Comparative ultrastructural neuropathology of naturally occurring bovine spongiform encephalopathy and experimentally induced scrapie and Creutzfeldt-Jakob disease

Diminished Neuronal ESCRT-0 Function Exacerbates AMPA Receptor Derangement and Accelerates Prion-Induced Neurodegeneration

Endolysosomal defects in neurons are central to the pathogenesis of prion and other neurodegenerative disorders. In prion disease, prion oligomers traffic through the multivesicular body (MVB) and are routed for degradation in lysosomes or for release in exosomes, yet how prions impact proteostatic pathways is unclear. We found that prion-affected human and mouse brain showed a marked reduction in Hrs and STAM1 (ESCRT-0), which route ubiquitinated membrane proteins from early endosomes into MVBs. To determine how the reduction in ESCRT-0 impacts prion conversion and cellular toxicity in vivo, we prion-challenged conditional knockout mice (male and female) having Hrs deleted from neurons, astrocytes, or microglia. The neuronal, but not astrocytic or microglial, Hrs-depleted mice showed a shortened survival and an acceleration in synaptic derangements, including an accumulation of ubiquitinated proteins, deregulation of phosphorylated AMPA and metabotropic glutamate receptors, and profoundly altered synaptic structure, all of which occurred later in the prion-infected control mice. Finally, we found that neuronal Hrs (nHrs) depletion increased surface levels of the cellular prion protein, PrPC, which may contribute to the rapidly advancing disease through neurotoxic signaling. Taken together, the reduced Hrs in the prion-affected brain hampers ubiquitinated protein clearance at the synapse, exacerbates postsynaptic glutamate receptor deregulation, and accelerates neurodegeneration.SIGNIFICANCE STATEMENT Prion diseases are rapidly progressive neurodegenerative disorders characterized by prion aggregate spread through the central nervous system. Early disease features include ubiquitinated protein accumulation and synapse loss. Here, we investigate how prion aggregates alter ubiquitinated protein clearance pathways (ESCRT) in mouse and human prion-infected brain, discovering a marked reduction in Hrs. Using a prion-infection mouse model with neuronal Hrs (nHrs) depleted, we show that low neuronal Hrs is detrimental and markedly shortens survival time while accelerating synaptic derangements, including ubiquitinated protein accumulation, indicating that Hrs loss exacerbates prion disease progression. Additionally, Hrs depletion increases the surface distribution of prion protein (PrPC), linked to aggregate-induced neurotoxic signaling, suggesting that Hrs loss in prion disease accelerates disease through enhancing PrPC-mediated neurotoxic signaling.

Ultrastructural changes in the progress of natural Scrapie regardless fixation protocol

Because few studies regarding ultrastructural pathological changes associated with natural prion diseases have been performed, the present study primarily intended to determine consistent lesions at the subcellular level and to demonstrate whether these changes are evident regardless of the fixation protocol. Thus far, no assessment method has been developed for classifying the possible variations according to the disease stage, although such an assessment would contribute to clarifying the pathogenesis of this neurodegenerative disease. Therefore, animals at different disease stages were included here. This study presents the first description of lesions associated with natural Scrapie in the cerebellum. Vacuolation, which preferentially occurs around Purkinje cells and which displays a close relation with glial cells, is one of the most novel observations provided in this study. The disruption of hypolemmal cisterns in this neuronal type and the presence of a primary cilium in the granular layer both represent the first findings concerning prion diseases. The possibility of including samples regardless of their fixation protocol is confirmed in this work. Therefore, a high proportion of tissue bank samples that are currently being wasted can be included in ultrastructural studies, which constitute a valuable source for information regarding physiological and pathological samples.

Frontotemporal dementia caused by CHMP2B mutations

CHMP2B mutations are a rare cause of autosomal dominant frontotemporal dementia (FTD). The best studied example is frontotemporal dementia linked to chromosome 3 (FTD-3) which occurs in a large Danish family, with a further CHMP2B mutation identified in an unrelated Belgian familial FTD patient. These mutations lead to C-terminal truncations of the CHMP2B protein and we will review recent advances in our understanding of the molecular effects of these mutant truncated proteins on vesicular fusion events within the endosome-lysosome and autophagy degradation pathways. We will also review the clinical features of FTD caused by CHMP2B truncation mutations as well as new brain imaging and neuropathological findings. Finally, we collate the current data on CHMP2B missense mutations, which have been reported in FTD and motor neuron disease.

Prion protein disease and neuropathology of prion disease

Human prion diseases, in common with other neurodegenerative diseases, may be sporadic or inherited and are characterized by the accumulation of cellular proteins accompanied by neuronal death and synaptic loss. Prion diseases are, however, unique in being transmissible. Central to the pathogenesis of all forms of prion disease is the prion protein. This article provides a brief overview of the biology of human prion diseases followed by a more in-depth discussion of the neuropathology of these diseases, including features of neuroradiologic relevance.

Tubulovesicular structures are a consistent (and unexplained) finding in the brains of humans with prion diseases

Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker disease (GSS) and Fatal Familial Insomnia (FFI) are slow neurodegenerative disorders classified as transmissible spongiform encephalopathies (TSEs) or prion diseases, which appear in sporadic, hereditary or environmentally acquired forms. Tubulovesicular structures (TVS) are ultrastructural particles of unknown origin and chemical composition found in the brains of both animal and human forms of transmissible spongiform encephalopathies or prion diseases. In this paper, we report the results of a search for TVS in a total of 13 cases of sporadic Creutzfeldt-Jakob disease, three cases of Gerstmann-Sträussler-Scheinker disease, two cases of Fatal Familial Insomnia, and individual cases of familial, iatrogenic, and variant CJD (vCJD). TVS were found in all but one sporadic and one familial case of CJD. As controls, we examined 15 cases of Alzheimer's disease (AD), two cases of Pick's disease, and one case of multiple system atrophy. TVS were not present in any of these cases. This study confirms the TSE-specificity of TVS, the morphology of which suggests a possible pathogenetic role and relationship to recently described virion-like arrays of 25nm particles in scrapie-infected tissue cultures.

Inhibition of P53-related apoptosis had no effect on PrPSc accumulation and prion disease incubation time

Results from several laboratories indicate that apoptosis via the P53 pathway is involved in prion disease pathogenesis. Prion diseases, among them scrapie and BSE, are a group of fatal neurodegenerative disorders associated with the conversion of PrP(C) to PrP(Sc), its conformational abnormal isoform. In this work, we tested whether an established anti-apoptotic reagent, PFT, which has been shown in different systems to inhibit P53 activity, can delay the outbreak of prion disease in infected animals. Our findings indicate that although PFT efficiently reduced caspase 3 expression in brains from scrapie sick hamsters, as well as inhibited PrP(Sc) accumulation in cell culture, it had no effect on disease incubation time or PrP(Sc) accumulation in vivo. We conclude that the P53 dependent apoptosis may not be an obligatory mechanism for prion disease-induced cell death.

Sub-cellular pathology of scrapie: coated pits are increased in PrP codon 136 alanine homozygous scrapie-affected sheep

Sub-cellular studies of transmissible spongiform encephalopathies (TSEs) have been carried out on several animal species and human beings. However, studies of optimal perfusion-fixed tissues have largely been confined to examination of rodents. Using a recently developed technique, heads of scrapie-affected sheep and controls were perfusion fixed with mixed aldehydes. The obexes were immunohistochemically labelled with PrP antibodies, and the dorsal motor nucleus of the vagal nerve was examined by electron microscopy. Irregular neuritic profiles with highly invaginated membranes, associated with coated pits were found in all scrapie-affected sheep, but not in controls. Interestingly, they were consistently more frequent in the homozygous A(136) sheep. This is the first report describing sub-cellular differences in pathology associated with different PrP genotypes. Rarely, amorphous material, or sparse fibrillar structures, were present in the extracellular space. The changes were often associated with irregular plasmalemma and frequent coated pits. Vacuolation typical of TSEs, dystrophic neurites and variable gliosis were present. Herniation of membranes and organelles from apparently healthy processes into adjacent vacuoles and dendrites was also observed. We suggest that the increase in coated pits and plasmalemma invagination is related to an attempted internalisation of aggregated disease-specific PrP, or protofilaments, from the extracellular space.

Neuropathology of prion diseases

In prion diseases, neuropathology has remained the most important tool to give a definite diagnosis, and neuropathological research has contributed significantly to our current pathogenetic understanding. Immunohistochemistry for the disease-associated prion protein (PrP(Sc)) is indispensable for the neuropathological confirmation of prion diseases. The amount and distribution of PrP(Sc) deposits do not always correlate with type and severity of local tissue damage. PrP(Sc) deposition occurs only where neuronal parenchyma is present; in scarred infarctions with prominent gliosis, PrP(Sc) does not accumulate. Early, severe and selective loss affects a subset of inhibitory GABAergic neurons both in human and experimental prion diseases. The central pathogenetic cascade includes oxidative stress to neurons and their apoptosis. New patterns of PrP(Sc) immunoreactivity include granular ganglionic and tiny adaxonal PrP(Sc) deposits in peripheral nervous tissue, and dendritic cells and macrophages in vessel walls, suggesting that mobile haematogenous cells may be involved in spread of prions.

Tubulovesicular structures: what are they really?

The tubulovesicular structures (TVS) are the only structures unique at the level of thin-section electron microscopy for all TSEs so far examined. They were first described in NIH Swiss mice infected intracerebrally with the "Chandler" strain of scrapie by David-Ferreira et al. in 1968 [Proc. Soc. Exp. Biol. Med. 127:313-320]. TVS were described as "particles and rods ranging in diameter from 320 to 360 A(o)." The exact topology of TVS is not entirely clear. In most published electron micrographs, TVS appeared as spheres measuring between 20 and 40 nm in diameter. The number of neuritic processes containing TVS increases through the incubation period and has been shown to correlate with the incubation period and titre of infectivity in three longitudinal disease studies of scrapie and CJD. These studies, therefore, suggest that TVS may represent a primary pathogenetic event rather than a pathological product of disease. The predominant theory of the scrapie agent is now the "prion hypothesis" and its derivatives, which implies that a conformationally altered abnormal isoform (PrP(Sc) or PrP*) of a normal cellular membrane glycoprotein (PrP(c)) is the agent and its accumulation merely mimicks replication. If an abnormal fraction of PrP is indeed the infectious agent, (although it is no longer suggested in some quarters that protease resistant fraction of PrP(Sc) is the agent). The absence of stainable PrP in TVS, however, would indicate that they are not the ultrastructural correlate of the agent. However, TVS appear to be specific and unique to the TSEs, appearing before the earliest pathological changes and increasing in line with incubation period or titre. The very existence of TVS and their correlation with infectivity, therefore, urgently needs an explanation.

Heightened expression of tumor necrosis factor alpha, interleukin 1 alpha, and glial fibrillary acidic protein in experimental Creutzfeldt-Jakob disease in mice

The ultrastructural pathology of myelinated axons in mice infected experimentally with the Fujisaki strain of Creutzfeldt-Jakob disease (CJD) virus is characterized by myelin sheath vacuolation that closely resembles that induced in murine spinal cord organotypic cultures by tumor necrosis factor alpha (TNF-alpha), a cytokine produced by astrocytes and macrophages. To clarify the role of TNF-alpha in experimental CJD, we investigated the expression of TNF-alpha in brain tissues from CJD virus-infected mice at weekly intervals after inoculation by reverse transcription-coupled PCR, Northern and Western blot analyses, and immunocytochemical staining. Neuropathological findings by electron microscopy, as well as expression of interleukin 1 alpha and glial fibrillary acidic protein, were concurrently monitored. As determined by reverse transcription-coupled PCR, the expression of TNF-alpha, interleukin 1 alpha, and glial fibrillary acidic protein was increased by approximately 200-fold in the brains of CJD virus-inoculated mice during the course of disease. By contrast, beta-actin expression remained unchanged. Progressively increased expression of TNF-alpha in CJD virus-infected brain tissues was verified by Northern and Western blot analyses, and astrocytes in areas with striking myelin sheath vacuolation were intensely stained with an antibody against murine TNF-alpha. The collective findings of TNF-alpha overexpression during the course of clinical disease suggest that TNF-alpha may mediate the myelin sheath vacuolation observed in experimental CJD.

Spongy degeneration in the zitter rat: ultrastructural and immunohistochemical studies

Pathological changes in the grey matter of the zitter rat were examined by electron microscopy and immunohistochemistry to investigate the pathogenesis of spongy degeneration. Vacuole formation was first detected in the pons and the outer thalamus at 2 weeks of age. The vacuoles arose from the periaxonal or inter-myelinic spaces as well as the cytoplasm of some oligodendrocytes or astrocytes. With increasing age, some dendrites and the cytoplasm of neurons developed an electron lucent area with sparse organelles and the vacuoles occasionally fused together. Although spongy degeneration gradually extended to the entire CNS, no inflammatory or phagocytotic cell infiltration and no viral particles were detected. Glial fibrillary acidic protein immunoreactivity increased transiently in the vacuolated areas from 2 to 15 weeks of age (maximal at 7 weeks of age). Although zitter rats older than 65 weeks showed some reactive astrocytes in vacuolated areas, their numbers and the intensity of immunostaining decreased with advanced vacuolation suggesting astrocytic hypofunction in response to tissue damage. Immunoreactivity for synaptophysin was weaker in the zitter rats than in the control rats throughout the observation period, which suggested that synapse formation was disturbed in the zitter rats, probably due to a combination of hypomyelination and vacuole formation in the grey matter. These findings suggest that an unknown genetic abnormality, probably related to cell membrane biosynthesis or cell-to-cell interactions, produces both hypomyelination and spongy degeneration in the zitter rat.

Neuroaxonal dystrophy in experimental Creutzfeldt-Jakob disease: electron microscopical and immunohistochemical demonstration of neurofilament accumulations within affected neurites

Neuroaxonal dystrophy is a feature of neuronal degeneration encountered in all subacute spongiform "virus" encephalopathies, including scrapie and Creutzfeldt-Jakob disease (CJD). By immunohistochemical techniques, the accumulation of 200 kDa neurofilament protein was demonstrated in affected neurites in murine CJD. These neurites exhibited the ultrastructural features of dystrophic neurites encountered in other neurodegenerative disorders, particularly Alzheimer's disease. These findings support the hypothesis that impairment of slow axoplasmic transport is a common pathogenetic mechanism for CJD and many other neurodegenerative conditions.

The neuropathology and epidemiology of bovine spongiform encephalopathy

Bovine spongiform encephalopathy (BSE), defined originally from its characteristic neuropathology, retains a place of particular interest in the scrapie-like or prion disease group, presenting uniquely an example of such diseases occurring as a nationwide food-borne epidemic in Great Britain. Comprehensive monitoring of the epidemic, both pathologically and epidemiologically, has facilitated our present understanding of the disease. BSE presents the classical neuropathological features of the transmissible spongiform encephalopathies. Although particularly similar to natural scrapie of sheep, BSE has, unlike scrapie, a stereotypic lesion profile from which it has been concluded that host and agent factors, including probably the strain of agent, which influence the profile, are constant in this disease. Neuronal loss in BSE may make an important but hitherto inapparent contribution to functional deficits. Preliminary ultrastructural studies have confirmed light microscopic features of brain changes in BSE but have as yet not established significant new findings. Immunohistochemical studies of PrP accumulation reveal distinctive forms and distributions of immunolabelling, confirming features reported previously in experimental models of scrapie, including perineuronal and perineuritic "synapse-like" reactivity. The histopathological diagnosis of BSE, validated on a single section of the medulla for the statutory diagnosis of large numbers of cases, is supplemented where necessary by fibril (SAF) examination which performs similarly to the histological diagnosis in the majority of cases. Epidemiological studies of BSE have supported the pathological findings that there is no detectable variation in susceptibility within the cattle population. The detailed monitoring of the epidemic has revealed the expected effects on the incidence as a result of statutory measures intended to prevent food-borne exposure after July 1988. The main effect has been a reduction in the national incidence during 1993 which has been continued into 1994. Analytical studies have not revealed any means of transmission, other than the food-borne source, capable of maintaining the epidemic in Great Britain. An international comparison of risk factors for the occurrence of BSE indicates that an epidemic of similar magnitude outside the British Isles is unlikely.

Transmissible cerebral amyloidoses as a model for Alzheimer's disease. An ultrastructural perspective

Alzheimer's disease, a prototypic nontransmissible cerebral amyloidosis, has no adequate experimental model. Several pathogenetic events, however, may be modeled and accurately studied in the transmissible cerebral amyloidoses of kuru, Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker disease, and scrapie. The common neuropathological denominator in both types of cerebral amyloidoses is the presence of stellate kuru plaques, senile plaques, and pure neuritic plaques. These amyloid plaques consist of amyloid fibers, dystrophic neurites, and reactive astrocytes in different proportions. Microglial cells, which are regarded as amyloid producer/processor cells in Alzheimer's disease, may play the same function in the transmissible cerebral amyloidoses. In both transmissible and nontransmissible amyloidoses, the impairment of axonal transport leads to accumulation of abnormally phosphorylated cytoskeleton proteins (such as neurofilament proteins and microtubule-associated protein tau), which eventually produce dystrophic neurites observed as parts of plaque or as isolated pathological structures.