurons

Simple Summary

Prion diseases are neurodegenerative disorders affecting humans and animals. The development of in vitro cellular models from naturally susceptible species like humans or ruminants can potentially make a great contribution to the study of many aspects of these diseases, including the ability of prions to infect and replicate in cells and therapeutics. Our study shows for the first time how ovine mesenchymal stem cells derived from bone marrow and their neural-like progeny are able to react to scrapie prion infection in vitro and assesses the effects of this infection on cell viability and proliferation. Finally, we observe that the differentiation of ovine mesenchymal stem cells into neuron-like cells makes them more permissive to prion infection.

Abstract

Scrapie is a prion disease affecting sheep and goats and it is considered a prototype of transmissible spongiform encephalopathies (TSEs). Mesenchymal stem cells (MSCs) have been proposed as candidates for developing in vitro models of prion diseases. Murine MSCs are able to propagate prions after previous mouse-adaptation of prion strains and, although ovine MSCs express the cellular prion protein (PrP^C), their susceptibility to prion infection has never been investigated. Here, we analyze the potential of ovine bone marrow-derived MSCs (oBM-MSCs), in growth and neurogenic conditions, to be infected by natural scrapie and propagate prion particles (PrP^Sc) in vitro, as well as the effect of this infection on cell viability and proliferation. Cultures were kept for 48–72 h in contact with homogenates of central nervous system (CNS) samples from scrapie or control sheep. In growth conditions, oBM-MSCs initially maintained detectable levels of PrP^Sc post-inoculation, as determined by Western blotting and ELISA. However, the PrP^Sc signal weakened and was lost over time. oBM-MSCs infected with scrapie displayed lower cell doubling and higher doubling times than those infected with control inocula. On the other hand, in neurogenic conditions, oBM-MSCs not only maintained detectable levels of PrP^Sc post-inoculation, as determined by ELISA, but this PrP^Sc signal also increased progressively over time. Finally, inoculation with CNS extracts seems to induce the proliferation of oBM-MSCs in both growth and neurogenic conditions. Our results suggest that oBM-MSCs respond to prion infection by decreasing their proliferation capacity and thus might not be permissive to prion replication, whereas ovine MSC-derived neuron-like cells seem to maintain and replicate PrP^Sc.

Human cerebral organoids as a therapeutic drug screening model for Creutzfeldt-Jakob disease

Creutzfeldt-Jakob Disease (CJD) is a fatal, currently incurable, neurodegenerative disease. The search for candidate treatments would be greatly facilitated by the availability of human cell-based models of prion disease. Recently, an induced pluripotent stem cell derived human cerebral organoid model was shown to take up and propagate human CJD prions. This model offers new opportunities to screen drug candidates for the treatment of human prion diseases in an entirely human genetic background. Here we provide the first evidence that human cerebral organoids can be a viable model for CJD drug screening by using an established anti-prion compound, pentosan polysulfate (PPS). PPS delayed prion propagation in a prophylactic-like treatment paradigm and also alleviated propagation when applied following establishment of infection in a therapeutic-like treatment paradigm. This study demonstrates the utility of cerebral organoids as the first human 3D cell culture system for screening therapeutic drug candidates for human prion diseases.

Neuroinflammation in Prion Disease

Neuroinflammation, typically manifest as microglial activation and astrogliosis accompanied by transcriptomic alterations, represents a common hallmark of various neurodegenerative conditions including prion diseases. Microglia play an overall neuroprotective role in prion disease, whereas reactive astrocytes with aberrant phenotypes propagate prions and contribute to prion-induced neurodegeneration. The existence of heterogeneous subpopulations and dual functions of microglia and astrocytes in prion disease make them potential targets for therapeutic intervention. A variety of neuroinflammation-related molecules are involved in prion pathogenesis. Therapeutics targeting neuroinflammation represents a novel approach to combat prion disease. Deciphering neuroinflammation in prion disease will deepen our understanding of pathogenesis of other neurodegenerative disorders.

Astrocytes in rare neurological conditions: Morphological and functional considerations

Astrocytes are a population of central nervous system (CNS) cells with distinctive morphological and functional characteristics that differ within specific areas of the brain and are widely distributed throughout the CNS. There are mainly two types of astrocytes, protoplasmic and fibrous, which differ in morphologic appearance and location. Astrocytes are important cells of the CNS that not only provide structural support, but also modulate synaptic activity, regulate neuroinflammatory responses, maintain the blood-brain barrier, and supply energy to neurons. As a result, astrocytic disruption can lead to widespread detrimental effects and can contribute to the pathophysiology of several neurological conditions. The characteristics of astrocytes in more common neuropathologies such as Alzheimer's and Parkinson's disease have significantly been described and continue to be widely studied. However, there still exist numerous rare neurological conditions in which astrocytic involvement is unknown and needs to be explored. Accordingly, this review will summarize functional and morphological changes of astrocytes in various rare neurological conditions based on current knowledge thus far and highlight remaining neuropathologies where astrocytic involvement has yet to be investigated.

Non-cell autonomous astrocyte-mediated neuronal toxicity in prion diseases

Under normal conditions, astrocytes perform a number of important physiological functions centered around neuronal support and synapse maintenance. In neurodegenerative diseases including Alzheimer’s, Parkinson’s and prion diseases, astrocytes acquire reactive phenotypes, which are sustained throughout the disease progression. It is not known whether in the reactive states associated with prion diseases, astrocytes lose their ability to perform physiological functions and whether the reactive states are neurotoxic or, on the contrary, neuroprotective. The current work addresses these questions by testing the effects of reactive astrocytes isolated from prion-infected C57BL/6J mice on primary neuronal cultures. We found that astrocytes isolated at the clinical stage of the disease exhibited reactive, pro-inflammatory phenotype, which also showed downregulation of genes involved in neurogenic and synaptogenic functions. In astrocyte-neuron co-cultures, astrocytes from prion-infected animals impaired neuronal growth, dendritic spine development and synapse maturation. Toward examining the role of factors secreted by reactive astrocytes, astrocyte-conditioned media was found to have detrimental effects on neuronal viability and synaptogenic functions via impairing synapse integrity, and by reducing spine size and density. Reactive microglia isolated from prion-infected animals were found to induce phenotypic changes in primary astrocytes reminiscent to those observed in prion-infected mice. In particular, astrocytes cultured with reactive microglia-conditioned media displayed hypertrophic morphology and a downregulation of genes involved in neurogenic and synaptogenic functions. In summary, the current study provided experimental support toward the non-cell autonomous mechanisms behind neurotoxicity in prion diseases and demonstrated that the astrocyte reactive phenotype associated with prion diseases is synaptotoxic.

From Cell Culture to Organoids-Model Systems for Investigating Prion Strain Characteristics

Prion diseases are the hallmark protein folding neurodegenerative disease. Their transmissible nature has allowed for the development of many different cellular models of disease where prion propagation and sometimes pathology can be induced. This review examines the range of simple cell cultures to more complex neurospheres, organoid, and organotypic slice cultures that have been used to study prion disease pathogenesis and to test therapeutics. We highlight the advantages and disadvantages of each system, giving special consideration to the importance of strains when choosing a model and when interpreting results, as not all systems propagate all strains, and in some cases, the technique used, or treatment applied, can alter the very strain properties being studied.

Current and future applications of induced pluripotent stem cell-based models to study pathological proteins in neurodegenerative disorders

Neurodegenerative disorders emerge from the failure of intricate cellular mechanisms, which ultimately lead to the loss of vulnerable neuronal populations. Research conducted across several laboratories has now provided compelling evidence that pathogenic proteins can also contribute to non-cell autonomous toxicity in several neurodegenerative contexts, including Alzheimer's, Parkinson's, and Huntington's diseases as well as Amyotrophic Lateral Sclerosis. Given the nearly ubiquitous nature of abnormal protein accumulation in such disorders, elucidating the mechanisms and routes underlying these processes is essential to the development of effective treatments. To this end, physiologically relevant human in vitro models are critical to understand the processes surrounding uptake, release and nucleation under physiological or pathological conditions. This review explores the use of human-induced pluripotent stem cells (iPSCs) to study prion-like protein propagation in neurodegenerative diseases, discusses advantages and limitations of this model, and presents emerging technologies that, combined with the use of iPSC-based models, will provide powerful model systems to propel fundamental research forward.

Administration of FK506 from Late Stage of Disease Prolongs Survival of Human Prion-Inoculated Mice

Human prion diseases are etiologically categorized into three forms: sporadic, genetic, and infectious. Sporadic Creutzfeldt-Jakob disease (sCJD) is the most common type of human prion disease that manifests as subacute progressive dementia. No effective therapy for sCJD is currently available. Potential therapeutic compounds are frequently tested in rodents infected with mouse-adapted prions that differ from human prions. However, therapeutic effect varies depending on the prion strain, which is one of the reasons why candidate compounds have shown little effect in sCJD patients. We previously reported that intraperitoneal administration of FK506 was able to prolong the survival of mice infected with a mouse-adapted prion by suppressing the accumulation of abnormal prion protein (PrP) and inhibiting the activation of microglia. In this study, we tested oral administration of FK506 in knock-in mice expressing chimeric human prion protein (KiChM) that were infected with sCJD to determine if this compound is also effective against a clinically relevant human prion, i.e., one that has not been adapted to mice. Treatment with FK506, started either just before or just after disease onset, suppressed typical sCJD pathology (gliosis) and slightly but significantly prolonged the survival of sCJD-inoculated mice. It would be worthwhile to conduct a clinical trial using FK506, which has been safety-approved and is widely used as a mild immunosuppressant.

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.

Decreased IL-1β-induced CCL20 response in human iPSC-astrocytes in schizophrenia: Potential attenuating effects on recruitment of regulatory T cells

Schizophrenia (SCZ) is a severe mental disorder with a high heritability. Although its pathophysiology is mainly unknown, dysregulated immune activation and inflammation have recently emerged as possible candidates in the underlying mechanisms of SCZ. Previous studies suggest that aberrant inflammasome activation, glia dysregulation, and brain inflammation may be involved in the pathophysiology of the disorder. Here, we studied the effects of inflammatory modulation on human induced pluripotent stem cell (iPSC)-derived astrocytes generated from SCZ patients and healthy controls (CTRL). Inflammasome activation was mimicked by short-term IL-1β exposure, and gene expression were measured with high-coverage RNA-Seq to ensure a global characterization of the transcriptional effects of the treatment. IL-1β exposure modulated several pathways involved in innate immune responses, cell cycle regulation, and metabolism in both SCZ and CTRL astrocytes. Significant differences were found in the expression of HILPDA and CCL20 genes, both of which had reduced up-regulation upon IL-1β treatment in SCZ astrocytes compared to CTRL astrocytes. CCL20 data were further validated and confirmed using qPCR, ELISA, and regulatory T lymphocyte (T_reg) migration assays. Additionally, we found significantly decreased mRNA expression of the T_reg-specific marker FOXP3 in the blood of a large cohort of SCZ patients (n = 484) compared to CTRL (n = 472). Since CCL20 is a specific chemoattractant for CD4⁺CD25⁺CCR6⁺ T_regs, which are crucially involved in anti-inflammatory responses during brain (auto)inflammation, our results imply a plausible role for an altered astroglia-CCL20-CCR6-T_reg axis in SCZ pathophysiology.

Prion infection, transmission, and cytopathology modeled in a low-biohazard human cell line

Expanding the toolbox of prion research to a low-biohazard, scalable human cell model.

Transmission of prion infectivity to susceptible murine cell lines has simplified prion titration assays and has greatly reduced the need for animal experimentation. However, murine cell models suffer from technical and biological constraints. Human cell lines might be more useful, but they are much more biohazardous and are often poorly infectible. Here, we describe the human clonal cell line hovS, which lacks the human PRNP gene and expresses instead the ovine PRNP VRQ allele. HovS cells were highly susceptible to the PG127 strain of sheep-derived murine prions, reaching up to 90% infected cells in any given culture and were maintained in a continuous infected state for at least 14 passages. Infected hovS cells produced proteinase K–resistant prion protein (PrP^Sc), pelletable PrP aggregates, and bona fide infectious prions capable of infecting further generations of naïve hovS cells and mice expressing the VRQ allelic variant of ovine PrP^C. Infection in hovS led to prominent cytopathic vacuolation akin to the spongiform changes observed in individuals suffering from prion diseases. In addition to expanding the toolbox for prion research to human experimental genetics, the hovS cell line provides a human-derived system that does not require human prions. Hence, the manipulation of scrapie-infected hovS cells may present fewer biosafety hazards than that of genuine human prions.

An astrocyte cell line that differentially propagates murine prions

Prion diseases are fatal infectious neurodegenerative disorders in human and animals caused by misfolding of the cellular prion protein (PrP^C) into the pathological isoform PrP^Sc. Elucidating the molecular and cellular mechanisms underlying prion propagation may help to develop disease interventions. Cell culture systems for prion propagation have greatly advanced molecular insights into prion biology, but translation of in vitro to in vivo findings is often disappointing. A wider range of cell culture systems might help overcome these shortcomings. Here, we describe an immortalized mouse neuronal astrocyte cell line (C8D1A) that can be infected with murine prions. Both PrP^C protein and mRNA levels in astrocytes were comparable with those in neuronal and non-neuronal cell lines permitting persistent prion infection. We challenged astrocytes with three mouse-adapted prion strains (22L, RML, and ME7) and cultured them for six passages. Immunoblotting results revealed that the astrocytes propagated 22L prions well over all six passages, whereas ME7 prions did not replicate, and RML prions replicated only very weakly after five passages. Immunofluorescence analysis indicated similar results for PrP^Sc. Interestingly, when we used prion conversion activity as a readout in real-time quaking-induced conversion assays with RML-infected cell lysates, we observed a strong signal over all six passages, comparable with that for 22L-infected cells. These data indicate that the C8D1A cell line is permissive to prion infection. Moreover, the propagated prions differed in conversion and proteinase K–resistance levels in these astrocytes. We propose that the C8D1A cell line could be used to decipher prion strain biology.

Enhanced phosphorylation of PERK in primary cultured neurons as an autonomous neuronal response to prion infection

Conversion of cellular prion protein (PrP^C) into the pathogenic isoform of prion protein (PrP^Sc) in neurons is one of the key pathophysiological events in prion diseases. However, the molecular mechanism of neurodegeneration in prion diseases has yet to be fully elucidated because of a lack of suitable experimental models for analyzing neuron-autonomous responses to prion infection. In the present study, we used neuron-enriched primary cultures of cortical and thalamic mouse neurons to analyze autonomous neuronal responses to prion infection. PrP^Sc levels in neurons increased over the time after prion infection; however, no obvious neuronal losses or neurite alterations were observed. Interestingly, a finer analysis of individual neurons co-stained with PrP^Sc and phosphorylated protein kinase RNA-activated-like endoplasmic reticulum (ER) kinase (p-PERK), the early cellular response of the PERK-eukaryotic initiation factor 2 (eIF2α) pathway, demonstrated a positive correlation between the number of PrP^Sc granular stains and p-PERK granular stains, in cortical neurons at 21 dpi. Although the phosphorylation of PERK was enhanced in prion-infected cortical neurons, there was no sign of subsequent translational repression of synaptic protein synthesis or activations of downstream unfolded protein response (UPR) in the PERK-eIF2α pathway. These results suggest that PrP^Sc production in neurons induces ER stress in a neuron-autonomous manner; however, it does not fully activate UPR in prion-infected neurons. Our findings provide insights into the autonomous neuronal responses to prion propagation and the involvement of neuron-non-autonomous factor(s) in the mechanisms of neurodegeneration in prion diseases.

In vitro Models of Neurodegenerative Diseases

Neurodegenerative diseases are progressive degenerative conditions characterized by the functional deterioration and ultimate loss of neurons. These incurable and debilitating diseases affect millions of people worldwide, and therefore represent a major global health challenge with severe implications for individuals and society. Recently, several neuroprotective drugs have failed in human clinical trials despite promising pre-clinical data, suggesting that conventional cell cultures and animal models cannot precisely replicate human pathophysiology. To bridge the gap between animal and human studies, three-dimensional cell culture models have been developed from human or animal cells, allowing the effects of new therapies to be predicted more accurately by closely replicating some aspects of the brain environment, mimicking neuronal and glial cell interactions, and incorporating the effects of blood flow. In this review, we discuss the relative merits of different cerebral models, from traditional cell cultures to the latest high-throughput three-dimensional systems. We discuss their advantages and disadvantages as well as their potential to investigate the complex mechanisms of human neurodegenerative diseases. We focus on in vitro models of the most frequent age-related neurodegenerative disorders, such as Parkinson’s disease, Alzheimer’s disease and prion disease, and on multiple sclerosis, a chronic inflammatory neurodegenerative disease affecting young adults.

Markers of A1 astrocytes stratify to molecular sub-types in sporadic Creutzfeldt-Jakob disease brain

Astrocytes are glial cells of the central nervous system that become reactive under conditions of stress. The functional properties of reactive astrocytes depend on their stimulus that induces the upregulation of specific genes. Reactive astrocytes are a neuropathological feature of prion disorders; however, their role in the disease pathogenesis is not well understood. Here, we describe our studies of one polarization state of reactive astrocytes, termed A1 astrocytes, in the frontal cortex region of 35 human sporadic Creutzfeldt-Jakob disease brains encompassing a range of molecular sub-types. Examination of two mRNA markers of A1 astrocytes, C3 and GBP2, revealed a strong linear correlation between the two following their log-normalization (P = 0.0011). Both markers were found upregulated in the sporadic Creutzfeldt-Jakob disease brain compared with age-matched control tissues (P = 0.0029 and 0.0002, for C3log and GBP2log, respectively), and stratifying samples based on codon 129 genotype revealed that C3log is highest in homozygous methionine and lowest in homozygous valine patients, which followed a linear trend (P = 0.027). Upon assessing other disease parameters, a significant positive correlation was found between GBP2log and disease duration (P = 0.031). These findings provide evidence for a divergence in the astrocytic environment amongst patients with sporadic Creutzfeldt-Jakob disease based on molecular sub-type parameters of disease. While more research will be needed to determine the global changes in the genomic profiles and resulting functional properties of reactive astrocytes in disease, considering the evidence demonstrating that A1 astrocytes harbour neurotoxic properties, the changes seen in C3log and GBP2log in the current study may reflect differences in pathogenic mechanisms amongst the sporadic Creutzfeldt-Jakob disease sub-types associated with the A1 polarization state.

Cellular models for discovering prion disease therapeutics: Progress and challenges

Prions, which cause fatal neurodegenerative disorders such as Creutzfeldt-Jakob disease, are misfolded and infectious protein aggregates. Currently, there are no treatments available to halt or even delay the progression of prion disease in the brain. The infectious nature of prions has resulted in animal paradigms that accurately recapitulate all aspects of prion disease, and these have proven to be instrumental for testing the efficacy of candidate therapeutics. Nonetheless, infection of cultured cells with prions provides a much more powerful system for identifying molecules capable of interfering with prion propagation. Certain lines of cultured cells can be chronically infected with various types of mouse prions, and these models have been used to unearth candidate anti-prion drugs that are at least partially efficacious when administered to prion-infected rodents. However, these studies have also revealed that not all types of prions are equal, and that drugs active against mouse prions are not necessarily effective against prions from other species. Despite some recent progress, the number of cellular models available for studying non-mouse prions remains limited. In particular, human prions have proven to be particularly challenging to propagate in cultured cells, which has severely hindered the discovery of drugs for Creutzfeldt-Jakob disease. In this review, we summarize the cellular models that are presently available for discovering and testing drugs capable of blocking the propagation of prions and highlight challenges that remain on the path towards developing therapies for prion disease.

Region-specific glial homeostatic signature in prion diseases is replaced by a uniform neuroinflammation signature, common for brain regions and prion strains with different cell tropism

Chronic neuroinflammation is recognized as a major neuropathological hallmark in a broad spectrum of neurodegenerative diseases including Alzheimer's, Parkinson's, Frontal Temporal Dementia, Amyotrophic Lateral Sclerosis, and prion diseases. Both microglia and astrocytes exhibit region-specific homeostatic transcriptional identities, which under chronic neurodegeneration, transform into reactive phenotypes in a region- and disease-specific manner. Little is known about region-specific identity of glia in prion diseases. The current study was designed to determine whether the region-specific homeostatic signature of glia changes with the progression of prion diseases, and whether these changes occur in a region-dependent or universal manner. Also of interest was whether different prion strains give rise to different reactive phenotypes. To answer these questions, we analyzed gene expression in the thalamus, cortex, hypothalamus and hippocampus of mice infected with 22L and ME7 prion strains using a Nanostring Neuroinflammation panel at the subclinical, early clinical and advanced stages of the disease. We found that at the preclinical stage of the disease, the region-specific homeostatic identities were preserved. However, with the appearance of clinical signs, the region-specific signatures were partially lost and replaced with a neuroinflammation signature. While the same sets of genes were activated by both prion strains, the timing of neuroinflammation and the degree of activation in different brain regions was strain-specific. Changes in astrocyte function scored at the top of the activated pathways. Moreover, clustering analysis suggested that the astrocyte function pathway responded to prion infection prior to the Activated Microglia or Neuron and Neurotransmission pathways. The current work established neuroinflammation gene expression signature associated with prion diseases. Our results illustrate that with the disease progression, the region-specific homeostatic transcriptome signatures are replaced by the region-independent neuroinflammation signature, which is common for prion strains with different cell tropism. The prion-associated neuroinflammation signature identified in the current study overlapped only partially with the microglia degenerative phenotype and the disease-associated microglia phenotype reported for animal models of other neurodegenerative diseases.

Region-Specific Response of Astrocytes to Prion Infection

Chronic neuroinflammation involves reactive microgliosis and astrogliosis, and is regarded as a common pathological hallmark of neurodegenerative diseases including Alzheimer’s, Parkinson’s, ALS and prion diseases. Reactive astrogliosis, routinely observed immunohistochemically as an increase in glial fibrillary acidic protein (GFAP) signal, is a well-documented feature of chronic neuroinflammation associated with neurodegenerative diseases. Recent studies on single-cell transcriptional profiling of a mouse brain revealed that, under normal conditions, several distinct subtypes of astrocytes with regionally specialized distribution exist. However, it remains unclear whether astrocytic response to pro-inflammatory pathological conditions is uniform across whole brain or is region-specific. The current study compares the response of microglia and astrocytes to prions in mice infected with 22L mouse-adapted prion strain. While the intensity of reactive microgliosis correlated well with the extent of PrP^Sc deposition, reactive astrogliosis displayed a different, region-specific pattern. In particular, the thalamus and stratum oriens of hippocampus, which are both affected by 22L prions, displayed strikingly different response of astrocytes to PrP^Sc. Astrocytes in stratum oriens of hippocampus responded to accumulation of PrP^Sc with visible hypertrophy and increased GFAP, while in the thalamus, despite stronger PrP^Sc signal, the increase of GFAP was milder than in hippocampus, and the change in astrocyte morphology was less pronounced. The current study suggests that astrocyte response to prion infection is heterogeneous and, in part, defined by brain region. Moreover, the current work emphasizes the needs for elucidating region-specific changes in functional states of astrocytes and exploring the impact of these changes to chronic neurodegeneration.

Discrimination of Prion Strain Targeting in the Central Nervous System via Reactive Astrocyte Heterogeneity in CD44 Expression

Prion diseases or transmissible spongiform encephalopathies are fatal, progressive, neurodegenerative, protein-misfolding disorders. Prion diseases may arise spontaneously, be inherited genetically or be acquired by infection and affect a variety of mammalian species including humans. Prion infections in the central nervous system (CNS) cause extensive neuropathology, including abnormal accumulations of misfolded host prion protein, vacuolar change resulting in sponge-like (spongiform) appearance of CNS tissue, neurodegeneration and reactive glial responses. Many different prion agent strains exist and these can differ based on disease duration, clinical signs and the targeting and distribution of the neuropathology in distinct brain areas. Reactive astrocytes are a prominent feature in the prion disease affected CNS as revealed by distinct morphological changes and upregulation of glial fibrillary acidic protein (GFAP). The CD44 antigen is a transmembrane glycoprotein involved in cell-cell interactions, cell adhesion and migration. Here we show that CD44 is also highly expressed in a subset of reactive astrocytes in regions of the CNS targeted by prions. Astrocyte heterogeneity revealed by differential CD44 upregulation occurs coincident with the earliest neuropathological changes during the pre-clinical phase of disease, and is not affected by the route of infection. The expression and distribution of CD44 was compared in brains from a large collection of 15 distinct prion agent strains transmitted to mice of different prion protein (Prnp) genotype backgrounds. Our data show that the pattern of CD44 upregulation observed in the hippocampus in each prion agent strain and host Prnp genotype combination was unique. Many mouse-adapted prion strains and hosts have previously been characterized based on the pattern of the distribution of the spongiform pathology or the misfolded PrP deposition within the brain. Our data show that CD44 expression also provides a reliable discriminatory marker of prion infection with a greater dynamic range than misfolded prion protein deposition, aiding strain identification. Together, our data reveal CD44 as a novel marker to detect reactive astrocyte heterogeneity during CNS prion disease and for enhanced identification of distinct prion agent strains.