Recombinant prion protein induces a new transmissible prion disease in wild-type animals

Prion disease is a neurodegenerative malady, which is believed to be transmitted via a prion protein in its abnormal conformation (PrP^Sc). Previous studies have failed to demonstrate that prion disease could be induced in wild-type animals using recombinant prion protein (rPrP) produced in Escherichia coli. Here, we report that prion infectivity was generated in Syrian hamsters after inoculating full-length rPrP that had been converted into the cross-β-sheet amyloid form and subjected to annealing. Serial transmission gave rise to a disease phenotype with highly unique clinical and neuropathological features. Among them were the deposition of large PrP^Sc plaques in subpial and subependymal areas in brain and spinal cord, very minor lesioning of the hippocampus and cerebellum, and a very slow progression of disease after onset of clinical signs despite the accumulation of large amounts of PrP^Sc in the brain. The length of the clinical duration is more typical of human and large animal prion diseases, than those of rodents. Our studies establish that transmissible prion disease can be induced in wild-type animals by inoculation of rPrP and introduce a valuable new model of prion diseases.     

Virus-induced alterations of membrane lipids affect the incorporation of PrP Sc into cells

Prion diseases are fatal neurodegenerative disorders characterized by long incubation periods. To investigate whether concurrent diseases can modify the clinical outcome of prion‐affected subjects, we tested the effect of viral infection on the binding and internalization of PrP^Sc, essential steps of prion propagation. To this effect, we added scrapie brain homogenate or purified PrP^Sc to fibroblasts previously infected with minute virus of mice (MVM), a mouse parvovirus. We show here that the rate of incorporation of PrP^Sc into MVM‐infected cells was significantly higher than that observed for naïve cells. Immunostaining of cells and immunoblotting of subcellular fractions using antibodies recognizing PrP and LysoTracker, a lysosomal marker, revealed that in both control and MVM‐infected cells the incorporated PrP^Sc was associated mostly with lysosomes. Interestingly, floatation gradient analysis revealed that the majority of the PrP^Sc internalized into MVM‐infected cells shifted toward raft‐containing low‐density fractions. Concomitantly, the MVM‐infected cells demonstrated increased levels of the glycosphingolipid GM1 (an essential raft lipid component) throughout the gradient and a shift in caveolin 1 (a raft protein marker) toward lighter membrane fractions compared with noninfected cells. Our results suggest that the effect of viral infection on membrane lipid composition may promote the incorporation of exogenous PrP^Sc into rafts. Importantly, membrane rafts are believed to be the conversion site of PrP^C to PrP^Sc; therefore, the association of exogenous PrP^Sc with such membrane microdomains may facilitate prion infection. © 2008 Wiley‐Liss, Inc.     

Molecular biology and pathology of prion strains in sporadic human prion diseases

Prion diseases are believed to propagate by the mechanism involving self-perpetuating conformational conversion of the normal form of the prion protein, PrP^C, to the misfolded, pathogenic state, PrP^Sc. One of the most intriguing aspects of these disorders is the phenomenon of prion strains. It is believed that strain properties are fully encoded in distinct conformations of PrP^Sc. Strains are of practical relevance to human prion diseases as their diversity may explain the unusual heterogeneity of these disorders. The first insight into the molecular mechanisms underlying heterogeneity of human prion diseases was provided by the observation that two distinct disease phenotypes and their associated PrP^Sc conformers co-distribute with distinct PrP genotypes as determined by the methionine/valine polymorphism at codon 129 of the PrP gene. Subsequent studies identified six possible combinations of the three genotypes (determined by the polymorphic codon 129) and two common PrP^Sc conformers (named types 1 and 2) as the major determinants of the phenotype in sporadic human prion diseases. This scenario implies that each 129 genotype–PrP^Sc type combination would be associated with a distinct disease phenotype and prion strain. However, notable exceptions have been found. For example, two genotype–PrP^Sc type combinations are linked to the same phenotype, and conversely, the same combination was found to be associated with two distinct phenotypes. Furthermore, in some cases, PrP^Sc conformers naturally associated with distinct phenotypes appear, upon transmission, to lose their phenotype-determining strain characteristics. Currently it seems safe to assume that typical sporadic prion diseases are associated with at least six distinct prion strains. However, the intrinsic characteristics that distinguish at least four of these strains remain to be identified.     

Continuous intraventricular infusion of pentosan polysulfate: Clinical trial against prion diseases

Prion diseases are progressive neurological disorders due to abnormal prion protein (PrP^Sc) deposition in the central nervous system. At present, there is no effective treatment available for any form of prion disease. Pentosan polysulfate (PPS) has been shown to prolong significantly the incubation period in mice with PrP^Sc infection when administered to the cerebral ventricles in preclinical trials. In human studies conducted in European countries and Japan, intraventricular PPS was administered to patients with different forms of prion disease and was well tolerated. We report 11 patients with prion disease treated with intraventricular PPS at Fukuoka University from 2004. Cases included three familial CJD (two with V180I mutation, one GSS with P102L mutation), two iatrogenic CJD, and six sporadic CJD cases. At present, average survival period after treatment was 24.2 months (range, 4–49). Seven cases died of sepsis and pneumonia. Subdural effusion with various degrees was seen on CT scan in most cases. Except for these, adverse effects did not occur in the treatment period. Although our preliminary study of the new treatment with PPS by continuous intraventricular infusion showed no apparent improvement of clinical features in patients with prion disease, the possibility of extended survival in some patients receiving long‐term PPS was suggested.     

On the biology of prions

Summary Prions cause scrapie and Creutzfeldt-Jakob disease (CJD); these infectious pathogens are composed largely, if not entirely, of protein molecules. No prion-specific polynucleotide has been identified. Purified preparations of scrapie prions contain high titers (10^9.5 ID₅₀/ml), one protein (PrP 27-30) and amyloid rods (10–20 nm in diameter ×100–200 nm in length). Considerable evidence indicates that PrP 27-30 is required for and inseparable from scrapie infectivity. PrP 27-30 is encoded by a cellular gene and is derived from a larger protein, denoted PrP^Sc or PrP 33-35^Sc, by protease digestion. A cellular isoform, designated PrP^C or PrP 33-35^C, is encoded by the same gene as PrP^Sc and both proteins appear to be translated from the same 2.1 kb mRNA. Monoclonal antibodies to PrP 27-30, as well as antisera to PrP synthetic peptides, specifically react with both PrP^C and PrP^Sc, establishing their relatedness. PrP^C is digested by proteinase K, while PrP^Sc is converted to PrP 27-30 under the same conditions. Prion proteins are synthesized with signal peptides and are integrated into membranes. Detergent extraction of microsomal membranes isolated from scrapie-infected hamster brains solubilizes PrP^C but induces PrP^Sc to polymerize into amyloid rods. This procedure allows separation of the two prion protein isoforms and the demonstration that PrP^Sc accumulates during scrapie infection, while the level of PrP^C does not change. The prion amyloid rods generated by detergent extraction are identical morphologically, except for length, to extracellular collections of prion amyloid filaments which form plaques in scrapie- and CJD-infected brains. The prion amyloid plaques stain with antibodies to PrP 27-30 and PrP peptides. PrP 33-35^C does not accumulate in the extracellular space. Prion rods composed of PrP 27-30 can be dissociated into phospholipid vesicles with full retention of scrapie infectivity. The murine PrP gene (Prn-p) is linked to thePrn-i gene which controls the length of the scrapie incubation period. Prolonged incubation times are a cardinal feature of scrapie and CJD. While the central role of PrP^Sc in scrapie pathogenesis is well established, the chemical as well as conformational differences between PrP^C and PrP^Sc are unknown but probably arise from post-translational modifications.Supported by research grants from the National Institutes of Health (AG 02132 and NS 14069) and a Senator Jacob Javits Center of Excellence in Neuroscience (NS 22786) as well as by gifts from RJR-Nabisco, Inc. and Sherman Fairchild FoundationThis review is based upon a plenary lecture entitled Biology and Neuropathology of Prions presented at the Xth International Congress of Neuropathology, Stockholm, Sweden, September 11, 1986, and is dedicated to the memory of Peter Wilhelm Lampert (1929–1986)     

Accumulation of prion protein in the vagus nerve in creutzfeldt–jakob disease

Disease‐associated proteins are thought to propagate along neuronal processes in neurodegenerative diseases. To detect disease‐associated prion protein (PrP^Sc) in the vagus nerve in different forms and molecular subtypes of Creutzfeldt–Jakob disease (CJD), we applied 3 different anti‐PrP antibodies. We screened the vagus nerve in 162 sporadic and 30 genetic CJD cases. Four of 31 VV‐2 type sporadic CJD and 7 of 30 genetic CJD cases showed vagal PrP^Sc immunodeposits with distinct morphology. Thus, PrP^Sc in CJD affects the vagus nerve analogously to α‐synuclein in Parkinson disease. The morphologically diverse deposition of PrP^Sc in genetic and sporadic CJD argues against uniform mechanisms of propagation of PrP^Sc. Ann Neurol 2019;85:782–787     

Disease associated prion protein may deposit in the peripheral nervous system in human transmissible spongiform encephalopathies

There is increasing evidence indicating involvement of the peripheral nervous system (PNS) in the pathogenesis of transmissible spongiform encephalopathies (TSEs). Immunocytochemically detectable deposits of TSE-specific abnormal prion protein (PrP^sc) are considered as a surrogate marker for infectivity. We used anti-PrP immunocytochemistry to trace PrP^sc deposition in spinal and enteric ganglia, and peripheral nerve in Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker disease (GSS), and fatal familial insomnia. Discrete PrP^sc deposits were detectable only in a few posterior root nerve fibers in an adaxonal location in one of nine CJD and the one GSS patients examined. Follicular dendritic cells of the gut and enteric nervous system were not labeled. Thus, PrP^sc may spread to the PNS in different forms of human prion disease. In contrast to our observations in experimental scrapie (Groschup et al., Acta Neuropathol, this issue), the deposits were scant. Possible explanations for this discrepancy comprise strain difference, or centripetal (experimental scrapie) versus centrifugal (sporadic and genetic human prion diseases) spread of PrP^sc, resulting in different patterns and amounts of PrP^sc accumulation in the PNS.     

Differential solubility of prions is associated in manifold phenotypes

The main feature of prion diseases is the accumulation of infectious proteins (PrP^Sc). Since PrP^Sc results from conversion of cellular prion proteins (PrP^C), differential expressed PrP^C types may play an important role in the formation and conversion efficiency to specific PrP^Sc forms. However, little is known about the PrP^C expression, regulation and differentiation. Here, we demonstrate a new type of differentiation of overlapping PrP^C isoforms in brain homogenates using differential SDS solubility. Low and highly soluble PrP^C were detected along with various types of protein which are present in the brain of non-infected humans, sheep and cattle. Our findings provide evidence for the existence of several overlapping PrP^C proteins exhibiting distinct glycotypes. The selection of defined PrP^C types offers new possibilities for identifying highly efficient converting proteins and provides the potential for disease control.     

Quantification of surviving cerebellar granule neurones and abnormal prion protein (PrPSc) deposition in sporadic Creutzfeldt-Jakob disease supports a pathogenic role for small PrPSc deposits common to the various molecular subtypes

B. A. Faucheux, E. Morain, V. Diouron, J.‐P. Brandel, D. Salomon, V. Sazdovitch, N. Privat, J.‐L. Laplanche, J.‐J. Hauw and S. Haïk (2011) Neuropathology and Applied Neurobiology 37, 500–512 Quantification of surviving cerebellar granule neurones and abnormal prion protein (PrP^Sc) deposition in sporadic Creutzfeldt–Jakob disease supports a pathogenic role for small PrP^Sc deposits common to the various molecular subtypes Aims: Neuronal death is a major neuropathological hallmark in prion diseases. The association between the accumulation of the disease‐related prion protein (PrP^Sc) and neuronal loss varies within the wide spectrum of prion diseases and their experimental models. In this study, we investigated the relationships between neuronal loss and PrP^Sc deposition in the cerebellum from cases of the six subtypes of sporadic Creutzfeldt–Jakob disease (sCJD; n = 100) that can be determined according to the M129V polymorphism of the human prion protein gene (PRNP) and PrP^Sc molecular types. Methods: The numerical density of neurones was estimated with a computer‐assisted image analysis system and the accumulation of PrP^Sc deposits was scored. Results: The scores of PrP^Sc immunoreactive deposits of the punctate type (synaptic type) were correlated with neurone counts – the higher the score the higher the neuronal loss – in all sCJD subtypes. Large 5‐ to 50‐µm‐wide deposits (focal type) were found in sCJD‐MV2 and sCJD‐VV2 subtypes, and occasionally in a few cases of the other studied groups. By contrast, the highest scores for 5‐ to 50‐µm‐wide deposits observed in sCJD‐MV2 subtype were not associated with higher neuronal loss. In addition, these scores were inversely correlated with neuronal counts in the sCJD‐VV2 subtype. Conclusions: These results support a putative pathogenic role for small PrP^Sc deposits common to the various sCJD subtypes. Furthermore, the observation of a lower loss of neurones associated with PrP^Sc type‐2 large deposits is consistent with a possible ‘protective’ role of aggregated deposits in both sCJD‐MV2 and sCJD‐VV2 subtypes.     

Subtyping of human cellular prion proteins and their differential solubility

A human form of a prion disorder is the Creutzfeldt-Jakob disease. A hallmark of the disease is the accumulation of misfolded prion proteins (PrP^Sc), which exist as heterogeneous subtypes. PrP^Sc is formed by protein conversion from the host-encoded cellular prion (PrP^C), which is expressed and modified to various isoforms. Little is known about variation in PrP^C; however, it is assumed that PrP^C types play important roles in the formation of PrP^Sc. In this study, we separated distinct human PrP^C subtypes on the basis of differential protein solubilities in detergent solutions. Single and sequential application of the detergents Triton X-100, octyl-glucopyranoside and CHAPS facilitated high solubility of glycosylated PrP^C isoforms, whereas high proportions of nonglycosylated PrP^C remained non-soluble. Most proteins became highly soluble with laurylsarcosine and sodium dodecyl sulphate. Our findings demonstrate that the solubility characteristics of heterogeneous PrP^C overlap in human brains and convey distinct solubility subtypes. Differentiation by solubility experiments can therefore provide valuable information on prion protein composition, facilitate the separation of subtypes, and offer new prospects for conversion specificity of distinct isoforms.     

Molecular pathogenesis of prion diseases

Prion diseases such as Creutzfeldt-Jakob disease in humans, scrapie in sheep, and BSE in cattle are transmissible and fatal neurodegenerative diseases. The infectious agent of these diseases has been designated as “prion”. It consists mainly and perhaps exclusively of a conformational variant of a physiological glycoprotein, the cellular prion, protein, PrP^C, which is a copper-binding protein of the cell surface. In spite of the wealth of biochemical and biophysical information, the conformational transition from PrP^C to PrP^Sc, the infectious isoform of the prion protein, is not well understood. Nerve cell loss in prion diseases may be caused by neurotoxic effects of the prion protein. Certain properties of the prion protein such as the apparent form of its glycosylation and conformational properties reflected by the preferential site of digestion with proteinase K are associated with particular phenotypes of prion disease. The appearance of a new variant of Creutzfeldt-Jakob disease in humans, which is most likely caused by the consumption of BSE-infected food in the UK, is cause for major concern particularly since there is no known effective treatment of prion diseases.     

A novel seven-octapeptide repeat insertion in the prion protein gene (PRNP) in a Dutch pedigree with Gerstmann�CStr?ussler�CScheinker disease phenotype: comparison with similar cases from the literature

Human prion diseases can be sporadic, inherited or acquired by infection and show considerable phenotypic heterogeneity. We describe the clinical, histopathological and pathological prion protein (PrP^Sc) characteristics of a Dutch family with a novel 7-octapeptide repeat insertion (7-OPRI) in PRNP, the gene encoding the prion protein (PrP). Clinical features were available in four, neuropathological features in three and biochemical characteristics in two members of this family. The clinical phenotype was characterized by slowly progressive cognitive decline, personality change, lethargy, depression with anxiety and panic attacks, apraxia and a hypokinetic-rigid syndrome. Neuropathological findings consisted of numerous multi- and unicentric amyloid plaques throughout the cerebrum and cerebellum with varying degrees of spongiform degeneration. Genetic and molecular studies were performed in two male family members. One of them was homozygous for valine and the other heterozygous for methionine and valine at codon 129 of PRNP. Sequence analysis identified a novel 168?bp insertion [R2?CR2?CR2?CR2?CR3g?CR2?CR2] in the octapeptide repeat region of PRNP. Both patients carried the mutation on the allele with valine at codon 129. Western blot analysis showed type 1 PrP^Sc in both patients and detected a smaller ~8?kDa PrP^Sc fragment in the cerebellum in one patient. The features of this Dutch kindred define an unusual neuropathological phenotype and a novel PRNP haplotype among the previously documented 7-OPRI mutations, further expanding the spectrum of genotype?Cphenotype correlations in inherited prion diseases.     

Prion infection of mice transgenic for human APPSwe: increased accumulation of cortical formic acid extractable Aβ(1–42) and rapid scrapie disease development

Neuropathological, epidemiological and experimental data indicate a potential interrelationship between Alzheimer's disease and prion diseases. Proteolytic processing of amyloid precursor protein (APP) by β-secretase was recently suggested to be controlled by prion protein expression. Here, we characterized the prion infection of Tg2576 mice, which overexpress the human APP_Swe protein. Prion infection of Tg2576-mice led to an early death of the animals, which was preceded by a relatively short symptomatic stage. However, disease-associated gliosis and deposition of misfolded prion protein PrP^Sc were identical in infected Tg2576-mice and non-transgenic littermate controls. To analyze the effect of prion infection on APP processing and generation of β-amyloid we determined cortical levels of SDS- and formic acid (FA)-extractable forms of β-amyloid (1–40) and (1–42) by ELISA. Formic acid-extractable Aβ (1–42) levels were 10-fold higher in infected versus uninfected Tg2576 mice whereas other forms of Aβ were essentially unaffected by the prion infection. Hence, the experimental model demonstrates that a prion infection of the CNS promotes selectively formation of FA-extractable Aβ(1–42) in Tg2576 mice.     

Genetic Creutzfeldt–Jakob disease and fatal familial insomnia: insights into phenotypic variability and disease pathogenesis

Human prion diseases are a group of rare neurodegenerative disorders characterized by the conversion of the constitutively expressed prion protein, PrP^C, into an abnormally aggregated isoform, called PrP^Sc. While most people who develop a prion disease have no identifiable cause and a few acquire the disease through an identified source of infection, about 10–15% of patients are affected by a genetic form and carry either a point mutation or an insertion of octapeptide repeats in the prion protein gene. Prion diseases show the highest extent of phenotypic heterogeneity among neurodegenerative disorders and comprise three major disease entities with variable though overlapping phenotypic features: Creutzfeldt–Jakob disease (CJD), fatal insomnia and the Gerstmann–Sträussler–Scheinker syndrome. Both CJD and fatal insomnia are fully transmissible diseases, a feature that led to the isolation and characterization of different strains of the agent or prion showing distinctive clinical and neuropathological features after transmission to syngenic animals. Here, we review the current knowledge of the effects of the pathogenic mutations linked to genetic CJD and fatal familial insomnia on the prion protein metabolism and physicochemical properties, the disease phenotype and the strain characteristics. The data derived from studies in vitro and from those using cell and animal models are compared with those obtained from the analyses of the naturally occurring disease. The extent of phenotypic variation in genetic prion disease is analyzed in comparison to that of the sporadic disease, which has recently been the topic of a systematic and detailed characterization.     

Phenotypic variability of sporadic human prion disease and its molecular basis: past,present, and future

Human prion diseases are rare neurodegenerative disorders related to prion protein misfolding that can occur as sporadic, familial or acquired forms. In comparison to other more common neurodegenerative disorders, prion diseases show a wider range of phenotypic variation and largely transmit to experimental animals, a feature that led to the isolation and characterization of different strains of the transmissible agent or prion with distinct biological properties. Biochemically distinct PrP^Sc types have been demonstrated which differ in their size after proteinase cleavage, glycosylation pattern, and possibly other features related to their conformation. These PrP^Sc types, possibly enciphering the prion strains, together with the naturally occurring polymorphism at codon 129 in the prion protein gene have a major influence on the disease phenotype. In the sporadic form, the most common but perhaps least understood form of human prion disease, there are at least six major combinations of codon 129 genotype and prion protein isotype, which are significantly related to distinctive clinical–pathological subgroups of the disease. In this review, we provide an update on the current knowledge and classification of the disease subtypes of the sporadic human prion diseases as defined by molecular features and pathological changes. Furthermore, we discuss the molecular basis of phenotypic variability taking into account the results of recent transmission studies that shed light on the extent of prion strain variation in humans.     

Cellular and sub-cellular pathology of animal prion diseases: relationship between morphological changes,accumulation of abnormal prion protein and clinical disease

The transmissible spongiform encephalopathies (TSEs) or prion diseases of animals are characterised by CNS spongiform change, gliosis and the accumulation of disease-associated forms of prion protein (PrP^d). Particularly in ruminant prion diseases, a wide range of morphological types of PrP^d depositions are found in association with neurons and glia. When light microscopic patterns of PrP^d accumulations are correlated with sub-cellular structure, intracellular PrP^d co-localises with lysosomes while non-intracellular PrP^d accumulation co-localises with cell membranes and the extracellular space. Intracellular lysosomal PrP^d is N-terminally truncated, but the site at which the PrP^d molecule is cleaved depends on strain and cell type. Different PrP^d cleavage sites are found for different cells infected with the same agent indicating that not all PrP^d conformers code for different prion strains. Non-intracellular PrP^d is full-length and is mainly found on plasma-lemmas of neuronal perikarya and dendrites and glia where it may be associated with scrapie-specific membrane pathology. These membrane changes appear to involve a redirection of the predominant axonal trafficking of normal cellular PrP and an altered endocytosis of PrP^d. PrP^d is poorly excised from membranes, probably due to increased stabilisation on the membrane of PrP^d complexed with other membrane ligands. PrP^d on plasma-lemmas may also be transferred to other cells or released to the extracellular space. It is widely assumed that PrP^d accumulations cause neurodegenerative changes that lead to clinical disease. However, when different animal prion diseases are considered, neurological deficits do not correlate well with any morphological type of PrP^d accumulation or perturbation of PrP^d trafficking. Non-PrP^d-associated neurodegenerative changes in TSEs include vacuolation, tubulovesicular bodies and terminal axonal degeneration. The last of these correlates well with early neurological disease in mice, but such changes are absent from large animal prion disease. Thus, the proximate cause of clinical disease in animal prion disease is uncertain, but may not involve PrP^d.     

Novel Compounds Identified by Structure-Based Prion Disease Drug Discovery Using In Silico Screening Delay the Progression of an Illness in Prion-Infected Mice

The accumulation of abnormal prion protein (PrP^Sc) produced by the structure conversion of PrP (PrP^C) in the brain induces prion disease. Although the conversion process of the protein is still not fully elucidated, it has been known that the intramolecular chemical bridging in the most fragile pocket of PrP, known as the “hot spot,” stabilizes the structure of PrP^C and inhibits the conversion process. Using our original structure-based drug discovery algorithm, we identified the low molecular weight compounds that predicted binding to the hot spot. NPR-130 and NPR-162 strongly bound to recombinant PrP in vitro, and fragment molecular orbital (FMO) analysis indicated that the high affinity of those candidates to the PrP is largely dependent on nonpolar interactions, such as van der Waals interactions. Those NPRs showed not only significant reduction of the PrP^Sc levels but also remarkable decrease of the number of aggresomes in persistently prion-infected cells. Intriguingly, treatment with those candidate compounds significantly prolonged the survival period of prion-infected mice and suppressed prion disease-specific pathological damage, such as vacuole degeneration, PrP^Sc accumulation, microgliosis, and astrogliosis in the brain, suggesting their possible clinical use. Our results indicate that in silico drug discovery using NUDE/DEGIMA may be widely useful to identify candidate compounds that effectively stabilize the protein.Electronic supplementary materialThe online version of this article (10.1007/s13311-020-00903-9) contains supplementary material, which is available to authorized users.     

Microglia and the pathogenesis of spongiform encephalopathies

Alterations in the phenotype and function of microglia, the resident mononuclear phagocytes of the central nervous system, are among the earliest indications of pathology within the brain and spinal cord. The prion diseases, also known as spongiform encephalopathies, are fatal neurodegenerative disorders with sporadic, genetic or acquired infectious manifestations. A hallmark of all prion diseases is the aberrant metabolism and resulting accumulation of the prion protein. Conversion of the normal cellular protein [PrP^c] into the abnormal pathogenic (or disease-causing) isoform [PrP^Sc] involves a conformational alteration whereby the α-helical content is transformed into β-sheet. The histological characteristics of these disorders are spongiform change, astrocytosis, neuronal loss and progressive accumulation of the protease-resistant prion isoform. An additional upregulation in microglial response has been reported in Kuru, Creutzfeldt–Jakob disease (CJD), Gerstmann–Sträussler–Scheinker syndrome (GSS), scrapie, in transgenic murine models and in culture, where microglial activation often accompanies prion protein deposition and neuronal loss. This article will review the roles of microglia in spongiform encephalopathies.